From 8ff4713bcf124f891932ca25d8b4c02cff712c60 Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Tue, 10 Dec 2024 09:03:32 +0100
Subject: [PATCH 1/6] Represent the arity of uncurried function definitions
 directly in the AST.

---
 analysis/src/CompletionFrontEnd.ml            |   2 +-
 analysis/src/DumpAst.ml                       |   2 +-
 analysis/src/Xform.ml                         |   6 +-
 compiler/frontend/ast_compatible.ml           |   4 +-
 compiler/frontend/ast_compatible.mli          |   7 +-
 compiler/frontend/ast_derive_js_mapper.ml     |   6 +-
 compiler/frontend/ast_derive_projector.ml     |   6 +-
 compiler/frontend/ast_pat.ml                  |   4 +-
 compiler/frontend/ast_uncurry_gen.ml          |   4 +-
 compiler/frontend/bs_ast_mapper.ml            |   4 +-
 compiler/frontend/bs_builtin_ppx.ml           |   4 +-
 compiler/ml/ast_async.ml                      |   6 +-
 compiler/ml/ast_helper.ml                     |   3 +-
 compiler/ml/ast_helper.mli                    |   1 +
 compiler/ml/ast_iterator.ml                   |   2 +-
 compiler/ml/ast_mapper.ml                     |   4 +-
 compiler/ml/ast_mapper_from0.ml               |  40 +-
 compiler/ml/ast_mapper_to0.ml                 |  29 +-
 compiler/ml/ast_uncurried.ml                  |  42 +-
 compiler/ml/depend.ml                         |   2 +-
 compiler/ml/parsetree.ml                      |   3 +-
 compiler/ml/pprintast.ml                      |  22 +-
 compiler/ml/printast.ml                       |   7 +-
 compiler/ml/typecore.ml                       |  13 +-
 compiler/ml/untypeast.ml                      |   7 +-
 compiler/syntax/src/jsx_v4.ml                 |  53 +-
 compiler/syntax/src/res_ast_debugger.ml       |   2 +-
 compiler/syntax/src/res_comments_table.ml     |  14 +-
 compiler/syntax/src/res_core.ml               |  11 +-
 compiler/syntax/src/res_parsetree_viewer.ml   |  25 +-
 compiler/syntax/src/res_printer.ml            |   6 +-
 scripts/test_syntax.sh                        |   2 +-
 .../errors/expressions/expected/array.res.txt |   3 +-
 .../errors/expressions/expected/arrow.res.txt |   8 +-
 .../errors/expressions/expected/block.res.txt |  39 +-
 .../expressions/expected/consecutive.res.txt  |   6 +-
 .../expressions/expected/emptyBlock.res.txt   |   2 +-
 .../expected/implementation.res.txt           |   5 +-
 .../other/expected/labelledParameters.res.txt |  14 +-
 .../errors/structure/expected/gh16B.res.txt   |  26 +-
 .../structure/expected/letBinding.res.txt     |   4 +-
 .../typeDef/expected/inlineRecord.res.txt     |  15 +-
 .../typexpr/expected/objectSpread.res.txt     |   2 +-
 .../expected/UncurriedAlways.res.txt          |   4 +-
 .../expected/UncurriedByDefault.res.txt       | 113 ++-
 .../expressions/expected/apply.res.txt        |  17 +-
 .../expressions/expected/argument.res.txt     |  14 +-
 .../expressions/expected/arrow.res.txt        | 239 +++---
 .../expressions/expected/async.res.txt        |  42 +-
 .../expressions/expected/await.res.txt        |  20 +-
 .../expressions/expected/binary.res.txt       |   3 +-
 .../expected/binaryNoEs6Arrow.res.txt         |   7 +-
 .../expressions/expected/block.res.txt        |  90 +--
 .../expected/bracedOrRecord.res.txt           |  22 +-
 .../expressions/expected/coerce.res.txt       |   4 +-
 .../expressions/expected/extension.res.txt    |   2 +-
 .../expected/firstClassModule.res.txt         |  93 ++-
 .../grammar/expressions/expected/jsx.res.txt  | 207 ++---
 .../expected/locallyAbstractTypes.res.txt     |  40 +-
 .../expressions/expected/record.res.txt       |   8 +-
 .../expected/tupleVsDivision.res.txt          |   7 +-
 .../expressions/expected/uncurried.res.txt    |  50 +-
 .../expected/underscoreApply.res.txt          |  52 +-
 .../grammar/modexpr/expected/apply.res.txt    |   7 +-
 .../expected/firstClassModules.res.txt        |  22 +-
 .../grammar/modexpr/expected/functor.res.txt  |  19 +-
 .../grammar/pattern/expected/any.res.txt      |  14 +-
 .../grammar/pattern/expected/array.res.txt    |  10 +-
 .../grammar/pattern/expected/constant.res.txt |  45 +-
 .../pattern/expected/constructor.res.txt      |  37 +-
 .../grammar/pattern/expected/dict.res.txt     |  45 +-
 .../pattern/expected/exception.res.txt        |  16 +-
 .../pattern/expected/extension.res.txt        |  17 +-
 .../expected/firstClassModules.res.txt        |  32 +-
 .../grammar/pattern/expected/list.res.txt     |  34 +-
 .../pattern/expected/polyvariants.res.txt     |  65 +-
 .../grammar/pattern/expected/record.res.txt   |  40 +-
 .../grammar/pattern/expected/tuple.res.txt    |  17 +-
 .../grammar/pattern/expected/unit.res.txt     |  20 +-
 .../pattern/expected/variantSpreads.res.txt   |  91 +--
 .../grammar/pattern/expected/variants.res.txt |  34 +-
 .../signature/expected/itemExtension.res.txt  |   2 +-
 .../expected/standAloneAttribute.res.txt      |   2 +-
 .../structure/expected/itemExtension.res.txt  |   2 +-
 .../structure/expected/letBinding.res.txt     |  14 +-
 .../expected/modExprExtension.res.txt         |   3 +-
 .../expected/moduleTypeExtension.res.txt      |   2 +-
 .../expected/standaloneAttribute.res.txt      |   2 +-
 .../expected/polyvariant.res.txt              |   2 +-
 .../expected/objectTypeSpreading.res.txt      |  29 +-
 .../typexpr/expected/typeconstr.res.txt       |   2 +-
 .../grammar/typexpr/expected/unit.res.txt     |  14 +-
 .../expected/equalAfterBinaryExpr.res.txt     | 285 ++++---
 .../expected/nonRecTypes.res.txt              | 739 +++++++++---------
 .../expected/templateEof.res.txt              |   7 +-
 .../expression/expected/emptyBlock.res.txt    |   2 +-
 .../recovery/expression/expected/list.res.txt |  19 +-
 .../expr/expected/underscoreApply.res.txt     |   2 +-
 tests/tools_tests/ppx/TestPpx.res             |   6 +
 .../src/expected/TestPpx.res.jsout            |  17 +
 100 files changed, 1616 insertions(+), 1601 deletions(-)

diff --git a/analysis/src/CompletionFrontEnd.ml b/analysis/src/CompletionFrontEnd.ml
index 8cae05d0ca..6337c0b5bb 100644
--- a/analysis/src/CompletionFrontEnd.ml
+++ b/analysis/src/CompletionFrontEnd.ml
@@ -1318,7 +1318,7 @@ let completionWithParser1 ~currentFile ~debug ~offset ~path ~posCursor
             match exprToContextPath lhs with
             | Some contextPath -> setResult (Cpath (CPObj (contextPath, label)))
             | None -> ())
-        | Pexp_fun (lbl, defaultExpOpt, pat, e) ->
+        | Pexp_fun (lbl, defaultExpOpt, pat, e, _) ->
           let oldScope = !scope in
           (match (!processingFun, !currentCtxPath) with
           | None, Some ctxPath -> processingFun := Some (ctxPath, 0)
diff --git a/analysis/src/DumpAst.ml b/analysis/src/DumpAst.ml
index 8e21853308..b7ad602f32 100644
--- a/analysis/src/DumpAst.ml
+++ b/analysis/src/DumpAst.ml
@@ -213,7 +213,7 @@ and printExprItem expr ~pos ~indentation =
       | None -> ""
       | Some expr -> "," ^ printExprItem expr ~pos ~indentation)
     ^ ")"
-  | Pexp_fun (arg, _maybeDefaultArgExpr, pattern, nextExpr) ->
+  | Pexp_fun (arg, _maybeDefaultArgExpr, pattern, nextExpr, _) ->
     "Pexp_fun(\n"
     ^ addIndentation (indentation + 1)
     ^ "arg: "
diff --git a/analysis/src/Xform.ml b/analysis/src/Xform.ml
index b765e511c9..f2a91382b9 100644
--- a/analysis/src/Xform.ml
+++ b/analysis/src/Xform.ml
@@ -261,7 +261,7 @@ module AddBracesToFn = struct
         | _ -> false
       in
       (match e.pexp_desc with
-      | Pexp_fun (_, _, _, bodyExpr)
+      | Pexp_fun (_, _, _, bodyExpr, _)
         when Loc.hasPos ~pos bodyExpr.pexp_loc
              && isBracedExpr bodyExpr = false
              && isFunction bodyExpr = false ->
@@ -303,10 +303,10 @@ module AddTypeAnnotation = struct
     in
     let rec processFunction ~argNum (e : Parsetree.expression) =
       match e.pexp_desc with
-      | Pexp_fun (argLabel, _, pat, e)
+      | Pexp_fun (argLabel, _, pat, e, _)
       | Pexp_construct
           ( {txt = Lident "Function$"},
-            Some {pexp_desc = Pexp_fun (argLabel, _, pat, e)} ) ->
+            Some {pexp_desc = Pexp_fun (argLabel, _, pat, e, _)} ) ->
         let isUnlabeledOnlyArg =
           argNum = 1 && argLabel = Nolabel
           &&
diff --git a/compiler/frontend/ast_compatible.ml b/compiler/frontend/ast_compatible.ml
index 338a66acc1..94170e4e40 100644
--- a/compiler/frontend/ast_compatible.ml
+++ b/compiler/frontend/ast_compatible.ml
@@ -63,11 +63,11 @@ let app3 ?(loc = default_loc) ?(attrs = []) fn arg1 arg2 arg3 : expression =
       Pexp_apply (fn, [(Nolabel, arg1); (Nolabel, arg2); (Nolabel, arg3)]);
   }
 
-let fun_ ?(loc = default_loc) ?(attrs = []) pat exp =
+let fun_ ?(loc = default_loc) ?(attrs = []) ~arity pat exp =
   {
     pexp_loc = loc;
     pexp_attributes = attrs;
-    pexp_desc = Pexp_fun (Nolabel, None, pat, exp);
+    pexp_desc = Pexp_fun (Nolabel, None, pat, exp, arity);
   }
 
 let const_exp_string ?(loc = default_loc) ?(attrs = []) ?delimiter (s : string)
diff --git a/compiler/frontend/ast_compatible.mli b/compiler/frontend/ast_compatible.mli
index 8bda0e0275..e2b68f601e 100644
--- a/compiler/frontend/ast_compatible.mli
+++ b/compiler/frontend/ast_compatible.mli
@@ -72,7 +72,12 @@ val apply_labels :
 *)
 
 val fun_ :
-  ?loc:Location.t -> ?attrs:attrs -> pattern -> expression -> expression
+  ?loc:Location.t ->
+  ?attrs:attrs ->
+  arity:int option ->
+  pattern ->
+  expression ->
+  expression
 
 (* val opt_label : string -> Asttypes.arg_label *)
 
diff --git a/compiler/frontend/ast_derive_js_mapper.ml b/compiler/frontend/ast_derive_js_mapper.ml
index b845c43749..70f4babb77 100644
--- a/compiler/frontend/ast_derive_js_mapper.ml
+++ b/compiler/frontend/ast_derive_js_mapper.ml
@@ -167,7 +167,7 @@ let init () =
               in
               let to_js_body body =
                 Ast_comb.single_non_rec_value pat_to_js
-                  (Ast_compatible.fun_
+                  (Ast_compatible.fun_ ~arity:None
                      (Pat.constraint_ (Pat.var pat_param) core_type)
                      body)
               in
@@ -211,7 +211,7 @@ let init () =
                 in
                 let from_js =
                   Ast_comb.single_non_rec_value pat_from_js
-                    (Ast_compatible.fun_ (Pat.var pat_param)
+                    (Ast_compatible.fun_ ~arity:None (Pat.var pat_param)
                        (if create_type then
                           Exp.let_ Nonrecursive
                             [Vb.mk (Pat.var pat_param) (exp_param +: new_type)]
@@ -253,7 +253,7 @@ let init () =
                            app2 unsafe_index_get_exp exp_map exp_param
                          else app1 erase_type_exp exp_param);
                       Ast_comb.single_non_rec_value pat_from_js
-                        (Ast_compatible.fun_ (Pat.var pat_param)
+                        (Ast_compatible.fun_ ~arity:None (Pat.var pat_param)
                            (let result =
                               app2 unsafe_index_get_exp rev_exp_map exp_param
                             in
diff --git a/compiler/frontend/ast_derive_projector.ml b/compiler/frontend/ast_derive_projector.ml
index c84e935e92..6884dbe497 100644
--- a/compiler/frontend/ast_derive_projector.ml
+++ b/compiler/frontend/ast_derive_projector.ml
@@ -46,7 +46,7 @@ let init () =
                   ->
                     let txt = "param" in
                     Ast_comb.single_non_rec_value ?attrs:gentype_attrs pld_name
-                      (Ast_compatible.fun_
+                      (Ast_compatible.fun_ ~arity:None
                          (Pat.constraint_ (Pat.var {txt; loc}) core_type)
                          (Exp.field
                             (Exp.ident {txt = Lident txt; loc})
@@ -108,7 +108,9 @@ let init () =
                              annotate_type
                          in
                          Ext_list.fold_right vars exp (fun var b ->
-                             Ast_compatible.fun_ (Pat.var {loc; txt = var}) b)
+                             Ast_compatible.fun_ ~arity:None
+                               (Pat.var {loc; txt = var})
+                               b)
                          |> handle_uncurried_accessor_tranform ~loc ~arity))
               | Ptype_abstract | Ptype_open ->
                 Ast_derive_util.not_applicable tdcl.ptype_loc deriving_name;
diff --git a/compiler/frontend/ast_pat.ml b/compiler/frontend/ast_pat.ml
index b3789d8e35..91a099c33f 100644
--- a/compiler/frontend/ast_pat.ml
+++ b/compiler/frontend/ast_pat.ml
@@ -35,7 +35,7 @@ let is_unit_cont ~yes ~no (p : t) =
 let arity_of_fun (pat : Parsetree.pattern) (e : Parsetree.expression) =
   let rec aux (e : Parsetree.expression) =
     match e.pexp_desc with
-    | Pexp_fun (_, _, _, e) -> 1 + aux e (*FIXME error on optional*)
+    | Pexp_fun (_, _, _, e, _) -> 1 + aux e (*FIXME error on optional*)
     (* | Pexp_fun _
        -> Location.raise_errorf
            ~loc:e.pexp_loc "Label is not allowed in JS object" *)
@@ -45,7 +45,7 @@ let arity_of_fun (pat : Parsetree.pattern) (e : Parsetree.expression) =
 
 let rec labels_of_fun (e : Parsetree.expression) =
   match e.pexp_desc with
-  | Pexp_fun (l, _, _, e) -> l :: labels_of_fun e
+  | Pexp_fun (l, _, _, e, _) -> l :: labels_of_fun e
   | _ -> []
 
 let rec is_single_variable_pattern_conservative (p : t) =
diff --git a/compiler/frontend/ast_uncurry_gen.ml b/compiler/frontend/ast_uncurry_gen.ml
index f0eef18c5b..364ff66633 100644
--- a/compiler/frontend/ast_uncurry_gen.ml
+++ b/compiler/frontend/ast_uncurry_gen.ml
@@ -36,7 +36,7 @@ let to_method_callback loc (self : Bs_ast_mapper.mapper) label
     match Ast_attributes.process_attributes_rev body.pexp_attributes with
     | Nothing, attrs -> (
       match body.pexp_desc with
-      | Pexp_fun (arg_label, _, arg, body) ->
+      | Pexp_fun (arg_label, _, arg, body, _) ->
         Bs_syntaxerr.optional_err loc arg_label;
         aux ((arg_label, self.pat self arg, attrs) :: acc) body
       | _ -> (self.expr self body, acc))
@@ -45,7 +45,7 @@ let to_method_callback loc (self : Bs_ast_mapper.mapper) label
   let result, rev_extra_args = aux [(label, self_pat, [])] body in
   let body =
     Ext_list.fold_left rev_extra_args result (fun e (label, p, attrs) ->
-        Ast_helper.Exp.fun_ ~loc ~attrs label None p e)
+        Ast_helper.Exp.fun_ ~loc ~attrs ~arity:None label None p e)
   in
   let arity = List.length rev_extra_args in
   let arity_s = string_of_int arity in
diff --git a/compiler/frontend/bs_ast_mapper.ml b/compiler/frontend/bs_ast_mapper.ml
index 2fda74cbab..f63531e6b2 100644
--- a/compiler/frontend/bs_ast_mapper.ml
+++ b/compiler/frontend/bs_ast_mapper.ml
@@ -315,8 +315,8 @@ module E = struct
            sub vbs)
         (sub.expr sub e)
     (* #end *)
-    | Pexp_fun (lab, def, p, e) ->
-      fun_ ~loc ~attrs lab
+    | Pexp_fun (lab, def, p, e, arity) ->
+      fun_ ~loc ~attrs ~arity lab
         (map_opt (sub.expr sub) def)
         (sub.pat sub p) (sub.expr sub e)
     | Pexp_function pel -> function_ ~loc ~attrs (sub.cases sub pel)
diff --git a/compiler/frontend/bs_builtin_ppx.ml b/compiler/frontend/bs_builtin_ppx.ml
index 02ec4b1c90..5a0c31d22d 100644
--- a/compiler/frontend/bs_builtin_ppx.ml
+++ b/compiler/frontend/bs_builtin_ppx.ml
@@ -130,7 +130,7 @@ let expr_mapper ~async_context ~in_function_def (self : mapper)
     let body = Ast_async.add_async_attribute ~async body in
     let res = self.expr self body in
     {e with pexp_desc = Pexp_newtype (s, res)}
-  | Pexp_fun (label, _, pat, body) -> (
+  | Pexp_fun (label, _, pat, body, _arity) -> (
     let async = Ast_attributes.has_async_payload e.pexp_attributes <> None in
     match Ast_attributes.process_attributes_rev e.pexp_attributes with
     | Nothing, _ ->
@@ -594,7 +594,7 @@ let rec structure_mapper ~await_context (self : mapper) (stru : Ast_structure.t)
             | Pexp_ifthenelse (_, then_expr, Some else_expr) ->
               aux then_expr @ aux else_expr
             | Pexp_construct (_, Some expr) -> aux expr
-            | Pexp_fun (_, _, _, expr) | Pexp_newtype (_, expr) -> aux expr
+            | Pexp_fun (_, _, _, expr, _) | Pexp_newtype (_, expr) -> aux expr
             | _ -> acc
           in
           aux pvb_expr @ spelunk_vbs acc tl
diff --git a/compiler/ml/ast_async.ml b/compiler/ml/ast_async.ml
index e8ed63a736..b6b4e46b3d 100644
--- a/compiler/ml/ast_async.ml
+++ b/compiler/ml/ast_async.ml
@@ -40,14 +40,14 @@ let add_async_attribute ~async (body : Parsetree.expression) =
 
 let rec add_promise_to_result ~loc (e : Parsetree.expression) =
   match e.pexp_desc with
-  | Pexp_fun (label, eo, pat, body) ->
+  | Pexp_fun (label, eo, pat, body, arity) ->
     let body = add_promise_to_result ~loc body in
-    {e with pexp_desc = Pexp_fun (label, eo, pat, body)}
+    {e with pexp_desc = Pexp_fun (label, eo, pat, body, arity)}
   | _ -> add_promise_type ~loc ~async:true e
 
 let make_function_async ~async (e : Parsetree.expression) =
   if async then
     match e.pexp_desc with
-    | Pexp_fun (_, _, {ppat_loc}, _) -> add_promise_to_result ~loc:ppat_loc e
+    | Pexp_fun (_, _, {ppat_loc}, _, _) -> add_promise_to_result ~loc:ppat_loc e
     | _ -> assert false
   else e
diff --git a/compiler/ml/ast_helper.ml b/compiler/ml/ast_helper.ml
index 52aa1719ff..01c5ea0e13 100644
--- a/compiler/ml/ast_helper.ml
+++ b/compiler/ml/ast_helper.ml
@@ -150,7 +150,8 @@ module Exp = struct
   let ident ?loc ?attrs a = mk ?loc ?attrs (Pexp_ident a)
   let constant ?loc ?attrs a = mk ?loc ?attrs (Pexp_constant a)
   let let_ ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_let (a, b, c))
-  let fun_ ?loc ?attrs a b c d = mk ?loc ?attrs (Pexp_fun (a, b, c, d))
+  let fun_ ?loc ?attrs ~arity a b c d =
+    mk ?loc ?attrs (Pexp_fun (a, b, c, d, arity))
   let function_ ?loc ?attrs a = mk ?loc ?attrs (Pexp_function a)
   let apply ?loc ?attrs a b = mk ?loc ?attrs (Pexp_apply (a, b))
   let match_ ?loc ?attrs a b = mk ?loc ?attrs (Pexp_match (a, b))
diff --git a/compiler/ml/ast_helper.mli b/compiler/ml/ast_helper.mli
index 856b35abb9..cfb761485d 100644
--- a/compiler/ml/ast_helper.mli
+++ b/compiler/ml/ast_helper.mli
@@ -132,6 +132,7 @@ module Exp : sig
   val fun_ :
     ?loc:loc ->
     ?attrs:attrs ->
+    arity:int option ->
     arg_label ->
     expression option ->
     pattern ->
diff --git a/compiler/ml/ast_iterator.ml b/compiler/ml/ast_iterator.ml
index b524431edf..6f20cd9650 100644
--- a/compiler/ml/ast_iterator.ml
+++ b/compiler/ml/ast_iterator.ml
@@ -282,7 +282,7 @@ module E = struct
     | Pexp_let (_r, vbs, e) ->
       List.iter (sub.value_binding sub) vbs;
       sub.expr sub e
-    | Pexp_fun (_lab, def, p, e) ->
+    | Pexp_fun (_lab, def, p, e, _) ->
       iter_opt (sub.expr sub) def;
       sub.pat sub p;
       sub.expr sub e
diff --git a/compiler/ml/ast_mapper.ml b/compiler/ml/ast_mapper.ml
index 4b1d4dca22..83fb0c0e56 100644
--- a/compiler/ml/ast_mapper.ml
+++ b/compiler/ml/ast_mapper.ml
@@ -278,8 +278,8 @@ module E = struct
     | Pexp_constant x -> constant ~loc ~attrs x
     | Pexp_let (r, vbs, e) ->
       let_ ~loc ~attrs r (List.map (sub.value_binding sub) vbs) (sub.expr sub e)
-    | Pexp_fun (lab, def, p, e) ->
-      fun_ ~loc ~attrs lab
+    | Pexp_fun (lab, def, p, e, arity) ->
+      fun_ ~loc ~attrs ~arity lab
         (map_opt (sub.expr sub) def)
         (sub.pat sub p) (sub.expr sub e)
     | Pexp_function pel -> function_ ~loc ~attrs (sub.cases sub pel)
diff --git a/compiler/ml/ast_mapper_from0.ml b/compiler/ml/ast_mapper_from0.ml
index 76ba9e5392..036ecb3755 100644
--- a/compiler/ml/ast_mapper_from0.ml
+++ b/compiler/ml/ast_mapper_from0.ml
@@ -285,7 +285,7 @@ module E = struct
     | Pexp_let (r, vbs, e) ->
       let_ ~loc ~attrs r (List.map (sub.value_binding sub) vbs) (sub.expr sub e)
     | Pexp_fun (lab, def, p, e) ->
-      fun_ ~loc ~attrs lab
+      fun_ ~loc ~attrs ~arity:None lab
         (map_opt (sub.expr sub) def)
         (sub.pat sub p) (sub.expr sub e)
     | Pexp_function pel -> function_ ~loc ~attrs (sub.cases sub pel)
@@ -295,8 +295,42 @@ module E = struct
       match_ ~loc ~attrs (sub.expr sub e) (sub.cases sub pel)
     | Pexp_try (e, pel) -> try_ ~loc ~attrs (sub.expr sub e) (sub.cases sub pel)
     | Pexp_tuple el -> tuple ~loc ~attrs (List.map (sub.expr sub) el)
-    | Pexp_construct (lid, arg) ->
-      construct ~loc ~attrs (map_loc sub lid) (map_opt (sub.expr sub) arg)
+    | Pexp_construct (lid, arg) -> (
+      let lid1 = map_loc sub lid in
+      let arg1 = map_opt (sub.expr sub) arg in
+      let exp1 = construct ~loc ~attrs lid1 arg1 in
+      match lid.txt with
+      | Lident "Function$" -> (
+        let rec attributes_to_arity (attrs : Parsetree.attributes) =
+          match attrs with
+          | ( {txt = "res.arity"},
+              PStr
+                [
+                  {
+                    pstr_desc =
+                      Pstr_eval
+                        ( {pexp_desc = Pexp_constant (Pconst_integer (arity, _))},
+                          _ );
+                  };
+                ] )
+            :: _ ->
+            int_of_string arity
+          | _ :: rest -> attributes_to_arity rest
+          | [] -> assert false
+        in
+        match arg1 with
+        | Some ({pexp_desc = Pexp_fun (l, eo, p, e, _)} as e1) ->
+          let arity = attributes_to_arity attrs in
+          {
+            e1 with
+            pexp_desc =
+              Pexp_construct
+                ( lid1,
+                  Some {e with pexp_desc = Pexp_fun (l, eo, p, e, Some arity)}
+                );
+          }
+        | _ -> exp1)
+      | _ -> exp1)
     | Pexp_variant (lab, eo) ->
       variant ~loc ~attrs lab (map_opt (sub.expr sub) eo)
     | Pexp_record (l, eo) ->
diff --git a/compiler/ml/ast_mapper_to0.ml b/compiler/ml/ast_mapper_to0.ml
index 2ad44c96b2..260b0de940 100644
--- a/compiler/ml/ast_mapper_to0.ml
+++ b/compiler/ml/ast_mapper_to0.ml
@@ -283,7 +283,7 @@ module E = struct
     | Pexp_constant x -> constant ~loc ~attrs (map_constant x)
     | Pexp_let (r, vbs, e) ->
       let_ ~loc ~attrs r (List.map (sub.value_binding sub) vbs) (sub.expr sub e)
-    | Pexp_fun (lab, def, p, e) ->
+    | Pexp_fun (lab, def, p, e, _) ->
       fun_ ~loc ~attrs lab
         (map_opt (sub.expr sub) def)
         (sub.pat sub p) (sub.expr sub e)
@@ -294,8 +294,31 @@ module E = struct
       match_ ~loc ~attrs (sub.expr sub e) (sub.cases sub pel)
     | Pexp_try (e, pel) -> try_ ~loc ~attrs (sub.expr sub e) (sub.cases sub pel)
     | Pexp_tuple el -> tuple ~loc ~attrs (List.map (sub.expr sub) el)
-    | Pexp_construct (lid, arg) ->
-      construct ~loc ~attrs (map_loc sub lid) (map_opt (sub.expr sub) arg)
+    | Pexp_construct (lid, arg) -> (
+      let exp0 =
+        construct ~loc ~attrs (map_loc sub lid) (map_opt (sub.expr sub) arg)
+      in
+      match lid.txt with
+      | Lident "Function$" -> (
+        match arg with
+        | Some {pexp_desc = Pexp_fun (_, _, _, _, Some arity)} ->
+          let arity_to_attributes arity =
+            [
+              ( Location.mknoloc "res.arity",
+                Parsetree0.PStr
+                  [
+                    Ast_helper0.Str.eval
+                      (Ast_helper0.Exp.constant
+                         (Pconst_integer (string_of_int arity, None)));
+                  ] );
+            ]
+          in
+          {
+            exp0 with
+            pexp_attributes = arity_to_attributes arity @ exp0.pexp_attributes;
+          }
+        | _ -> assert false)
+      | _ -> exp0)
     | Pexp_variant (lab, eo) ->
       variant ~loc ~attrs lab (map_opt (sub.expr sub) eo)
     | Pexp_record (l, eo) ->
diff --git a/compiler/ml/ast_uncurried.ml b/compiler/ml/ast_uncurried.ml
index 3d36fcc656..883a8d7ded 100644
--- a/compiler/ml/ast_uncurried.ml
+++ b/compiler/ml/ast_uncurried.ml
@@ -19,36 +19,14 @@ let uncurried_type ~loc ~arity t_arg =
   let t_arity = arity_type ~loc arity in
   Ast_helper.Typ.constr ~loc {txt = Lident "function$"; loc} [t_arg; t_arity]
 
-let arity_to_attributes arity =
-  [
-    ( Location.mknoloc "res.arity",
-      Parsetree.PStr
-        [
-          Ast_helper.Str.eval
-            (Ast_helper.Exp.constant
-               (Pconst_integer (string_of_int arity, None)));
-        ] );
-  ]
-
-let rec attributes_to_arity (attrs : Parsetree.attributes) =
-  match attrs with
-  | ( {txt = "res.arity"},
-      PStr
-        [
-          {
-            pstr_desc =
-              Pstr_eval
-                ({pexp_desc = Pexp_constant (Pconst_integer (arity, _))}, _);
-          };
-        ] )
-    :: _ ->
-    int_of_string arity
-  | _ :: rest -> attributes_to_arity rest
-  | _ -> assert false
-
 let uncurried_fun ~loc ~arity fun_expr =
+  let fun_expr =
+    match fun_expr.Parsetree.pexp_desc with
+    | Pexp_fun (l, eo, p, e, _) ->
+      {fun_expr with pexp_desc = Pexp_fun (l, eo, p, e, Some arity)}
+    | _ -> assert false
+  in
   Ast_helper.Exp.construct ~loc
-    ~attrs:(arity_to_attributes arity)
     (Location.mknoloc (Longident.Lident "Function$"))
     (Some fun_expr)
 
@@ -59,7 +37,13 @@ let expr_is_uncurried_fun (expr : Parsetree.expression) =
 
 let expr_extract_uncurried_fun (expr : Parsetree.expression) =
   match expr.pexp_desc with
-  | Pexp_construct ({txt = Lident "Function$"}, Some e) -> e
+  | Pexp_construct ({txt = Lident "Function$"}, Some e) ->
+    let () =
+      match e.pexp_desc with
+      | Pexp_fun (_, _, _, _, Some _arity) -> ()
+      | _ -> assert false
+    in
+    e
   | _ -> assert false
 
 let core_type_is_uncurried_fun (typ : Parsetree.core_type) =
diff --git a/compiler/ml/depend.ml b/compiler/ml/depend.ml
index bf4da4cfde..a2c4e04b40 100644
--- a/compiler/ml/depend.ml
+++ b/compiler/ml/depend.ml
@@ -218,7 +218,7 @@ let rec add_expr bv exp =
   | Pexp_let (rf, pel, e) ->
     let bv = add_bindings rf bv pel in
     add_expr bv e
-  | Pexp_fun (_, opte, p, e) ->
+  | Pexp_fun (_, opte, p, e, _) ->
     add_opt add_expr bv opte;
     add_expr (add_pattern bv p) e
   | Pexp_function pel -> add_cases bv pel
diff --git a/compiler/ml/parsetree.ml b/compiler/ml/parsetree.ml
index 3da8d64769..060f9cf5b4 100644
--- a/compiler/ml/parsetree.ml
+++ b/compiler/ml/parsetree.ml
@@ -225,7 +225,8 @@ and expression_desc =
        let rec P1 = E1 and ... and Pn = EN in E   (flag = Recursive)
     *)
   | Pexp_function of case list (* function P1 -> E1 | ... | Pn -> En *)
-  | Pexp_fun of arg_label * expression option * pattern * expression
+  | Pexp_fun of
+      arg_label * expression option * pattern * expression * int option
     (* fun P -> E1                          (Simple, None)
        fun ~l:P -> E1                       (Labelled l, None)
        fun ?l:P -> E1                       (Optional l, None)
diff --git a/compiler/ml/pprintast.ml b/compiler/ml/pprintast.ml
index 56eab619d6..a360d3dfe6 100644
--- a/compiler/ml/pprintast.ml
+++ b/compiler/ml/pprintast.ml
@@ -543,9 +543,13 @@ and expression ctxt f x =
     | Pexp_let _ | Pexp_letmodule _ | Pexp_open _ | Pexp_letexception _
         when ctxt.semi ->
         paren true (expression reset_ctxt) f x
-    | Pexp_fun (l, e0, p, e) ->
-        pp f "@[<2>fun@;%a->@;%a@]"
-          (label_exp ctxt) (l, e0, p)
+    | Pexp_fun (l, e0, p, e, arity) ->
+        let arity_str = match arity with
+          | None -> ""
+          | Some arity -> "[arity:" ^ string_of_int arity ^ "]"
+          in
+        pp f "@[<2>fun@;%s%a->@;%a@]"
+          arity_str (label_exp ctxt) (l, e0, p)
           (expression ctxt) e
     | Pexp_function l ->
         pp f "@[<hv>function%a@]" (case_list ctxt) l
@@ -949,12 +953,16 @@ and binding ctxt f {pvb_pat=p; pvb_expr=x; _} =
   let rec pp_print_pexp_function f x =
     if x.pexp_attributes <> [] then pp f "=@;%a" (expression ctxt) x
     else match x.pexp_desc with
-      | Pexp_fun (label, eo, p, e) ->
+      | Pexp_fun (label, eo, p, e, arity) ->
+          let arity_str = match arity with
+            | None -> ""
+            | Some arity -> "arity:" ^ string_of_int arity
+          in
           if label=Nolabel then
-            pp f "%a@ %a" (simple_pattern ctxt) p pp_print_pexp_function e
+            pp f "%s%a@ %a" arity_str (simple_pattern ctxt) p pp_print_pexp_function e
           else
-            pp f "%a@ %a"
-              (label_exp ctxt) (label,eo,p) pp_print_pexp_function e
+            pp f "%s%a@ %a"
+              arity_str (label_exp ctxt) (label,eo,p) pp_print_pexp_function e
       | Pexp_newtype (str,e) ->
           pp f "(type@ %s)@ %a" str.txt pp_print_pexp_function e
       | _ -> pp f "=@;%a" (expression ctxt) x
diff --git a/compiler/ml/printast.ml b/compiler/ml/printast.ml
index 0977e7f8a2..a635fc0498 100644
--- a/compiler/ml/printast.ml
+++ b/compiler/ml/printast.ml
@@ -236,8 +236,13 @@ and expression i ppf x =
   | Pexp_function l ->
     line i ppf "Pexp_function\n";
     list i case ppf l
-  | Pexp_fun (l, eo, p, e) ->
+  | Pexp_fun (l, eo, p, e, arity) ->
     line i ppf "Pexp_fun\n";
+    let () =
+      match arity with
+      | None -> ()
+      | Some arity -> line i ppf "arity:%d\n" arity
+    in
     arg_label i ppf l;
     option i expression ppf eo;
     pattern i ppf p;
diff --git a/compiler/ml/typecore.ml b/compiler/ml/typecore.ml
index f9085caaa4..fba16114be 100644
--- a/compiler/ml/typecore.ml
+++ b/compiler/ml/typecore.ml
@@ -138,7 +138,7 @@ let iter_expression f e =
     | Pexp_ident _ | Pexp_new _ | Pexp_constant _ ->
       ()
     | Pexp_function pel -> List.iter case pel
-    | Pexp_fun (_, eo, _, e) ->
+    | Pexp_fun (_, eo, _, e, _) ->
       may expr eo;
       expr e
     | Pexp_apply (e, lel) ->
@@ -1914,7 +1914,7 @@ let rec approx_type env sty =
 let rec type_approx env sexp =
   match sexp.pexp_desc with
   | Pexp_let (_, _, e) -> type_approx env e
-  | Pexp_fun (p, _, _, e) ->
+  | Pexp_fun (p, _, _, e, _arity) ->
     let ty = if is_optional p then type_option (newvar ()) else newvar () in
     newty (Tarrow (p, ty, type_approx env e, Cok))
   | Pexp_function ({pc_rhs = e} :: _) ->
@@ -2374,7 +2374,7 @@ and type_expect_ ?type_clash_context ?in_function ?(recarg = Rejected) env sexp
         exp_attributes = sexp.pexp_attributes;
         exp_env = env;
       }
-  | Pexp_fun (l, Some default, spat, sbody) ->
+  | Pexp_fun (l, Some default, spat, sbody, _arity) ->
     assert (is_optional l);
     (* default allowed only with optional argument *)
     let open Ast_helper in
@@ -2414,7 +2414,7 @@ and type_expect_ ?type_clash_context ?in_function ?(recarg = Rejected) env sexp
     in
     type_function ?in_function loc sexp.pexp_attributes env ty_expected l
       [Exp.case pat body]
-  | Pexp_fun (l, None, spat, sbody) ->
+  | Pexp_fun (l, None, spat, sbody, _arity) ->
     type_function ?in_function loc sexp.pexp_attributes env ty_expected l
       [Ast_helper.Exp.case spat sbody]
   | Pexp_function caselist ->
@@ -2531,9 +2531,10 @@ and type_expect_ ?type_clash_context ?in_function ?(recarg = Rejected) env sexp
         exp_attributes = sexp.pexp_attributes;
         exp_env = env;
       }
-  | Pexp_construct (({txt = Lident "Function$"} as lid), sarg) ->
+  | Pexp_construct
+      ( ({txt = Lident "Function$"} as lid),
+        (Some {pexp_desc = Pexp_fun (_, _, _, _, Some arity)} as sarg) ) ->
     let state = Warnings.backup () in
-    let arity = Ast_uncurried.attributes_to_arity sexp.pexp_attributes in
     let uncurried_typ =
       Ast_uncurried.make_uncurried_type ~env ~arity (newvar ())
     in
diff --git a/compiler/ml/untypeast.ml b/compiler/ml/untypeast.ml
index 0812bc88f7..da064d8d77 100644
--- a/compiler/ml/untypeast.ml
+++ b/compiler/ml/untypeast.ml
@@ -312,7 +312,8 @@ let expression sub exp =
     (* One case, no guard: It's a fun. *)
     | Texp_function
         {arg_label; cases = [{c_lhs = p; c_guard = None; c_rhs = e}]; _} ->
-      Pexp_fun (arg_label, None, sub.pat sub p, sub.expr sub e)
+      let arity = assert false in
+      Pexp_fun (arg_label, None, sub.pat sub p, sub.expr sub e, arity)
     (* No label: it's a function. *)
     | Texp_function {arg_label = Nolabel; cases; _} ->
       Pexp_function (sub.cases sub cases)
@@ -320,13 +321,15 @@ let expression sub exp =
     | Texp_function {arg_label = (Labelled s | Optional s) as label; cases; _}
       ->
       let name = fresh_name s exp.exp_env in
+      let arity = assert false in
       Pexp_fun
         ( label,
           None,
           Pat.var ~loc {loc; txt = name},
           Exp.match_ ~loc
             (Exp.ident ~loc {loc; txt = Lident name})
-            (sub.cases sub cases) )
+            (sub.cases sub cases),
+          arity )
     | Texp_apply (exp, list) ->
       Pexp_apply
         ( sub.expr sub exp,
diff --git a/compiler/syntax/src/jsx_v4.ml b/compiler/syntax/src/jsx_v4.ml
index d80ff12f5c..e8fe6defd4 100644
--- a/compiler/syntax/src/jsx_v4.ml
+++ b/compiler/syntax/src/jsx_v4.ml
@@ -642,15 +642,17 @@ let transform_lowercase_call3 ~config mapper jsx_expr_loc call_expr_loc attrs
 let rec recursively_transform_named_args_for_make expr args newtypes core_type =
   match expr.pexp_desc with
   (* TODO: make this show up with a loc. *)
-  | Pexp_fun (Labelled "key", _, _, _) | Pexp_fun (Optional "key", _, _, _) ->
+  | Pexp_fun (Labelled "key", _, _, _, _) | Pexp_fun (Optional "key", _, _, _, _)
+    ->
     Jsx_common.raise_error ~loc:expr.pexp_loc
       "Key cannot be accessed inside of a component. Don't worry - you can \
        always key a component from its parent!"
-  | Pexp_fun (Labelled "ref", _, _, _) | Pexp_fun (Optional "ref", _, _, _) ->
+  | Pexp_fun (Labelled "ref", _, _, _, _) | Pexp_fun (Optional "ref", _, _, _, _)
+    ->
     Jsx_common.raise_error ~loc:expr.pexp_loc
       "Ref cannot be passed as a normal prop. Please use `forwardRef` API \
        instead."
-  | Pexp_fun (arg, default, pattern, expression)
+  | Pexp_fun (arg, default, pattern, expression, _arity)
     when is_optional arg || is_labelled arg ->
     let () =
       match (is_optional arg, pattern, default) with
@@ -700,7 +702,8 @@ let rec recursively_transform_named_args_for_make expr args newtypes core_type =
       ( Nolabel,
         _,
         {ppat_desc = Ppat_construct ({txt = Lident "()"}, _) | Ppat_any},
-        _expression ) ->
+        _expression,
+        _arity ) ->
     (args, newtypes, core_type)
   | Pexp_fun
       ( Nolabel,
@@ -709,7 +712,8 @@ let rec recursively_transform_named_args_for_make expr args newtypes core_type =
            ppat_desc =
              Ppat_var {txt} | Ppat_constraint ({ppat_desc = Ppat_var {txt}}, _);
          } as pattern),
-        _expression ) ->
+        _expression,
+        _arity ) ->
     if txt = "ref" then
       let type_ =
         match pattern with
@@ -721,7 +725,7 @@ let rec recursively_transform_named_args_for_make expr args newtypes core_type =
         newtypes,
         core_type )
     else (args, newtypes, core_type)
-  | Pexp_fun (Nolabel, _, pattern, _expression) ->
+  | Pexp_fun (Nolabel, _, pattern, _expression, _arity) ->
     Location.raise_errorf ~loc:pattern.ppat_loc
       "React: react.component refs only support plain arguments and type \
        annotations."
@@ -824,14 +828,18 @@ let modified_binding ~binding_loc ~binding_pat_loc ~fn_name binding =
          ( ((Labelled _ | Optional _) as label),
            default,
            pattern,
-           ({pexp_desc = Pexp_fun _} as internal_expression) );
+           ({pexp_desc = Pexp_fun _} as internal_expression),
+           arity );
     } ->
       let wrap, has_forward_ref, exp =
         spelunk_for_fun_expression internal_expression
       in
       ( wrap,
         has_forward_ref,
-        {expression with pexp_desc = Pexp_fun (label, default, pattern, exp)} )
+        {
+          expression with
+          pexp_desc = Pexp_fun (label, default, pattern, exp, arity);
+        } )
     (* let make = (()) => ... *)
     (* let make = (_) => ... *)
     | {
@@ -840,19 +848,26 @@ let modified_binding ~binding_loc ~binding_pat_loc ~fn_name binding =
          ( Nolabel,
            _default,
            {ppat_desc = Ppat_construct ({txt = Lident "()"}, _) | Ppat_any},
-           _internalExpression );
+           _internalExpression,
+           _arity );
     } ->
       ((fun a -> a), false, expression)
     (* let make = (~prop) => ... *)
     | {
      pexp_desc =
        Pexp_fun
-         ((Labelled _ | Optional _), _default, _pattern, _internalExpression);
+         ( (Labelled _ | Optional _),
+           _default,
+           _pattern,
+           _internalExpression,
+           _arity );
     } ->
       ((fun a -> a), false, expression)
     (* let make = (prop) => ... *)
-    | {pexp_desc = Pexp_fun (_nolabel, _default, pattern, _internalExpression)}
-      ->
+    | {
+     pexp_desc =
+       Pexp_fun (_nolabel, _default, pattern, _internalExpression, _arity);
+    } ->
       if !has_application then ((fun a -> a), false, expression)
       else
         Location.raise_errorf ~loc:pattern.ppat_loc
@@ -1001,12 +1016,12 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
       (* let make = React.forwardRef({
            let \"App" = (props, ref) => make({...props, ref: @optional (Js.Nullabel.toOption(ref))})
          })*)
-      Exp.fun_ nolabel None
+      Exp.fun_ ~arity:None nolabel None
         (match core_type_of_attr with
         | None -> make_props_pattern named_type_list
         | Some _ -> make_props_pattern typ_vars_of_core_type)
         (if has_forward_ref then
-           Exp.fun_ nolabel None
+           Exp.fun_ ~arity:None nolabel None
              (Pat.var @@ Location.mknoloc "ref")
              inner_expression
          else inner_expression)
@@ -1055,9 +1070,11 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
           ( _arg_label,
             _default,
             {ppat_desc = Ppat_construct ({txt = Lident "()"}, _)},
-            expr ) ->
+            expr,
+            _arity ) ->
         (patterns_with_label, patterns_with_nolabel, expr)
-      | Pexp_fun (arg_label, default, ({ppat_loc; ppat_desc} as pattern), expr)
+      | Pexp_fun
+          (arg_label, default, ({ppat_loc; ppat_desc} as pattern), expr, _arity)
         -> (
         let pattern_without_constraint =
           strip_constraint_unpack ~label:(get_label arg_label) pattern
@@ -1117,7 +1134,7 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
               Pat.constraint_ pattern (ref_type Location.none)
             | _ -> pattern
           in
-          Exp.fun_ Nolabel None pattern expr)
+          Exp.fun_ ~arity:None Nolabel None pattern expr)
         expression patterns_with_nolabel
     in
     (* ({a, b, _}: props<'a, 'b>) *)
@@ -1127,7 +1144,7 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
       | _ -> Pat.record (List.rev patterns_with_label) Open
     in
     let expression =
-      Exp.fun_ Nolabel None
+      Exp.fun_ ~arity:None Nolabel None
         (Pat.constraint_ record_pattern
            (Typ.constr ~loc:empty_loc
               {txt = Lident "props"; loc = empty_loc}
diff --git a/compiler/syntax/src/res_ast_debugger.ml b/compiler/syntax/src/res_ast_debugger.ml
index e3bd7f7bf7..0a6cf69a12 100644
--- a/compiler/syntax/src/res_ast_debugger.ml
+++ b/compiler/syntax/src/res_ast_debugger.ml
@@ -561,7 +561,7 @@ module SexpAst = struct
       | Pexp_function cases ->
         Sexp.list
           [Sexp.atom "Pexp_function"; Sexp.list (map_empty ~f:case cases)]
-      | Pexp_fun (arg_lbl, expr_opt, pat, expr) ->
+      | Pexp_fun (arg_lbl, expr_opt, pat, expr, _) ->
         Sexp.list
           [
             Sexp.atom "Pexp_fun";
diff --git a/compiler/syntax/src/res_comments_table.ml b/compiler/syntax/src/res_comments_table.ml
index 50ce987b1f..ac168be7fb 100644
--- a/compiler/syntax/src/res_comments_table.ml
+++ b/compiler/syntax/src/res_comments_table.ml
@@ -264,7 +264,7 @@ let fun_expr expr =
   let rec collect attrs_before acc expr =
     match expr with
     | {
-     pexp_desc = Pexp_fun (lbl, default_expr, pattern, return_expr);
+     pexp_desc = Pexp_fun (lbl, default_expr, pattern, return_expr, _);
      pexp_attributes = [];
     } ->
       let parameter = ([], lbl, default_expr, pattern) in
@@ -279,7 +279,7 @@ let fun_expr expr =
       in
       collect attrs_before (parameter :: acc) return_expr
     | {
-     pexp_desc = Pexp_fun (lbl, default_expr, pattern, return_expr);
+     pexp_desc = Pexp_fun (lbl, default_expr, pattern, return_expr, _);
      pexp_attributes = [({txt = "bs"}, _)] as attrs;
     } ->
       let parameter = (attrs, lbl, default_expr, pattern) in
@@ -287,7 +287,11 @@ let fun_expr expr =
     | {
      pexp_desc =
        Pexp_fun
-         (((Labelled _ | Optional _) as lbl), default_expr, pattern, return_expr);
+         ( ((Labelled _ | Optional _) as lbl),
+           default_expr,
+           pattern,
+           return_expr,
+           _ );
      pexp_attributes = attrs;
     } ->
       let parameter = (attrs, lbl, default_expr, pattern) in
@@ -296,7 +300,7 @@ let fun_expr expr =
   in
   match expr with
   | {
-      pexp_desc = Pexp_fun (Nolabel, _defaultExpr, _pattern, _returnExpr);
+      pexp_desc = Pexp_fun (Nolabel, _defaultExpr, _pattern, _returnExpr, _);
       pexp_attributes = attrs;
     } as expr ->
     collect attrs [] {expr with pexp_attributes = []}
@@ -1406,7 +1410,7 @@ and walk_expression expr t comments =
       in
       attach t.trailing call_expr.pexp_loc after_expr;
       walk_list (arguments |> List.map (fun (_, e) -> ExprArgument e)) t rest
-  | Pexp_fun (_, _, _, _) | Pexp_newtype _ -> (
+  | Pexp_fun (_, _, _, _, _) | Pexp_newtype _ -> (
     let _, parameters, return_expr = fun_expr expr in
     let comments =
       visit_list_but_continue_with_remaining_comments ~newline_delimited:false
diff --git a/compiler/syntax/src/res_core.ml b/compiler/syntax/src/res_core.ml
index dbd79c7852..610a37b734 100644
--- a/compiler/syntax/src/res_core.ml
+++ b/compiler/syntax/src/res_core.ml
@@ -525,7 +525,9 @@ let process_underscore_application args =
           (Ppat_var (Location.mkloc hidden_var loc))
           ~loc:Location.none
       in
-      let fun_expr = Ast_helper.Exp.fun_ ~loc Nolabel None pattern exp_apply in
+      let fun_expr =
+        Ast_helper.Exp.fun_ ~loc ~arity:(Some 1) Nolabel None pattern exp_apply
+      in
       Ast_uncurried.uncurried_fun ~loc ~arity:1 fun_expr
     | None -> exp_apply
   in
@@ -1594,7 +1596,8 @@ and parse_es6_arrow_expression ?(arrow_attrs = []) ?(arrow_start_pos = None)
             {attrs; label = lbl; expr = default_expr; pat; pos = start_pos} ->
           let loc = mk_loc start_pos end_pos in
           let fun_expr =
-            Ast_helper.Exp.fun_ ~loc ~attrs lbl default_expr pat expr
+            Ast_helper.Exp.fun_ ~loc ~attrs ~arity:(Some arity) lbl default_expr
+              pat expr
           in
           if term_param_num = 1 then
             ( term_param_num - 1,
@@ -2407,13 +2410,13 @@ and over_parse_constrained_or_coerced_or_arrow_expression p expr =
       let arrow1 =
         Ast_helper.Exp.fun_
           ~loc:(mk_loc expr.pexp_loc.loc_start body.pexp_loc.loc_end)
-          Asttypes.Nolabel None pat
+          ~arity:None Asttypes.Nolabel None pat
           (Ast_helper.Exp.constraint_ body typ)
       in
       let arrow2 =
         Ast_helper.Exp.fun_
           ~loc:(mk_loc expr.pexp_loc.loc_start body.pexp_loc.loc_end)
-          Asttypes.Nolabel None
+          ~arity:None Asttypes.Nolabel None
           (Ast_helper.Pat.constraint_ pat typ)
           body
       in
diff --git a/compiler/syntax/src/res_parsetree_viewer.ml b/compiler/syntax/src/res_parsetree_viewer.ml
index 038f19f06b..aa74b0992b 100644
--- a/compiler/syntax/src/res_parsetree_viewer.ml
+++ b/compiler/syntax/src/res_parsetree_viewer.ml
@@ -140,7 +140,8 @@ let rewrite_underscore_apply expr =
       ( Nolabel,
         None,
         {ppat_desc = Ppat_var {txt = "__x"}},
-        ({pexp_desc = Pexp_apply (call_expr, args)} as e) ) ->
+        ({pexp_desc = Pexp_apply (call_expr, args)} as e),
+        _ ) ->
     let new_args =
       List.map
         (fun arg ->
@@ -185,7 +186,8 @@ let fun_expr expr =
          ( Nolabel,
            None,
            {ppat_desc = Ppat_var {txt = "__x"}},
-           {pexp_desc = Pexp_apply _} );
+           {pexp_desc = Pexp_apply _},
+           _ );
     } ->
       (uncurried, attrs_before, List.rev acc, rewrite_underscore_apply expr)
     | {pexp_desc = Pexp_newtype (string_loc, rest); pexp_attributes = attrs} ->
@@ -193,7 +195,7 @@ let fun_expr expr =
       let param = NewTypes {attrs; locs = string_locs} in
       collect ~uncurried ~n_fun attrs_before (param :: acc) return_expr
     | {
-     pexp_desc = Pexp_fun (lbl, default_expr, pattern, return_expr);
+     pexp_desc = Pexp_fun (lbl, default_expr, pattern, return_expr, _);
      pexp_attributes = [];
     } ->
       let parameter =
@@ -231,9 +233,9 @@ let filter_parsing_attrs attrs =
       match attr with
       | ( {
             Location.txt =
-              ( "res.arity" | "res.braces" | "ns.braces" | "res.iflet"
-              | "res.namedArgLoc" | "res.ternary" | "res.async" | "res.await"
-              | "res.template" | "res.taggedTemplate" | "res.patVariantSpread"
+              ( "res.braces" | "ns.braces" | "res.iflet" | "res.namedArgLoc"
+              | "res.ternary" | "res.async" | "res.await" | "res.template"
+              | "res.taggedTemplate" | "res.patVariantSpread"
               | "res.dictPattern" );
           },
           _ ) ->
@@ -382,8 +384,8 @@ let has_attributes attrs =
       match attr with
       | ( {
             Location.txt =
-              ( "res.arity" | "res.braces" | "ns.braces" | "res.iflet"
-              | "res.ternary" | "res.async" | "res.await" | "res.template" );
+              ( "res.braces" | "ns.braces" | "res.iflet" | "res.ternary"
+              | "res.async" | "res.await" | "res.template" );
           },
           _ ) ->
         false
@@ -566,8 +568,8 @@ let is_printable_attribute attr =
   match attr with
   | ( {
         Location.txt =
-          ( "res.arity" | "res.iflet" | "res.braces" | "ns.braces" | "JSX"
-          | "res.async" | "res.await" | "res.template" | "res.ternary" );
+          ( "res.iflet" | "res.braces" | "ns.braces" | "JSX" | "res.async"
+          | "res.await" | "res.template" | "res.ternary" );
       },
       _ ) ->
     false
@@ -744,7 +746,8 @@ let is_underscore_apply_sugar expr =
       ( Nolabel,
         None,
         {ppat_desc = Ppat_var {txt = "__x"}},
-        {pexp_desc = Pexp_apply _} ) ->
+        {pexp_desc = Pexp_apply _},
+        _ ) ->
     true
   | _ -> false
 
diff --git a/compiler/syntax/src/res_printer.ml b/compiler/syntax/src/res_printer.ml
index 6fdb675b7e..a5fde5b0ae 100644
--- a/compiler/syntax/src/res_printer.ml
+++ b/compiler/syntax/src/res_printer.ml
@@ -2792,7 +2792,8 @@ and print_expression ~state (e : Parsetree.expression) cmt_tbl =
         ( Nolabel,
           None,
           {ppat_desc = Ppat_var {txt = "__x"}},
-          {pexp_desc = Pexp_apply _} )
+          {pexp_desc = Pexp_apply _},
+          _ )
     | Pexp_construct
         ( {txt = Lident "Function$"},
           Some
@@ -2802,7 +2803,8 @@ and print_expression ~state (e : Parsetree.expression) cmt_tbl =
                   ( Nolabel,
                     None,
                     {ppat_desc = Ppat_var {txt = "__x"}},
-                    {pexp_desc = Pexp_apply _} );
+                    {pexp_desc = Pexp_apply _},
+                    _ );
             } ) ->
       (* (__x) => f(a, __x, c) -----> f(a, _, c)  *)
       print_expression_with_comments ~state
diff --git a/scripts/test_syntax.sh b/scripts/test_syntax.sh
index 3e13b719ff..080a9e64a6 100755
--- a/scripts/test_syntax.sh
+++ b/scripts/test_syntax.sh
@@ -54,7 +54,7 @@ warningYellow='\033[0;33m'
 successGreen='\033[0;32m'
 reset='\033[0m'
 
-git diff --ignore-cr-at-eol $(find tests -name expected) >temp/diff.txt
+git diff --ignore-cr-at-eol $(find syntax_tests -name expected) >temp/diff.txt
 diff=$(cat temp/diff.txt)
 if [[ $diff = "" ]]; then
   printf "${successGreen}✅ No unstaged tests difference.${reset}\n"
diff --git a/tests/syntax_tests/data/parsing/errors/expressions/expected/array.res.txt b/tests/syntax_tests/data/parsing/errors/expressions/expected/array.res.txt
index e155f0f9d9..31dbf702e0 100644
--- a/tests/syntax_tests/data/parsing/errors/expressions/expected/array.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/expressions/expected/array.res.txt
@@ -8,5 +8,4 @@
 
   Did you forget a `]` here?
 
-let xs =
-  x.map ((Function$ (fun key -> [|key;(predicates.(key))|]))[@res.arity 1])
\ No newline at end of file
+let xs = x.map (Function$ (fun [arity:1]key -> [|key;(predicates.(key))|]))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/errors/expressions/expected/arrow.res.txt b/tests/syntax_tests/data/parsing/errors/expressions/expected/arrow.res.txt
index 07356025b6..921b269614 100644
--- a/tests/syntax_tests/data/parsing/errors/expressions/expected/arrow.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/expressions/expected/arrow.res.txt
@@ -9,7 +9,7 @@
   Did you forget a `,` here?
 
 ;;(Object.keys providers).reduce
-    ((Function$
-        (fun elements ->
-           fun providerId -> ((let x = 1 in let b = 2 in x + b)
-             [@res.braces ])))[@res.arity 2])
\ No newline at end of file
+    (Function$
+       (fun [arity:2]elements ->
+          fun [arity:1]providerId -> ((let x = 1 in let b = 2 in x + b)
+            [@res.braces ])))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/errors/expressions/expected/block.res.txt b/tests/syntax_tests/data/parsing/errors/expressions/expected/block.res.txt
index c360a4c9c0..636937df30 100644
--- a/tests/syntax_tests/data/parsing/errors/expressions/expected/block.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/expressions/expected/block.res.txt
@@ -64,26 +64,25 @@
   Looks like there might be an expression missing here
 
 let findThreadByIdLinearScan =
-  ((Function$
-      (fun ~threads:((threads)[@res.namedArgLoc ]) ->
-         fun ~id:((id)[@res.namedArgLoc ]) ->
-           ((Js.Array2.findi ThreadsModel.threads
-               ((Function$
-                   (fun thread ->
-                      fun i ->
-                        ((let thisId =
-                            match thread with
-                            | ServerData.OneToOne
-                                { otherPersonIDWhichIsAlsoThreadID } ->
-                                otherPersonIDWhichIsAlsoThreadID
-                            | Group { id } -> id
-                            | Unknown { id } ->
-                                (unknown.id |.u Js.String.make) |.u
-                                  FBID.ofStringUnsafe in
-                          thisId == id)
-                        [@res.braces ])))[@res.arity 2]))
-           [@res.braces ])))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]~threads:((threads)[@res.namedArgLoc ]) ->
+       fun [arity:1]~id:((id)[@res.namedArgLoc ]) ->
+         ((Js.Array2.findi ThreadsModel.threads
+             (Function$
+                (fun [arity:2]thread ->
+                   fun [arity:1]i ->
+                     ((let thisId =
+                         match thread with
+                         | ServerData.OneToOne
+                             { otherPersonIDWhichIsAlsoThreadID } ->
+                             otherPersonIDWhichIsAlsoThreadID
+                         | Group { id } -> id
+                         | Unknown { id } ->
+                             (unknown.id |.u Js.String.make) |.u
+                               FBID.ofStringUnsafe in
+                       thisId == id)
+                     [@res.braces ]))))
+         [@res.braces ]))
 let x = ((loop 0 (Nil |.u (push doc)))[@res.braces ])
 ;;match stack with
   | Empty -> [%rescript.exprhole ]
diff --git a/tests/syntax_tests/data/parsing/errors/expressions/expected/consecutive.res.txt b/tests/syntax_tests/data/parsing/errors/expressions/expected/consecutive.res.txt
index 1fe6364746..9d12fcc36f 100644
--- a/tests/syntax_tests/data/parsing/errors/expressions/expected/consecutive.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/expressions/expected/consecutive.res.txt
@@ -44,9 +44,9 @@
 
   consecutive expressions on a line must be separated by ';' or a newline
 
-let f = ((Function$ (fun a -> fun b -> a + 3))[@res.arity 2])
+let f = Function$ (fun [arity:2]a -> fun [arity:1]b -> a + 3)
 ;;b
-let f = ((Function$ (fun g -> fun h -> ((a + 3; b)[@res.braces ])))
-  [@res.arity 2])
+let f =
+  Function$ (fun [arity:2]g -> fun [arity:1]h -> ((a + 3; b)[@res.braces ]))
 let () = ((sideEffect1 (); sideEffect2 ())[@res.braces ])
 let () = ((let open Foo in let exception End  in x ())[@res.braces ])
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/errors/expressions/expected/emptyBlock.res.txt b/tests/syntax_tests/data/parsing/errors/expressions/expected/emptyBlock.res.txt
index 545f257b92..b08fdd7412 100644
--- a/tests/syntax_tests/data/parsing/errors/expressions/expected/emptyBlock.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/expressions/expected/emptyBlock.res.txt
@@ -1,2 +1,2 @@
 let x = {  }
-let f = ((Function$ (fun a -> fun b -> {  }))[@res.arity 2])
\ No newline at end of file
+let f = Function$ (fun [arity:2]a -> fun [arity:1]b -> {  })
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/errors/expressions/expected/implementation.res.txt b/tests/syntax_tests/data/parsing/errors/expressions/expected/implementation.res.txt
index 38fd3ae420..cb8406f831 100644
--- a/tests/syntax_tests/data/parsing/errors/expressions/expected/implementation.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/expressions/expected/implementation.res.txt
@@ -13,7 +13,8 @@
 module InstallerDownload =
   struct
     let make =
-      ((Function$ (fun () -> ((div ~children:[] ())[@res.braces ][@JSX ])))
-      [@res.arity 1])[@@react.component ]
+      Function$
+        (fun [arity:1]() -> ((div ~children:[] ())[@res.braces ][@JSX ]))
+      [@@react.component ]
   end
 module LicenseList = struct  end
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/errors/other/expected/labelledParameters.res.txt b/tests/syntax_tests/data/parsing/errors/other/expected/labelledParameters.res.txt
index 5ffc6661b8..ad41ffd983 100644
--- a/tests/syntax_tests/data/parsing/errors/other/expected/labelledParameters.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/other/expected/labelledParameters.res.txt
@@ -31,12 +31,12 @@
 
   A labeled parameter starts with a `~`. Did you mean: `~x`?
 
-let f = ((Function$ (fun x -> fun ?(y= 2) -> fun z -> (x + y) + z))
-  [@res.arity 3])
+let f =
+  Function$
+    (fun [arity:3]x -> fun [arity:2]?(y= 2) -> fun [arity:1]z -> (x + y) + z)
 let g =
-  ((Function$
-      (fun ~x:((x)[@res.namedArgLoc ]) ->
-         fun ?y:(((y)[@res.namedArgLoc ])= 2) ->
-           fun ~z:((z)[@res.namedArgLoc ]) -> (x + y) + z))
-  [@res.arity 3])
+  Function$
+    (fun [arity:3]~x:((x)[@res.namedArgLoc ]) ->
+       fun [arity:2]?y:(((y)[@res.namedArgLoc ])= 2) ->
+         fun [arity:1]~z:((z)[@res.namedArgLoc ]) -> (x + y) + z)
 type nonrec f = (x:int -> y:int -> int, [ `Has_arity2 ]) function$
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/errors/structure/expected/gh16B.res.txt b/tests/syntax_tests/data/parsing/errors/structure/expected/gh16B.res.txt
index 0c13bcdc3e..278e3ab31b 100644
--- a/tests/syntax_tests/data/parsing/errors/structure/expected/gh16B.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/structure/expected/gh16B.res.txt
@@ -15,12 +15,11 @@ open Ws
 let wss = Server.make { port = 82 }
 let address = wss |.u Server.address
 let log =
-  ((Function$
-      (fun msg ->
-         Js.log
-           (((((({js|> Server: |js})[@res.template ]) ^ msg)[@res.template ])
-               ^ (({js||js})[@res.template ]))[@res.template ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]msg ->
+       Js.log
+         (((((({js|> Server: |js})[@res.template ]) ^ msg)[@res.template ]) ^
+             (({js||js})[@res.template ]))[@res.template ]))
 ;;log
     (((((((((((((({js|Running on: |js})[@res.template ]) ^ address.address)
                 [@res.template ]) ^ (({js|:|js})[@res.template ]))
@@ -34,14 +33,13 @@ module ClientSet =
       (Belt.Id.MakeComparable)(struct
                                  type nonrec t = Client.t
                                  let cmp =
-                                   ((Function$
-                                       (fun a ->
-                                          fun b ->
-                                            ((compare
-                                                (a |.u Client.getUniqueId)
-                                                (b |.u Client.getUniqueId))
-                                            [@res.braces ])))
-                                   [@res.arity 2])
+                                   Function$
+                                     (fun [arity:2]a ->
+                                        fun [arity:1]b ->
+                                          ((compare
+                                              (a |.u Client.getUniqueId)
+                                              (b |.u Client.getUniqueId))
+                                          [@res.braces ]))
                                end)
     let empty = Belt.Set.make ~id:(((module T))[@res.namedArgLoc ])
   end
diff --git a/tests/syntax_tests/data/parsing/errors/structure/expected/letBinding.res.txt b/tests/syntax_tests/data/parsing/errors/structure/expected/letBinding.res.txt
index 6be6d414bf..6d99a83fc5 100644
--- a/tests/syntax_tests/data/parsing/errors/structure/expected/letBinding.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/structure/expected/letBinding.res.txt
@@ -99,8 +99,8 @@
 
 let rightResource =
   (ur.resources).find
-    ((Function$ (fun r -> r.account_id == ((connection.left).account).id))
-    [@res.arity 1])
+    (Function$
+       (fun [arity:1]r -> r.account_id == ((connection.left).account).id))
 let x = ((let field = p |.u parseFieldDeclaration in field)[@res.braces ])
 let t = ((let (_, _, token) = scanner |.u scan in token)[@res.braces ])
 let (keyTable : int Belt.Map.String.t) = [%rescript.exprhole ]
diff --git a/tests/syntax_tests/data/parsing/errors/typeDef/expected/inlineRecord.res.txt b/tests/syntax_tests/data/parsing/errors/typeDef/expected/inlineRecord.res.txt
index 3847940a87..85834693c6 100644
--- a/tests/syntax_tests/data/parsing/errors/typeDef/expected/inlineRecord.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/typeDef/expected/inlineRecord.res.txt
@@ -44,11 +44,10 @@ type nonrec user =
   name: string ;
   address: < street: string  ;country: string   >  }
 let make =
-  ((Function$
-      (fun
-         (props :
-           <
-             handleClick: (Click.t -> unit, [ `Has_arity1 ]) function$  ;
-             value: string   > )
-         -> render props))
-  [@res.arity 1])
\ No newline at end of file
+  Function$
+    (fun
+       [arity:1](props :
+                  <
+                    handleClick: (Click.t -> unit, [ `Has_arity1 ]) function$
+                       ;value: string   > )
+       -> render props)
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/errors/typexpr/expected/objectSpread.res.txt b/tests/syntax_tests/data/parsing/errors/typexpr/expected/objectSpread.res.txt
index f49d8f0df5..d429e17985 100644
--- a/tests/syntax_tests/data/parsing/errors/typexpr/expected/objectSpread.res.txt
+++ b/tests/syntax_tests/data/parsing/errors/typexpr/expected/objectSpread.res.txt
@@ -30,4 +30,4 @@ type nonrec u = private {
 type nonrec x =
   | Type of < a ;u: int   >  
 type nonrec u = < a ;u: int  ;v: int   > 
-let f = ((Function$ (fun (x : < a: int  ;b: int   > ) -> ()))[@res.arity 1])
\ No newline at end of file
+let f = Function$ (fun [arity:1](x : < a: int  ;b: int   > ) -> ())
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedAlways.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedAlways.res.txt
index ffd40ac996..8e6a52419c 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedAlways.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedAlways.res.txt
@@ -1,7 +1,7 @@
 [@@@uncurried ]
-let foo = ((Function$ (fun x -> fun y -> x + y))[@res.arity 2])
+let foo = Function$ (fun [arity:2]x -> fun [arity:1]y -> x + y)
 let z = foo 3 4
-let bar = ((Function$ (fun x -> fun y -> x + y))[@res.arity 2])
+let bar = Function$ (fun [arity:2]x -> fun [arity:1]y -> x + y)
 let b = bar 3 4
 let w = 3 |.u (foo 4)
 let a = 3 |.u (foo 4)
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedByDefault.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedByDefault.res.txt
index ee062011ea..4d86f16622 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedByDefault.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedByDefault.res.txt
@@ -1,24 +1,21 @@
 let cApp = foo 3
 let uApp = foo 3
-let cFun = ((Function$ (fun x -> 3))[@res.arity 1])
-let uFun = ((Function$ (fun x -> 3))[@res.arity 1])
+let cFun = Function$ (fun [arity:1]x -> 3)
+let uFun = Function$ (fun [arity:1]x -> 3)
 let mixFun =
-  ((Function$
-      (fun a ->
-         fun b ->
-           fun c ->
-             ((Function$
-                 (fun d ->
-                    fun e ->
-                      fun f -> ((Function$ (fun g -> fun h -> 4))
-                        [@res.arity 2])))
-             [@res.arity 3])))
-  [@res.arity 3])
+  Function$
+    (fun [arity:3]a ->
+       fun [arity:2]b ->
+         fun [arity:1]c ->
+           Function$
+             (fun [arity:3]d ->
+                fun [arity:2]e ->
+                  fun [arity:1]f ->
+                    Function$ (fun [arity:2]g -> fun [arity:1]h -> 4)))
 let bracesFun =
-  ((Function$ (fun x -> ((Function$ (fun y -> x + y))[@res.arity 1])))
-  [@res.arity 1])
-let cFun2 = ((Function$ (fun x -> fun y -> 3))[@res.arity 2])
-let uFun2 = ((Function$ (fun x -> fun y -> 3))[@res.arity 2])
+  Function$ (fun [arity:1]x -> Function$ (fun [arity:1]y -> x + y))
+let cFun2 = Function$ (fun [arity:2]x -> fun [arity:1]y -> 3)
+let uFun2 = Function$ (fun [arity:2]x -> fun [arity:1]y -> 3)
 type nonrec cTyp = (string -> int, [ `Has_arity1 ]) function$
 type nonrec uTyp = (string -> int, [ `Has_arity1 ]) function$
 type nonrec mixTyp =
@@ -75,26 +72,24 @@ type nonrec unested =
     function$
 let (uannpoly : ('a -> string, [ `Has_arity1 ]) function$) = xx
 let (uannint : (int -> string, [ `Has_arity1 ]) function$) = xx
-let _ = ((Function$ ((fun x -> 34)[@att ]))[@res.arity 1])
-let _ = ((Function$ ((fun x -> 34)[@res.async ][@att ]))[@res.arity 1])
-let _ = preserveAttr ((Function$ ((fun x -> 34)[@att ]))[@res.arity 1])
-let _ =
-  preserveAttr ((Function$ ((fun x -> 34)[@res.async ][@att ]))
-    [@res.arity 1])
+let _ = Function$ ((fun [arity:1]x -> 34)[@att ])
+let _ = Function$ ((fun [arity:1]x -> 34)[@res.async ][@att ])
+let _ = preserveAttr (Function$ ((fun [arity:1]x -> 34)[@att ]))
+let _ = preserveAttr (Function$ ((fun [arity:1]x -> 34)[@res.async ][@att ]))
 let t0 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t1 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t2 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t3 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t4 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t5 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t6 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 type nonrec arrowPath1 = (int -> string, [ `Has_arity1 ]) function$
 type nonrec arrowPath2 = (I.t -> string, [ `Has_arity1 ]) function$
 type nonrec arrowPath3 = (int -> string, [ `Has_arity1 ]) function$
@@ -107,31 +102,25 @@ type nonrec callback3 =
   (ReactEvent.Mouse.t -> unit, [ `Has_arity1 ]) function$ as 'callback
 let cApp = foo 3
 let uApp = foo 3
-let cFun = ((Function$ (fun x -> 3))[@res.arity 1])
-let uFun = ((Function$ (fun x -> 3))[@res.arity 1])
+let cFun = Function$ (fun [arity:1]x -> 3)
+let uFun = Function$ (fun [arity:1]x -> 3)
 let mixFun =
-  ((Function$
-      (fun a ->
-         ((Function$
-             (fun b ->
-                fun c ->
-                  ((Function$
-                      (fun d ->
-                         fun e ->
-                           fun f ->
-                             ((Function$
-                                 (fun g -> ((Function$ (fun h -> 4))
-                                    [@res.arity 1])))
-                             [@res.arity 1])))
-                  [@res.arity 3])))
-         [@res.arity 2])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]a ->
+       Function$
+         (fun [arity:2]b ->
+            fun [arity:1]c ->
+              Function$
+                (fun [arity:3]d ->
+                   fun [arity:2]e ->
+                     fun [arity:1]f ->
+                       Function$
+                         (fun [arity:1]g -> Function$ (fun [arity:1]h -> 4)))))
 let bracesFun =
-  ((Function$ (fun x -> ((Function$ (fun y -> x + y))[@res.arity 1])))
-  [@res.arity 1])
-let cFun2 = ((Function$ (fun x -> fun y -> 3))[@res.arity 2])
-let uFun2 = ((Function$ (fun x -> fun y -> 3))[@res.arity 2])
-let cFun2Dots = ((Function$ (fun x -> fun y -> 3))[@res.arity 2])
+  Function$ (fun [arity:1]x -> Function$ (fun [arity:1]y -> x + y))
+let cFun2 = Function$ (fun [arity:2]x -> fun [arity:1]y -> 3)
+let uFun2 = Function$ (fun [arity:2]x -> fun [arity:1]y -> 3)
+let cFun2Dots = Function$ (fun [arity:2]x -> fun [arity:1]y -> 3)
 type nonrec cTyp = (string -> int, [ `Has_arity1 ]) function$
 type nonrec uTyp = (string -> int, [ `Has_arity1 ]) function$
 type nonrec mixTyp =
@@ -190,20 +179,18 @@ type nonrec unested =
 let pipe1 = 3 |.u f
 let (uannpoly : ('a -> string, [ `Has_arity1 ]) function$) = xx
 let (uannint : (int -> string, [ `Has_arity1 ]) function$) = xx
-let _ = ((Function$ ((fun x -> 34)[@att ]))[@res.arity 1])
-let _ = ((Function$ ((fun x -> 34)[@res.async ][@att ]))[@res.arity 1])
-let _ = preserveAttr ((Function$ ((fun x -> 34)[@att ]))[@res.arity 1])
-let _ =
-  preserveAttr ((Function$ ((fun x -> 34)[@res.async ][@att ]))
-    [@res.arity 1])
+let _ = Function$ ((fun [arity:1]x -> 34)[@att ])
+let _ = Function$ ((fun [arity:1]x -> 34)[@res.async ][@att ])
+let _ = preserveAttr (Function$ ((fun [arity:1]x -> 34)[@att ]))
+let _ = preserveAttr (Function$ ((fun [arity:1]x -> 34)[@res.async ][@att ]))
 let t0 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t1 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t2 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 let t3 (type a) (type b) =
-  ((Function$ (fun (l : a list) -> fun (x : a) -> x :: l))[@res.arity 2])
+  Function$ (fun [arity:2](l : a list) -> fun [arity:1](x : a) -> x :: l)
 type nonrec arrowPath1 = (int -> string, [ `Has_arity1 ]) function$
 type nonrec arrowPath2 = (I.t -> string, [ `Has_arity1 ]) function$
 type nonrec arrowPath3 = (int -> string, [ `Has_arity1 ]) function$
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/apply.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/apply.res.txt
index 7170c39226..a08df40ebd 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/apply.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/apply.res.txt
@@ -1,10 +1,11 @@
-;;foo ((Function$ (fun _ -> bla))[@res.arity 1]) blaz
-;;foo ((Function$ (fun _ -> bla))[@res.arity 1]) blaz
-;;foo ((Function$ (fun _ -> bla))[@res.arity 1]) blaz
-;;foo ((Function$ (fun _ -> bla))[@res.arity 1]) ((Function$ (fun _ -> blaz))
-    [@res.arity 1])
-;;List.map ((Function$ (fun x -> x + 1))[@res.arity 1]) myList
-;;List.reduce ((Function$ (fun acc -> fun curr -> acc + curr))[@res.arity 2])
-    0 myList
+;;foo (Function$ (fun [arity:1]_ -> bla)) blaz
+;;foo (Function$ (fun [arity:1]_ -> bla)) blaz
+;;foo (Function$ (fun [arity:1]_ -> bla)) blaz
+;;foo (Function$ (fun [arity:1]_ -> bla))
+    (Function$ (fun [arity:1]_ -> blaz))
+;;List.map (Function$ (fun [arity:1]x -> x + 1)) myList
+;;List.reduce
+    (Function$ (fun [arity:2]acc -> fun [arity:1]curr -> acc + curr)) 0
+    myList
 let unitUncurried = apply ()
 ;;call ~a:(((((a)[@res.namedArgLoc ]) : int))[@res.namedArgLoc ])
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/argument.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/argument.res.txt
index a8e3f5e200..267d3cb084 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/argument.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/argument.res.txt
@@ -1,15 +1,15 @@
 let foo =
-  ((Function$
-      (fun ~a:((a)[@res.namedArgLoc ]) ->
-         (a (let __res_unit = () in __res_unit)) +. 1.))
-  [@res.arity 1])
-let a = ((Function$ (fun () -> 2))[@res.arity 1])
+  Function$
+    (fun [arity:1]~a:((a)[@res.namedArgLoc ]) ->
+       (a (let __res_unit = () in __res_unit)) +. 1.)
+let a = Function$ (fun [arity:1]() -> 2)
 let bar = foo ~a:((a)[@res.namedArgLoc ])
 let comparisonResult =
   compare currentNode.value ~targetValue:((targetValue)[@res.namedArgLoc ])
 ;;callback firstNode ~y:((y)[@res.namedArgLoc ])
 ;;document.createElementWithOptions {js|div|js}
-    (elementProps ~onClick:((Function$ (fun _ -> Js.log {js|hello world|js}))
-       [@res.namedArgLoc ][@res.arity 1]))
+    (elementProps
+       ~onClick:((Function$ (fun [arity:1]_ -> Js.log {js|hello world|js}))
+       [@res.namedArgLoc ]))
 ;;resolve ()
 ;;resolve (let __res_unit = () in __res_unit)
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/arrow.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/arrow.res.txt
index 9dfddce397..9060aa6494 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/arrow.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/arrow.res.txt
@@ -1,103 +1,102 @@
-let f = ((Function$ (fun x -> x + 1))[@res.arity 1])
-let f = ((Function$ (fun _ -> Js.log {js|test|js}))[@res.arity 1])
-let f = ((Function$ (fun () -> Js.log {js|unit|js}))[@res.arity 1])
-let f = ((Function$ (fun (Reducer (inst, comp)) -> inst.render comp))
-  [@res.arity 1])
-let f = ((Function$ (fun (Instance) -> ()))[@res.arity 1])
-let f = ((Function$ (fun a -> fun b -> a + b))[@res.arity 2])
-let f = ((Function$ (fun 1 -> fun 2 -> ()))[@res.arity 2])
-let f = ((Function$ (fun {js|stringPattern|js} -> ()))[@res.arity 1])
-let f =
-  ((Function$ (fun {js|stringPattern|js} -> fun {js|stringPattern|js} -> ()))
-  [@res.arity 2])
-let f = ((Function$ (fun () -> ()))[@res.arity 1])
-let f = ((Function$ (fun (a : int) -> fun (b : int) -> a + b))[@res.arity 2])
-let f = ((Function$ (fun _ -> fun _ -> ()))[@res.arity 2])
-let f = ((Function$ (fun [|a;b|] -> fun [|c;d|] -> ((a + b) + c) + d))
-  [@res.arity 2])
-let f = ((Function$ (fun { a } -> a + 1))[@res.arity 1])
-let f = ((Function$ (fun { a; b } -> fun { c; d } -> ((a + b) + c) + d))
-  [@res.arity 2])
-let f = ((Function$ (fun (a, b) -> a + b))[@res.arity 1])
-let f = ((Function$ (fun (a, b) -> fun (c, d) -> ((a + b) + c) + d))
-  [@res.arity 2])
-let f = ((Function$ (fun exception Terminate -> ()))[@res.arity 1])
-let f = ((Function$ (fun exception Terminate -> fun exception Exit -> ()))
-  [@res.arity 2])
-let f = ((Function$ (fun [] -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::xs) -> x + (xs |.u Belt.List.length)))
-  [@res.arity 1])
-let f = ((Function$ (fun (x : int) -> fun (y : int) -> x + y))[@res.arity 2])
-let f =
-  ((Function$
-      (fun ~a:((a)[@res.namedArgLoc ]) ->
-         fun ~b:((b)[@res.namedArgLoc ]) -> a + b))
-  [@res.arity 2])
-let f =
-  ((Function$
-      (fun ~a:((x)[@res.namedArgLoc ]) ->
-         fun ~b:((y)[@res.namedArgLoc ]) -> x + y))
-  [@res.arity 2])
-let f =
-  ((Function$
-      (fun ~a:(((x : int))[@res.namedArgLoc ]) ->
-         fun ~b:(((y : int))[@res.namedArgLoc ]) -> x + y))
-  [@res.arity 2])
-let f =
-  ((Function$
-      (fun ?a:(((a)[@res.namedArgLoc ])= 1) ->
-         fun ?b:(((b)[@res.namedArgLoc ])= 2) -> fun c -> (a + b) + c))
-  [@res.arity 3])
-let f =
-  ((Function$
-      (fun ?a:(((x)[@res.namedArgLoc ])= 1) ->
-         fun ?b:(((y)[@res.namedArgLoc ])= 2) -> fun c -> (x + y) + c))
-  [@res.arity 3])
-let f =
-  ((Function$
-      (fun ?a:((((x : int))[@res.namedArgLoc ])= 1) ->
-         fun ?b:((((y : int))[@res.namedArgLoc ])= 2) -> fun c -> (x + y) + c))
-  [@res.arity 3])
-let f =
-  ((Function$
-      (fun ?a:((a)[@res.namedArgLoc ]) ->
-         fun ?b:((b)[@res.namedArgLoc ]) ->
-           fun c ->
-             match (a, b) with | (Some a, Some b) -> (a + b) + c | _ -> 3))
-  [@res.arity 3])
-let f =
-  ((Function$
-      (fun ?a:((x)[@res.namedArgLoc ]) ->
-         fun ?b:((y)[@res.namedArgLoc ]) ->
-           fun c ->
-             match (x, y) with | (Some a, Some b) -> (a + b) + c | _ -> 3))
-  [@res.arity 3])
-let f =
-  ((Function$
-      (fun ?a:(((x : int option))[@res.namedArgLoc ]) ->
-         fun ?b:(((y : int option))[@res.namedArgLoc ]) ->
-           fun c ->
-             match (x, y) with | (Some a, Some b) -> (a + b) + c | _ -> 3))
-  [@res.arity 3])
-let f = ((Function$ (fun a -> fun b -> a + b))[@res.arity 2])
-let f = ((Function$ (fun () -> ()))[@res.arity 1])
-let f = ((Function$ (fun () -> ()))[@res.arity 1])
-let f = ((Function$ (fun a -> fun b -> fun c -> ()))[@res.arity 3])
-let f = ((Function$ (fun a -> fun b -> fun c -> fun d -> ()))[@res.arity 4])
-let f = ((Function$ (fun a -> fun b -> fun c -> ()))[@res.arity 3])
-let f =
-  ((Function$
-      (fun ~a:((a)[@res.namedArgLoc ][@attr ]) ->
-         fun b -> fun ~c:((c)[@res.namedArgLoc ][@attr ]) -> fun d -> ()))
-  [@res.arity 4])
-let f =
-  ((Function$
-      (fun ~a:((a)[@res.namedArgLoc ][@attr ]) ->
-         fun ((b)[@attrOnB ]) ->
-           fun ~c:((c)[@res.namedArgLoc ][@attr ]) ->
-             fun ((d)[@attrOnD ]) -> ()))
-  [@res.arity 4])
-let f = ((Function$ (fun list -> list ()))[@res.arity 1])
+let f = Function$ (fun [arity:1]x -> x + 1)
+let f = Function$ (fun [arity:1]_ -> Js.log {js|test|js})
+let f = Function$ (fun [arity:1]() -> Js.log {js|unit|js})
+let f = Function$ (fun [arity:1](Reducer (inst, comp)) -> inst.render comp)
+let f = Function$ (fun [arity:1](Instance) -> ())
+let f = Function$ (fun [arity:2]a -> fun [arity:1]b -> a + b)
+let f = Function$ (fun [arity:2]1 -> fun [arity:1]2 -> ())
+let f = Function$ (fun [arity:1]{js|stringPattern|js} -> ())
+let f =
+  Function$
+    (fun [arity:2]{js|stringPattern|js} ->
+       fun [arity:1]{js|stringPattern|js} -> ())
+let f = Function$ (fun [arity:1]() -> ())
+let f = Function$ (fun [arity:2](a : int) -> fun [arity:1](b : int) -> a + b)
+let f = Function$ (fun [arity:2]_ -> fun [arity:1]_ -> ())
+let f =
+  Function$
+    (fun [arity:2][|a;b|] -> fun [arity:1][|c;d|] -> ((a + b) + c) + d)
+let f = Function$ (fun [arity:1]{ a } -> a + 1)
+let f =
+  Function$
+    (fun [arity:2]{ a; b } -> fun [arity:1]{ c; d } -> ((a + b) + c) + d)
+let f = Function$ (fun [arity:1](a, b) -> a + b)
+let f =
+  Function$ (fun [arity:2](a, b) -> fun [arity:1](c, d) -> ((a + b) + c) + d)
+let f = Function$ (fun [arity:1]exception Terminate -> ())
+let f =
+  Function$
+    (fun [arity:2]exception Terminate -> fun [arity:1]exception Exit -> ())
+let f = Function$ (fun [arity:1][] -> ())
+let f = Function$ (fun [arity:1](x::xs) -> x + (xs |.u Belt.List.length))
+let f = Function$ (fun [arity:2](x : int) -> fun [arity:1](y : int) -> x + y)
+let f =
+  Function$
+    (fun [arity:2]~a:((a)[@res.namedArgLoc ]) ->
+       fun [arity:1]~b:((b)[@res.namedArgLoc ]) -> a + b)
+let f =
+  Function$
+    (fun [arity:2]~a:((x)[@res.namedArgLoc ]) ->
+       fun [arity:1]~b:((y)[@res.namedArgLoc ]) -> x + y)
+let f =
+  Function$
+    (fun [arity:2]~a:(((x : int))[@res.namedArgLoc ]) ->
+       fun [arity:1]~b:(((y : int))[@res.namedArgLoc ]) -> x + y)
+let f =
+  Function$
+    (fun [arity:3]?a:(((a)[@res.namedArgLoc ])= 1) ->
+       fun [arity:2]?b:(((b)[@res.namedArgLoc ])= 2) ->
+         fun [arity:1]c -> (a + b) + c)
+let f =
+  Function$
+    (fun [arity:3]?a:(((x)[@res.namedArgLoc ])= 1) ->
+       fun [arity:2]?b:(((y)[@res.namedArgLoc ])= 2) ->
+         fun [arity:1]c -> (x + y) + c)
+let f =
+  Function$
+    (fun [arity:3]?a:((((x : int))[@res.namedArgLoc ])= 1) ->
+       fun [arity:2]?b:((((y : int))[@res.namedArgLoc ])= 2) ->
+         fun [arity:1]c -> (x + y) + c)
+let f =
+  Function$
+    (fun [arity:3]?a:((a)[@res.namedArgLoc ]) ->
+       fun [arity:2]?b:((b)[@res.namedArgLoc ]) ->
+         fun [arity:1]c ->
+           match (a, b) with | (Some a, Some b) -> (a + b) + c | _ -> 3)
+let f =
+  Function$
+    (fun [arity:3]?a:((x)[@res.namedArgLoc ]) ->
+       fun [arity:2]?b:((y)[@res.namedArgLoc ]) ->
+         fun [arity:1]c ->
+           match (x, y) with | (Some a, Some b) -> (a + b) + c | _ -> 3)
+let f =
+  Function$
+    (fun [arity:3]?a:(((x : int option))[@res.namedArgLoc ]) ->
+       fun [arity:2]?b:(((y : int option))[@res.namedArgLoc ]) ->
+         fun [arity:1]c ->
+           match (x, y) with | (Some a, Some b) -> (a + b) + c | _ -> 3)
+let f = Function$ (fun [arity:2]a -> fun [arity:1]b -> a + b)
+let f = Function$ (fun [arity:1]() -> ())
+let f = Function$ (fun [arity:1]() -> ())
+let f = Function$ (fun [arity:3]a -> fun [arity:2]b -> fun [arity:1]c -> ())
+let f =
+  Function$
+    (fun [arity:4]a ->
+       fun [arity:3]b -> fun [arity:2]c -> fun [arity:1]d -> ())
+let f = Function$ (fun [arity:3]a -> fun [arity:2]b -> fun [arity:1]c -> ())
+let f =
+  Function$
+    (fun [arity:4]~a:((a)[@res.namedArgLoc ][@attr ]) ->
+       fun [arity:3]b ->
+         fun [arity:2]~c:((c)[@res.namedArgLoc ][@attr ]) ->
+           fun [arity:1]d -> ())
+let f =
+  Function$
+    (fun [arity:4]~a:((a)[@res.namedArgLoc ][@attr ]) ->
+       fun [arity:3]((b)[@attrOnB ]) ->
+         fun [arity:2]~c:((c)[@res.namedArgLoc ][@attr ]) ->
+           fun [arity:1]((d)[@attrOnD ]) -> ())
+let f = Function$ (fun [arity:1]list -> list ())
 ;;match colour with
   | Red when
       (l = l') ||
@@ -106,35 +105,33 @@ let f = ((Function$ (fun list -> list ()))[@res.arity 1])
       -> (t1, t2)
   | _ -> ()
 let arr =
-  [|((Function$ ((fun _ -> doThings ())))
-    [@res.arity 1]);((Function$ ((fun _ -> doThings ())))
-    [@res.arity 1]);((Function$ ((fun _ -> doThings ())))[@res.arity 1])|]
+  [|(Function$ ((fun [arity:1]_ -> doThings ())));(Function$
+                                                     ((fun [arity:1]_ ->
+                                                         doThings ())));(
+    Function$ ((fun [arity:1]_ -> doThings ())))|]
 let list =
-  [((Function$ ((fun _ -> doThings ())))
-  [@res.arity 1]);
-  ((Function$ ((fun _ -> doThings ())))
-  [@res.arity 1]);
-  ((Function$ ((fun _ -> doThings ())))
-  [@res.arity 1])]
+  [Function$ ((fun [arity:1]_ -> doThings ()));
+  Function$ ((fun [arity:1]_ -> doThings ()));
+  Function$ ((fun [arity:1]_ -> doThings ()))]
 let tuple =
-  (((Function$ (fun _ -> doThings ()))[@res.arity 1]),
-    ((Function$ (fun _ -> doThings ()))[@res.arity 1]),
-    ((Function$ (fun _ -> doThings ()))[@res.arity 1]))
-;;((Function$ (fun _ -> doThings ()))[@res.arity 1])
+  ((Function$ (fun [arity:1]_ -> doThings ())),
+    (Function$ (fun [arity:1]_ -> doThings ())),
+    (Function$ (fun [arity:1]_ -> doThings ())))
+;;Function$ (fun [arity:1]_ -> doThings ())
 let x =
   Constructore
-    (((Function$ (fun _ -> copyChecklistItemCB ()))[@res.arity 1]),
-      ((Function$ (fun _ -> copyChecklistItemCB ()))[@res.arity 1]))
+    ((Function$ (fun [arity:1]_ -> copyChecklistItemCB ())),
+      (Function$ (fun [arity:1]_ -> copyChecklistItemCB ())))
 let y =
   `Constructore
-    (((Function$ (fun _ -> copyChecklistItemCB ()))[@res.arity 1]),
-      ((Function$ (fun _ -> copyChecklistItemCB ()))[@res.arity 1]))
-let f = ((Function$ (fun list -> list + 1))[@res.arity 1])
+    ((Function$ (fun [arity:1]_ -> copyChecklistItemCB ())),
+      (Function$ (fun [arity:1]_ -> copyChecklistItemCB ())))
+let f = Function$ (fun [arity:1]list -> list + 1)
 let foo = (() : unit)
 type nonrec u = unit
 let un = (() : u)
 type nonrec ('a, 'b) d = ('a * 'b)
-let c = ((Function$ (fun () -> ((1, 2) : ('a, 'b) d)))[@res.arity 1])
-let fn = ((Function$ (fun f -> f))[@res.arity 1])
+let c = Function$ (fun [arity:1]() -> ((1, 2) : ('a, 'b) d))
+let fn = Function$ (fun [arity:1]f -> f)
 type nonrec f = (int -> unit, [ `Has_arity1 ]) function$
-let a = fn (((Function$ (fun _ -> ()))[@res.arity 1]) : f)
\ No newline at end of file
+let a = fn (Function$ (fun [arity:1]_ -> ()) : f)
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
index 5bc1666203..d76ad3ec36 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
@@ -1,19 +1,17 @@
 let greetUser =
-  ((Function$
-      ((fun userId ->
-          ((let name = ((getUserName userId)[@res.await ]) in
-            ({js|Hello |js} ^ name) ^ {js|!|js})
-          [@res.braces ]))
-      [@res.async ]))
-  [@res.arity 1])
-;;((Function$ ((fun () -> 123)[@res.async ]))[@res.arity 1])
-let fetch = ((Function$ ((fun url -> browserFetch url)[@res.async ]))
-  [@res.braces ][@res.arity 1])
+  Function$
+    ((fun [arity:1]userId ->
+        ((let name = ((getUserName userId)[@res.await ]) in
+          ({js|Hello |js} ^ name) ^ {js|!|js})
+        [@res.braces ]))
+    [@res.async ])
+;;Function$ ((fun [arity:1]() -> 123)[@res.async ])
+let fetch =
+  ((Function$ ((fun [arity:1]url -> browserFetch url)[@res.async ]))
+  [@res.braces ])
 let fetch2 =
-  ((((Function$ (((fun url -> browserFetch url))[@res.async ]))
-    [@res.arity 1]);
-    ((Function$ (((fun url -> browserFetch2 url))[@res.async ]))
-    [@res.arity 1]))
+  ((Function$ (((fun [arity:1]url -> browserFetch url))[@res.async ]);
+    Function$ (((fun [arity:1]url -> browserFetch2 url))[@res.async ]))
   [@res.braces ])
 let async =
   ((let f = async () in
@@ -22,19 +20,21 @@ let async =
     async.async;
     { async = (async.(async)) };
     (result |.u async) |.u
-      (mapAsync ((Function$ (fun a -> doStuff a))[@res.arity 1])))
+      (mapAsync (Function$ (fun [arity:1]a -> doStuff a))))
   [@res.braces ])
 let f =
   ((if isPositive
-    then ((Function$ ((fun a -> fun b -> (a + b : int))[@res.async ]))
-      [@res.arity 2])
-    else ((Function$ (((fun c -> fun d -> (c - d : int)))[@res.async ]))
-      [@res.arity 2]))
+    then
+      Function$ ((fun [arity:2]a -> fun [arity:1]b -> (a + b : int))
+        [@res.async ])
+    else
+      Function$ (((fun [arity:2]c -> fun [arity:1]d -> (c - d : int)))
+        [@res.async ]))
   [@res.ternary ])
 let foo = async ~a:((34)[@res.namedArgLoc ])
 let bar =
-  ((Function$ ((fun ~a:((a)[@res.namedArgLoc ]) -> a + 1)[@res.async ]))
-  [@res.arity 1])
+  Function$ ((fun [arity:1]~a:((a)[@res.namedArgLoc ]) -> a + 1)
+    [@res.async ])
 let ex1 = ((3)[@res.await ]) + ((4)[@res.await ])
 let ex2 = ((3)[@res.await ]) ** ((4)[@res.await ])
 let ex3 = ((foo |.u (bar ~arg:((arg)[@res.namedArgLoc ])))[@res.await ])
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/await.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/await.res.txt
index 3ccb4c9d5e..12c2c1810f 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/await.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/await.res.txt
@@ -19,21 +19,19 @@ let () = ((((delay 10)[@res.await ]); ((delay 20)[@res.await ]))
 let forEach = ((Js.Import Belt.List.forEach)[@res.await ][@a ][@b ])
 module M = ((Belt.List)[@res.await ][@a ][@b ])
 let f =
-  ((Function$
-      (fun () ->
-         ((let module M = ((Belt.List)[@res.await ][@a ][@b ]) in M.forEach)
-         [@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]() ->
+       ((let module M = ((Belt.List)[@res.await ][@a ][@b ]) in M.forEach)
+       [@res.braces ]))
 let () = ((let module M = ((Belt.List)[@res.await ][@a ][@b ]) in M.forEach)
   [@res.braces ])
 module type BeltList  = module type of Belt.List
 let f =
-  ((Function$
-      (fun () ->
-         ((let module M = (((Belt.List : BeltList))[@res.await ][@a ]
-             [@b ]) in M.forEach)
-         [@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]() ->
+       ((let module M = (((Belt.List : BeltList))[@res.await ][@a ][@b ]) in
+           M.forEach)
+       [@res.braces ]))
 let () =
   ((let module M = (((Belt.List : BeltList))[@res.await ][@a ][@b ]) in
       M.forEach)
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/binary.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/binary.res.txt
index 7d4bc6327e..7c2881e167 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/binary.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/binary.res.txt
@@ -21,5 +21,4 @@ let x = a -. b
 let _ = ((Constructor (a, b); `Constructor (a, b))[@res.braces ])
 ;;(library.getBalance account) |.u
     (Promise.Js.catch
-       ((Function$ (fun _ -> ((Promise.resolved None)[@res.braces ])))
-       [@res.arity 1]))
\ No newline at end of file
+       (Function$ (fun [arity:1]_ -> ((Promise.resolved None)[@res.braces ]))))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/binaryNoEs6Arrow.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/binaryNoEs6Arrow.res.txt
index 77cf1dcb5f..9e4fb25ad9 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/binaryNoEs6Arrow.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/binaryNoEs6Arrow.res.txt
@@ -23,7 +23,7 @@
     do () done
 ;;((div
       ~onClick:((Function$
-                   (fun event ->
+                   (fun [arity:1]event ->
                       ((match videoContainerRect with
                         | Some videoContainerRect ->
                             let newChapter =
@@ -31,7 +31,6 @@
                               Video.chapter) in
                             { a; b } |.u onChange
                         | _ -> ())
-                      [@res.braces ])))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 1]) ~children:[] ())
-  [@JSX ])
+                      [@res.braces ])))[@res.namedArgLoc ][@res.braces ])
+      ~children:[] ())[@JSX ])
 ;;if inclusions.(index) <- (uid, url) then onChange inclusions
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/block.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/block.res.txt
index 30daf26a58..556b20feda 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/block.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/block.res.txt
@@ -1,12 +1,11 @@
 let b =
   ((let module Array = Belt.Array in
-      ([|1;2|] |.u (Array.map ((Function$ (fun x -> x + 1))[@res.arity 1])))
-        |.u Js.log)
+      ([|1;2|] |.u (Array.map (Function$ (fun [arity:1]x -> x + 1)))) |.u
+        Js.log)
   [@res.braces ])
 let b =
   ((let open Belt.Array in
-      ([|1;2|] |.u (map ((Function$ (fun x -> x + 1))[@res.arity 1]))) |.u
-        Js.log)
+      ([|1;2|] |.u (map (Function$ (fun [arity:1]x -> x + 1)))) |.u Js.log)
   [@res.braces ])
 let b = ((let exception QuitEarly  in raise QuitEarly)[@res.braces ])
 let b = ((let a = 1 in let b = 2 in a + b)[@res.braces ])
@@ -22,7 +21,7 @@ let b =
           let a = 1 in
           let b = 2 in
           sideEffect ();
-          (let x = (1 + 2) |.u ((Function$ (fun x -> x + 1))[@res.arity 1]) in
+          (let x = (1 + 2) |.u (Function$ (fun [arity:1]x -> x + 1)) in
            raise (Terminate x)))
   [@res.braces ])
 let b = ((f (); g (); h (); (let arr = [|1;2;3|] in ()))[@res.braces ])
@@ -43,52 +42,47 @@ let res =
 let nestedLet = ((let _ = 1 in ())[@res.braces ])
 let nestedLet = ((let _ = 1 in 2)[@res.braces ])
 let init =
-  ((Function$ (fun () -> ((foo (1 == 1); [%assert 1 == 2])[@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]() -> ((foo (1 == 1); [%assert 1 == 2])[@res.braces ]))
 let init =
-  ((Function$
-      (fun () -> (([%assert 1 == 2]; foo (1 == 1); [%assert 1 == 2])
-         [@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]() -> (([%assert 1 == 2]; foo (1 == 1); [%assert 1 == 2])
+       [@res.braces ]))
 let f =
-  ((Function$
-      (fun () -> ((let x = 1 in ((Function$ (fun _ -> ()))[@res.arity 1]))
-         [@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]() -> ((let x = 1 in Function$ (fun [arity:1]_ -> ()))
+       [@res.braces ]))
 let reifyStyle (type a) =
-  ((Function$
-      (fun (x : 'a) ->
-         (((let module Internal =
-              struct
-                type constructor
-                external canvasGradient : constructor = "CanvasGradient"
-                [@@val ]
-                external canvasPattern : constructor = "CanvasPattern"
-                [@@val ]
-                let instanceOf =
-                  ([%raw
-                     (({js|function(x,y) {return +(x instanceof y)}|js})
-                       [@res.template ])] : ('a -> constructor -> bool,
-                                              [ `Has_arity2 ]) function$)
-              end in
-              ((if (Js.typeof x) = {js|string|js}
-                then Obj.magic String
+  Function$
+    (fun [arity:1](x : 'a) ->
+       (((let module Internal =
+            struct
+              type constructor
+              external canvasGradient : constructor = "CanvasGradient"
+              [@@val ]
+              external canvasPattern : constructor = "CanvasPattern"[@@val ]
+              let instanceOf =
+                ([%raw
+                   (({js|function(x,y) {return +(x instanceof y)}|js})
+                     [@res.template ])] : ('a -> constructor -> bool,
+                                            [ `Has_arity2 ]) function$)
+            end in
+            ((if (Js.typeof x) = {js|string|js}
+              then Obj.magic String
+              else
+                if Internal.instanceOf x Internal.canvasGradient
+                then Obj.magic Gradient
                 else
-                  if Internal.instanceOf x Internal.canvasGradient
-                  then Obj.magic Gradient
+                  if Internal.instanceOf x Internal.canvasPattern
+                  then Obj.magic Pattern
                   else
-                    if Internal.instanceOf x Internal.canvasPattern
-                    then Obj.magic Pattern
-                    else
-                      raise
-                        (Invalid_argument
-                           {js|Unknown canvas style kind. Known values are: String, CanvasGradient, CanvasPattern|js})),
-                (Obj.magic x)))
-         [@res.braces ]) : (a style * a))))
-  [@res.arity 1])
+                    raise
+                      (Invalid_argument
+                         {js|Unknown canvas style kind. Known values are: String, CanvasGradient, CanvasPattern|js})),
+              (Obj.magic x)))
+       [@res.braces ]) : (a style * a)))
 let calc_fps =
-  ((Function$
-      (fun t0 ->
-         fun t1 -> ((let delta = (t1 -. t0) /. 1000. in 1. /. delta)
-           [@res.braces ])))
-  [@res.arity 2])
\ No newline at end of file
+  Function$
+    (fun [arity:2]t0 ->
+       fun [arity:1]t1 -> ((let delta = (t1 -. t0) /. 1000. in 1. /. delta)
+         [@res.braces ]))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/bracedOrRecord.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/bracedOrRecord.res.txt
index 1503dc741c..11388ed021 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/bracedOrRecord.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/bracedOrRecord.res.txt
@@ -3,7 +3,7 @@ let r = { a }
 let r = { a = expr }
 let r = { a = expr }
 let r = { a = expr; b = expr2 }
-let r = { f = ((Function$ (fun x -> x + b))[@res.arity 1]) }
+let r = { f = (Function$ (fun [arity:1]x -> x + b)) }
 let e = ((a)[@res.braces ])
 let e = ((a)[@res.braces ])
 let e = ((a; b ())[@res.braces ])
@@ -16,20 +16,18 @@ let e = ((a.(0))[@res.braces ])
 let e = ((f b)[@res.braces ])
 let e = (((a.b).c)[@res.braces ])
 let e = ((arr.(x) <- 20)[@res.braces ])
-let e = ((Function$ (fun x -> doStuff config (x + 1)))
-  [@res.braces ][@res.arity 1])
-let e = ((doStuff config ((Function$ (fun x -> x + 1))[@res.arity 1]))
+let e = ((Function$ (fun [arity:1]x -> doStuff config (x + 1)))
   [@res.braces ])
-let e =
-  ((if ((Function$ (fun x -> x + 1))[@res.arity 1]) then true else false)
+let e = ((doStuff config (Function$ (fun [arity:1]x -> x + 1)))
+  [@res.braces ])
+let e = ((if Function$ (fun [arity:1]x -> x + 1) then true else false)
   [@res.braces ][@res.ternary ])
 let e =
-  ((sideEffect ((Function$ (fun x -> x + 1))[@res.arity 1]);
-    logToAnalytics Shady.ml)
+  ((sideEffect (Function$ (fun [arity:1]x -> x + 1)); logToAnalytics Shady.ml)
+  [@res.braces ])
+let f = ((Function$ (fun [arity:1]event -> (event.target).value))
+  [@res.braces ])
+let f = ((Function$ (fun [arity:1]event -> ((event.target).value : string)))
   [@res.braces ])
-let f = ((Function$ (fun event -> (event.target).value))
-  [@res.braces ][@res.arity 1])
-let f = ((Function$ (fun event -> ((event.target).value : string)))
-  [@res.braces ][@res.arity 1])
 let x = ((let a = 1 in let b = 2 in a + b)[@res.braces ])
 ;;(([(({js|\n|js} |.u React.string)[@res.braces ])])[@JSX ])
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/coerce.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/coerce.res.txt
index 91dac0bf41..493f8badeb 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/coerce.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/coerce.res.txt
@@ -1,5 +1,5 @@
-let foo = ((Function$ (fun (x : int) -> (x :> int)))[@res.arity 1])
-let foo = ((Function$ (fun x -> ((x : t) :> int)))[@res.arity 1])
+let foo = Function$ (fun [arity:1](x : int) -> (x :> int))
+let foo = Function$ (fun [arity:1]x -> ((x : t) :> int))
 let _ = (x : int)
 let foo = ((x : int), (y :> float))
 let foo = ((x : int), (y :> float), (z :> int))
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/extension.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/extension.res.txt
index 5b19a89e43..137deda8e9 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/extension.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/extension.res.txt
@@ -1,5 +1,5 @@
 ;;[%expr ]
 ;;[%expr.extension ]
 ;;[%expr.extension.with.args {js|argument|js}]
-;;[%expr.extension.with.args ((Function$ (fun x -> f x))[@res.arity 1])]
+;;[%expr.extension.with.args Function$ (fun [arity:1]x -> f x)]
 let x = ([%raw {js|1|js}]) + ([%raw {js|2|js}])
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/firstClassModule.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/firstClassModule.res.txt
index e1f798dc53..f9f3af0c9c 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/firstClassModule.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/firstClassModule.res.txt
@@ -1,12 +1,11 @@
 let makeSet (type s) =
-  ((Function$
-      (fun cmp ->
-         ((let module S =
-             (Set.Make)(struct type nonrec t = s
-                               let compare = cmp end) in ((module
-             S) : (module Set.S with type elt = s)))
-         [@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]cmp ->
+       ((let module S =
+           (Set.Make)(struct type nonrec t = s
+                             let compare = cmp end) in ((module
+           S) : (module Set.S with type elt = s)))
+       [@res.braces ]))
 let three = ((module Three) : (module X_int))
 let numbers = [|three;(module Four)|]
 let numbers = (three, (module Four))
@@ -15,55 +14,51 @@ let numbers = [|three;(module struct let x = 4 end)|]
 let numbers = (three, (module struct let x = 4 end))
 let numbers = [three; (module struct let x = 4 end)]
 let plus =
-  ((Function$
-      (fun m1 ->
-         fun m2 -> ((((module
-           struct let x = (to_int m1) + (to_int m2) end) : (module X_int)))
-           [@res.braces ])))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]m1 ->
+       fun [arity:1]m2 -> ((((module
+         struct let x = (to_int m1) + (to_int m2) end) : (module X_int)))
+         [@res.braces ]))
 let plus =
-  ((Function$
-      (fun m1 ->
-         fun m2 -> ((module
-           struct let x = (to_int m1) + (to_int m2) end) : (module X_int))))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]m1 ->
+       fun [arity:1]m2 -> ((module
+         struct let x = (to_int m1) + (to_int m2) end) : (module X_int)))
 let unique_instance = ((module
   struct module Query_handler = Unique
          let this = Unique.create 0 end) : (module Query_handler_instance))
 let build_instance (type a) =
-  ((Function$
-      (fun ((module Q)  : (module Query_handler with type config = a)) ->
-         fun config -> ((module
-           struct module Query_handler = Q
-                  let this = Q.create config end) : (module
-                                                      Query_handler_instance))))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]((module Q)  : (module Query_handler with type config = a))
+       ->
+       fun [arity:1]config -> ((module
+         struct module Query_handler = Q
+                let this = Q.create config end) : (module
+                                                    Query_handler_instance)))
 let build_instance (type a) =
-  ((Function$
-      (fun ((module Q)  : (module Query_handler with type config = a)) ->
-         fun config -> ((((module
-           struct module Query_handler = Q
-                  let this = Q.create config end) : (module
-                                                      Query_handler_instance)))
-           [@res.braces ])))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]((module Q)  : (module Query_handler with type config = a))
+       ->
+       fun [arity:1]config -> ((((module
+         struct module Query_handler = Q
+                let this = Q.create config end) : (module
+                                                    Query_handler_instance)))
+         [@res.braces ]))
 let unique_instance = build_instance (module Unique) 0
 let build_dispatch_table =
-  ((Function$
-      (fun handlers ->
-         ((let table = Hashtbl.create (module String) in
-           List.iter handlers
-             ~f:((Function$
-                    (fun
-                       (((module I)  : (module Query_handler_instance)) as
-                          instance)
-                       ->
-                       Hashtbl.set table ~key:((I.Query_handler.name)
-                         [@res.namedArgLoc ]) ~data:((instance)
-                         [@res.namedArgLoc ])))
-             [@res.namedArgLoc ][@res.arity 1]) table)
-         [@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]handlers ->
+       ((let table = Hashtbl.create (module String) in
+         List.iter handlers
+           ~f:((Function$
+                  (fun
+                     [arity:1](((module I)  :
+                                 (module Query_handler_instance)) as instance)
+                     ->
+                     Hashtbl.set table ~key:((I.Query_handler.name)
+                       [@res.namedArgLoc ]) ~data:((instance)
+                       [@res.namedArgLoc ])))[@res.namedArgLoc ]) table)
+       [@res.braces ]))
 ;;(module Three)
 ;;((module Three) : (module X_int))
 ;;(module Teenager).(0)
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/jsx.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/jsx.res.txt
index b929c94c55..cd3f51ae10 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/jsx.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/jsx.res.txt
@@ -10,13 +10,13 @@ let _ = ((div ~className:(({js|menu|js})[@res.namedArgLoc ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div ~className:(({js|menu|js})[@res.namedArgLoc ])
-      ~onClick:((Function$ (fun _ -> Js.log {js|click|js}))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 1]) ~children:[] ())
+      ~onClick:((Function$ (fun [arity:1]_ -> Js.log {js|click|js}))
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div ~className:(({js|menu|js})[@res.namedArgLoc ])
-      ~onClick:((Function$ (fun _ -> Js.log {js|click|js}))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 1]) ~children:[] ())
+      ~onClick:((Function$ (fun [arity:1]_ -> Js.log {js|click|js}))
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ = ((Navbar.createElement ~children:[] ())[@JSX ])
 let _ = ((Navbar.createElement ~children:[] ())[@JSX ])
@@ -82,8 +82,7 @@ let _ =
                 [@JSX ])] ())
   [@JSX ])
 let _ = ((div ~children:child ())[@JSX ])
-let _ =
-  ((Foo.createElement ~children:((Function$ (fun a -> 1))[@res.arity 1]) ())
+let _ = ((Foo.createElement ~children:(Function$ (fun [arity:1]a -> 1)) ())
   [@JSX ])
 let _ =
   ((Foo.createElement ~children:((Foo2.createElement ~children:[] ())
@@ -151,30 +150,30 @@ let y =
   ((Routes.createElement ~path:((Routes.stateToPath state)
       [@res.namedArgLoc ]) ~isHistorical:((true)[@res.namedArgLoc ])
       ~onHashChange:((Function$
-                        (fun _oldPath ->
-                           fun _oldUrl ->
-                             fun newUrl ->
+                        (fun [arity:3]_oldPath ->
+                           fun [arity:2]_oldUrl ->
+                             fun [arity:1]newUrl ->
                                updater
-                                 ((Function$
-                                     (fun latestComponentBag ->
-                                        fun _ ->
-                                          ((let currentActualPath =
-                                              Routes.hashOfUri newUrl in
-                                            let pathFromState =
-                                              Routes.stateToPath
-                                                latestComponentBag.state in
-                                            ((if
-                                                currentActualPath =
-                                                  pathFromState
-                                              then None
-                                              else
-                                                dispatchEventless
-                                                  (State.UriNavigated
-                                                     currentActualPath)
-                                                  latestComponentBag ())
-                                              [@res.ternary ]))
-                                          [@res.braces ])))[@res.arity 2]) ()))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 3]) ~children:[] ())
+                                 (Function$
+                                    (fun [arity:2]latestComponentBag ->
+                                       fun [arity:1]_ ->
+                                         ((let currentActualPath =
+                                             Routes.hashOfUri newUrl in
+                                           let pathFromState =
+                                             Routes.stateToPath
+                                               latestComponentBag.state in
+                                           ((if
+                                               currentActualPath =
+                                                 pathFromState
+                                             then None
+                                             else
+                                               dispatchEventless
+                                                 (State.UriNavigated
+                                                    currentActualPath)
+                                                 latestComponentBag ())
+                                             [@res.ternary ]))
+                                         [@res.braces ]))) ()))
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let z =
   ((div
@@ -325,94 +324,101 @@ let _ =
       ~children:[child] ())
   [@JSX ])
 let _ =
-  ((div ~onClick:((Function$ (fun event -> handleChange event))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 1]) ~children:[] ())
+  ((div ~onClick:((Function$ (fun [arity:1]event -> handleChange event))
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div
       ~onClick:((Function$
-                   (fun eventWithLongIdent -> handleChange eventWithLongIdent))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 1]) ~children:[] ())
+                   (fun [arity:1]eventWithLongIdent ->
+                      handleChange eventWithLongIdent))
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div
       ~onClick:((Function$
-                   (fun event -> ((Js.log event; handleChange event)
-                      [@res.braces ])))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 1]) ~children:[] ())
+                   (fun [arity:1]event -> ((Js.log event; handleChange event)
+                      [@res.braces ])))[@res.namedArgLoc ][@res.braces ])
+      ~children:[] ())
   [@JSX ])
 let _ =
   ((StaticDiv.createElement
       ~onClick:((Function$
-                   (fun foo ->
-                      fun bar ->
-                        fun baz ->
-                          fun lineBreak ->
-                            fun identifier ->
+                   (fun [arity:5]foo ->
+                      fun [arity:4]bar ->
+                        fun [arity:3]baz ->
+                          fun [arity:2]lineBreak ->
+                            fun [arity:1]identifier ->
                               ((doStuff foo bar baz; bar lineBreak identifier)
                               [@res.braces ])))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 5]) ~children:[] ())
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((AttrDiv.createElement
-      ~onClick:((Function$ (fun event -> handleChange event))
-      [@res.namedArgLoc ][@res.braces ][@bar ][@res.arity 1]) ~children:[] ())
+      ~onClick:((Function$ (fun [arity:1]event -> handleChange event))
+      [@res.namedArgLoc ][@res.braces ][@bar ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((AttrDiv.createElement
       ~onClick:((Function$
-                   (fun eventLongIdentifier ->
+                   (fun [arity:1]eventLongIdentifier ->
                       handleChange eventLongIdentifier))
-      [@res.namedArgLoc ][@res.braces ][@bar ][@res.arity 1]) ~children:[] ())
+      [@res.namedArgLoc ][@res.braces ][@bar ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((StaticDivNamed.createElement
       ~onClick:((Function$
-                   (fun ~foo:((foo)[@res.namedArgLoc ]) ->
-                      fun ~bar:((bar)[@res.namedArgLoc ]) ->
-                        fun ~baz:((baz)[@res.namedArgLoc ]) ->
-                          fun ~lineBreak:((lineBreak)[@res.namedArgLoc ]) ->
-                            fun ~identifier:((identifier)[@res.namedArgLoc ])
-                              -> fun () -> bar lineBreak identifier))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 6]) ~children:[] ())
+                   (fun [arity:6]~foo:((foo)[@res.namedArgLoc ]) ->
+                      fun [arity:5]~bar:((bar)[@res.namedArgLoc ]) ->
+                        fun [arity:4]~baz:((baz)[@res.namedArgLoc ]) ->
+                          fun
+                            [arity:3]~lineBreak:((lineBreak)[@res.namedArgLoc
+                                                              ])
+                            ->
+                            fun
+                              [arity:2]~identifier:((identifier)[@res.namedArgLoc
+                                                                  ])
+                              -> fun [arity:1]() -> bar lineBreak identifier))
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div
       ~onClick:((Function$
-                   (fun e -> (((doStuff (); bar foo)[@res.braces ]) : event)))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 1]) ~children:[] ())
+                   (fun [arity:1]e -> (((doStuff (); bar foo)
+                      [@res.braces ]) : event)))
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div
       ~onClick:((Function$
-                   (fun e ->
-                      fun e2 -> (((doStuff (); bar foo)
+                   (fun [arity:2]e ->
+                      fun [arity:1]e2 -> (((doStuff (); bar foo)
                         [@res.braces ]) : event)))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 2]) ~children:[] ())
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div
       ~onClick:((Function$
-                   (fun foo ->
-                      fun bar ->
-                        fun baz ->
-                          fun superLongIdent ->
-                            fun breakLine -> (((doStuff (); bar foo)
+                   (fun [arity:5]foo ->
+                      fun [arity:4]bar ->
+                        fun [arity:3]baz ->
+                          fun [arity:2]superLongIdent ->
+                            fun [arity:1]breakLine -> (((doStuff (); bar foo)
                               [@res.braces ]) : (event * event2 * event3 *
                                                   event4 * event5))))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 5]) ~children:[] ())
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div
       ~onClick:((Function$
-                   (fun foo ->
-                      fun bar ->
-                        fun baz ->
-                          fun superLongIdent ->
-                            fun breakLine ->
+                   (fun [arity:5]foo ->
+                      fun [arity:4]bar ->
+                        fun [arity:3]baz ->
+                          fun [arity:2]superLongIdent ->
+                            fun [arity:1]breakLine ->
                               (doStuff () : (event * event2 * event3 * event4
                                               * event5))))
-      [@res.namedArgLoc ][@res.braces ][@res.arity 5]) ~children:[] ())
+      [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
   ((div
@@ -443,7 +449,7 @@ let _ =
   ((Animated.createElement ~initialValue:((0.0)[@res.namedArgLoc ])
       ~value:((value)[@res.namedArgLoc ])
       ~children:((Function$
-                    (fun value ->
+                    (fun [arity:1]value ->
                        ((div
                            ~style:((ReactDOMRe.Style.make
                                       ~width:(({js|20px|js})
@@ -455,13 +461,13 @@ let _ =
                                       ~backgroundColor:(({js|red|js})
                                       [@res.namedArgLoc ]))
                            [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
-                       [@JSX ])))[@res.braces ][@res.arity 1]) ())
+                       [@JSX ])))[@res.braces ]) ())
   [@JSX ])
 let _ =
   ((Animated.createElement ~initialValue:((0.0)[@res.namedArgLoc ])
       ~value:((value)[@res.namedArgLoc ])
       ~children:((Function$
-                    (fun value ->
+                    (fun [arity:1]value ->
                        (((div
                             ~style:((ReactDOMRe.Style.make
                                        ~width:(({js|20px|js})
@@ -474,14 +480,13 @@ let _ =
                                        [@res.namedArgLoc ]))
                             [@res.namedArgLoc ][@res.braces ]) ~children:[]
                             ())
-                       [@JSX ]) : ReasonReact.element)))
-      [@res.braces ][@res.arity 1]) ())
+                       [@JSX ]) : ReasonReact.element)))[@res.braces ]) ())
   [@JSX ])
 let _ =
   ((Animated.createElement ~initialValue:((0.0)[@res.namedArgLoc ])
       ~value:((value)[@res.namedArgLoc ])
       ~children:((Function$
-                    (fun value ->
+                    (fun [arity:1]value ->
                        ((div
                            ~style:((ReactDOMRe.Style.make
                                       ~width:(({js|20px|js})
@@ -493,14 +498,13 @@ let _ =
                                       ~backgroundColor:(({js|red|js})
                                       [@res.namedArgLoc ]))
                            [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
-                       [@res.braces ][@JSX ])))
-      [@res.braces ][@foo ][@res.arity 1]) ())
+                       [@res.braces ][@JSX ])))[@res.braces ][@foo ]) ())
   [@JSX ])
 let _ =
   ((Animated.createElement ~initialValue:((0.0)[@res.namedArgLoc ])
       ~value:((value)[@res.namedArgLoc ])
       ~children:((Function$
-                    (fun value ->
+                    (fun [arity:1]value ->
                        ((let width = {js|20px|js} in
                          let height = {js|20px|js} in
                          ((div
@@ -514,10 +518,10 @@ let _ =
                              [@res.namedArgLoc ][@res.braces ]) ~children:[]
                              ())
                            [@JSX ]))
-                       [@res.braces ])))[@res.braces ][@res.arity 1]) ())
+                       [@res.braces ])))[@res.braces ]) ())
   [@JSX ])
 let _ =
-  ((div ~callback:((reduce ((Function$ (fun () -> not state))[@res.arity 1]))
+  ((div ~callback:((reduce (Function$ (fun [arity:1]() -> not state)))
       [@res.namedArgLoc ][@res.braces ]) ~children:[] ())
   [@JSX ])
 let _ =
@@ -578,44 +582,47 @@ let _ =
       ~children:[((ReasonReact.array
                      (Array.of_list
                         (List.map
-                           ((Function$
-                               (fun possibleGradeValue ->
-                                  ((option
-                                      ~key:((string_of_int possibleGradeValue)
-                                      [@res.namedArgLoc ][@res.braces ])
-                                      ~value:((string_of_int
-                                                 possibleGradeValue)
-                                      [@res.namedArgLoc ][@res.braces ])
-                                      ~children:[((str
-                                                     (string_of_int
-                                                        possibleGradeValue))
-                                                [@res.braces ])] ())
-                                  [@JSX ])))[@res.arity 1])
+                           (Function$
+                              (fun [arity:1]possibleGradeValue ->
+                                 ((option
+                                     ~key:((string_of_int possibleGradeValue)
+                                     [@res.namedArgLoc ][@res.braces ])
+                                     ~value:((string_of_int
+                                                possibleGradeValue)
+                                     [@res.namedArgLoc ][@res.braces ])
+                                     ~children:[((str
+                                                    (string_of_int
+                                                       possibleGradeValue))
+                                               [@res.braces ])] ())
+                                 [@JSX ])))
                            (List.filter
-                              ((Function$ (fun g -> g <= state.maxGrade))
-                              [@res.arity 1]) possibleGradeValues))))
+                              (Function$
+                                 (fun [arity:1]g -> g <= state.maxGrade))
+                              possibleGradeValues))))
                 [@res.braces ])] ())[@JSX ])
 ;;((div ~children:[((Js.log (a <= 10))[@res.braces ])] ())[@JSX ])
 ;;((div
       ~children:[((div ~children:[((Js.log (a <= 10))[@res.braces ])] ())
                 [@JSX ])] ())[@JSX ])
 ;;((div
-      ~children:[((div ~onClick:((Function$ (fun _ -> Js.log (a <= 10)))
-                     [@res.namedArgLoc ][@res.braces ][@res.arity 1])
+      ~children:[((div
+                     ~onClick:((Function$
+                                  (fun [arity:1]_ -> Js.log (a <= 10)))
+                     [@res.namedArgLoc ][@res.braces ])
                      ~children:[((div
                                     ~children:[((Js.log (a <= 10))
                                               [@res.braces ])] ())
                                [@JSX ])] ())
                 [@JSX ])] ())[@JSX ])
 ;;((div ~children:element ())[@JSX ])
-;;((div ~children:((Function$ (fun a -> 1))[@res.braces ][@res.arity 1]) ())
+;;((div ~children:((Function$ (fun [arity:1]a -> 1))[@res.braces ]) ())
   [@JSX ])
 ;;((div ~children:((span ~children:[] ())[@JSX ]) ())[@JSX ])
 ;;((div ~children:[|a|] ())[@JSX ])
 ;;((div ~children:(1, 2) ())[@JSX ])
 ;;((div ~children:((array |.u f)[@res.braces ]) ())[@JSX ])
 ;;(([element])[@JSX ])
-;;(([((Function$ ((fun a -> 1)))[@res.braces ][@res.arity 1])])[@JSX ])
+;;(([((Function$ ((fun [arity:1]a -> 1)))[@res.braces ])])[@JSX ])
 ;;(([((span ~children:[] ())[@JSX ])])[@JSX ])
 ;;(([[|a|]])[@JSX ])
 ;;(([(1, 2)])[@JSX ])
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/locallyAbstractTypes.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/locallyAbstractTypes.res.txt
index 95bad3fb60..2e3d1e4b3b 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/locallyAbstractTypes.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/locallyAbstractTypes.res.txt
@@ -1,29 +1,31 @@
-let f (type t) = ((Function$ (fun (xs : t list) -> ()))[@res.arity 1])
+let f (type t) = Function$ (fun [arity:1](xs : t list) -> ())
 let f (type t) =
-  ((Function$ (fun (xs : t list) -> fun (type s) -> fun (ys : s list) -> ()))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2](xs : t list) -> fun (type s) ->
+       fun [arity:1](ys : s list) -> ())
 let f (type t) (type u) (type v) =
-  ((Function$ (fun (xs : (t * u * v) list) -> ()))[@res.arity 1])
+  Function$ (fun [arity:1](xs : (t * u * v) list) -> ())
 let f (type t) (type u) (type v) =
-  ((Function$
-      (fun (xs : (t * u * v) list) -> fun (type s) -> fun (type w) -> fun
-         (type z) -> fun (ys : (s * w * z) list) -> ()))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2](xs : (t * u * v) list) -> fun (type s) -> fun (type w) ->
+       fun (type z) -> fun [arity:1](ys : (s * w * z) list) -> ())
 let f = ((fun (type t) -> fun (type u) -> fun (type v) ->
-  ((Function$
-      (fun (xs : (t * u * v) list) -> ((fun (type s) -> fun (type w) -> fun
-         (type z) -> fun (ys : (s * w * z) list) -> ())[@attr2 ])))
-  [@res.arity 2]))[@attr ])
+  Function$
+    (fun [arity:2](xs : (t * u * v) list) -> ((fun (type s) -> fun (type w)
+       -> fun (type z) -> fun [arity:1](ys : (s * w * z) list) -> ())
+       [@attr2 ])))
+  [@attr ])
 let f = ((fun (type t) -> ((fun (type s) ->
-  ((Function$
-      (fun (xs : (t * s) list) -> ((fun (type u) -> ((fun (type v) -> fun
-         (type w) -> fun (ys : (u * v * w) list) -> ())[@attr ]))[@attr ])))
-  [@res.arity 2]))[@attr ]))[@attr ])
+  Function$
+    (fun [arity:2](xs : (t * s) list) -> ((fun (type u) -> ((fun (type v) ->
+       fun (type w) -> fun [arity:1](ys : (u * v * w) list) -> ())[@attr ]))
+       [@attr ])))
+  [@attr ]))[@attr ])
 let cancel_and_collect_callbacks :
   'a 'u 'c .
     (packed_callbacks list -> ('a, 'u, 'c) promise -> packed_callbacks list,
       [ `Has_arity2 ]) function$
   = fun (type x) ->
-  ((Function$
-      (fun callbacks_accumulator -> fun (p : (_, _, c) promise) -> ()))
-  [@res.arity 2])
\ No newline at end of file
+  Function$
+    (fun [arity:2]callbacks_accumulator ->
+       fun [arity:1](p : (_, _, c) promise) -> ())
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/record.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/record.res.txt
index 082bb3a3f9..002b2e837f 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/record.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/record.res.txt
@@ -13,10 +13,10 @@ let r = { expr with pexp_attributes = [||] }
 let r = { (make () : myRecord) with foo = bar }
 let r = { (make () : myRecord) with foo = bar }
 let r = { x = ?None; y = ?None; z = ?(None : tt) }
-let z = ((Function$ (fun name -> { name?; x = 3 }))[@res.arity 1])
-let z = ((Function$ (fun name -> { name?; x = 3 }))[@res.arity 1])
-let z = ((Function$ (fun name -> { name; x? }))[@res.arity 1])
-let zz = ((Function$ (fun name -> { name; x? }))[@res.arity 1])
+let z = Function$ (fun [arity:1]name -> { name?; x = 3 })
+let z = Function$ (fun [arity:1]name -> { name?; x = 3 })
+let z = Function$ (fun [arity:1]name -> { name; x? })
+let zz = Function$ (fun [arity:1]name -> { name; x? })
 let _ =
   match z with
   | { x? = None; y? = None; z? = (None : tt) } -> 11
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/tupleVsDivision.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/tupleVsDivision.res.txt
index 24f94f3599..46850dc5c8 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/tupleVsDivision.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/tupleVsDivision.res.txt
@@ -2,7 +2,6 @@
 ;;foo ()
 ;;(1, 2) |.u printTuple
 let f =
-  ((Function$
-      (fun () -> (((foo ()) / 1; foo (); (1, 2) |.u printTuple)
-         [@res.braces ])))
-  [@res.arity 1])
\ No newline at end of file
+  Function$
+    (fun [arity:1]() -> (((foo ()) / 1; foo (); (1, 2) |.u printTuple)
+       [@res.braces ]))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/uncurried.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/uncurried.res.txt
index db8486599f..1b6cb630d1 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/uncurried.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/uncurried.res.txt
@@ -1,30 +1,30 @@
-let f = ((Function$ (fun a -> fun b -> a + b))[@res.arity 2])
-let f = ((Function$ (fun a -> fun b -> a + b))[@res.arity 2])
-let f = ((Function$ (fun a -> fun b -> fun c -> fun d -> ((a + b) + c) + d))
-  [@res.arity 4])
+let f = Function$ (fun [arity:2]a -> fun [arity:1]b -> a + b)
+let f = Function$ (fun [arity:2]a -> fun [arity:1]b -> a + b)
 let f =
-  ((Function$
-      ((fun a ->
-          ((Function$
-              ((fun b ->
-                  ((Function$
-                      ((fun c -> ((Function$ ((fun d -> ())[@attr4 ]))
-                          [@res.arity 1]))
-                      [@attr3 ]))
-                  [@res.arity 1]))
-              [@attr2 ]))
-          [@res.arity 1]))
-      [@attr ]))
-  [@res.arity 1])
+  Function$
+    (fun [arity:4]a ->
+       fun [arity:3]b ->
+         fun [arity:2]c -> fun [arity:1]d -> ((a + b) + c) + d)
 let f =
-  ((Function$
-      (fun ((a)[@attr ]) ->
-         fun ((b)[@attr2 ]) -> fun ((c)[@attr3 ]) -> fun ((d)[@attr4 ]) -> ()))
-  [@res.arity 4])
+  Function$
+    ((fun [arity:1]a ->
+        Function$
+          ((fun [arity:1]b ->
+              Function$
+                ((fun [arity:1]c ->
+                    Function$ ((fun [arity:1]d -> ())[@attr4 ]))
+                [@attr3 ]))
+          [@attr2 ]))
+    [@attr ])
 let f =
-  ((Function$
-      (fun ((a)[@attr ]) ->
-         fun ((b)[@attr2 ]) -> fun ((c)[@attr3 ]) -> fun ((d)[@attr4 ]) -> ()))
-  [@res.arity 4])
+  Function$
+    (fun [arity:4]((a)[@attr ]) ->
+       fun [arity:3]((b)[@attr2 ]) ->
+         fun [arity:2]((c)[@attr3 ]) -> fun [arity:1]((d)[@attr4 ]) -> ())
+let f =
+  Function$
+    (fun [arity:4]((a)[@attr ]) ->
+       fun [arity:3]((b)[@attr2 ]) ->
+         fun [arity:2]((c)[@attr3 ]) -> fun [arity:1]((d)[@attr4 ]) -> ())
 ;;add 1 2
 ;;add 2 3 4 5 6 7 8 9 10
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/underscoreApply.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/underscoreApply.res.txt
index f26f2f3d08..86a8b0da28 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/underscoreApply.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/underscoreApply.res.txt
@@ -1,45 +1,39 @@
 let l =
   ([1; 2; 3] |.u
-     ((Function$
-         (fun __x ->
-            List.map ((Function$ (fun i -> i + 1))[@res.arity 1]) __x))
-     [@res.arity 1]))
+     (Function$
+        (fun [arity:1]__x ->
+           List.map (Function$ (fun [arity:1]i -> i + 1)) __x)))
     |.u
-    ((Function$
-        (fun __x ->
-           List.filter ((Function$ (fun i -> i > 0))[@res.arity 1]) __x))
-    [@res.arity 1])
+    (Function$
+       (fun [arity:1]__x ->
+          List.filter (Function$ (fun [arity:1]i -> i > 0)) __x))
 let l =
-  ((Function$ (fun i -> i + 1))[@res.arity 1]) |.u
-    ((Function$ (fun __x -> List.map __x [1; 2; 3]))[@res.arity 1])
-let x = ((Function$ (fun __x -> List.length __x))[@res.arity 1])
+  (Function$ (fun [arity:1]i -> i + 1)) |.u
+    (Function$ (fun [arity:1]__x -> List.map __x [1; 2; 3]))
+let x = Function$ (fun [arity:1]__x -> List.length __x)
 let nested =
-  ((Function$
-      (fun x -> ((Function$ (fun __x -> List.length __x))[@res.arity 1])))
-  [@res.arity 1])
-let incr = ((Function$ (fun ~v:((v)[@res.namedArgLoc ]) -> v + 1))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]x -> Function$ (fun [arity:1]__x -> List.length __x))
+let incr = Function$ (fun [arity:1]~v:((v)[@res.namedArgLoc ]) -> v + 1)
 let l1 =
   List.length
-    (List.map ((Function$ (fun __x -> incr ~v:__x))[@res.arity 1]) [1; 2; 3])
+    (List.map (Function$ (fun [arity:1]__x -> incr ~v:__x)) [1; 2; 3])
 let l2 =
   List.length
-    (List.map ((Function$ (fun __x -> incr ~v:__x))[@res.arity 1]) [1; 2; 3])
+    (List.map (Function$ (fun [arity:1]__x -> incr ~v:__x)) [1; 2; 3])
 let optParam =
-  ((Function$
-      (fun ?v:((v)[@res.namedArgLoc ]) ->
-         fun () -> ((if v = None then 0 else 1)[@res.ternary ])))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]?v:((v)[@res.namedArgLoc ]) ->
+       fun [arity:1]() -> ((if v = None then 0 else 1)[@res.ternary ]))
 let l1 =
   List.length
-    (List.map ((Function$ (fun __x -> optParam ?v:__x ()))[@res.arity 1])
+    (List.map (Function$ (fun [arity:1]__x -> optParam ?v:__x ()))
        [Some 1; None; Some 2])
 let l2 =
   List.length
-    (List.map ((Function$ (fun __x -> optParam ?v:__x ()))[@res.arity 1])
+    (List.map (Function$ (fun [arity:1]__x -> optParam ?v:__x ()))
        [Some 1; None; Some 2])
-;;((Function$
-      (fun __x ->
-         underscoreWithComments
-           ((Function$ (fun x -> ((something ())[@res.braces ])))
-           [@res.arity 1]) __x))[@res.arity 1])
\ No newline at end of file
+;;Function$
+    (fun [arity:1]__x ->
+       underscoreWithComments
+         (Function$ (fun [arity:1]x -> ((something ())[@res.braces ]))) __x)
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/modexpr/expected/apply.res.txt b/tests/syntax_tests/data/parsing/grammar/modexpr/expected/apply.res.txt
index 0de90cceb6..22f9a13cc2 100644
--- a/tests/syntax_tests/data/parsing/grammar/modexpr/expected/apply.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/modexpr/expected/apply.res.txt
@@ -12,7 +12,6 @@ module X = (F)((A : SetLike))
 module X = ((F)((A : SetLike)))((B : TreeLike))
 module X = ((F)((A : SetLike)))((B : TreeLike))
 let someFunctorAsFunction =
-  ((Function$
-      (fun (x : (module MT)) -> ((module (SomeFunctor)((val
-         x))) : (module ResT))))
-  [@res.arity 1])
\ No newline at end of file
+  Function$
+    (fun [arity:1](x : (module MT)) -> ((module (SomeFunctor)((val
+       x))) : (module ResT)))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/modexpr/expected/firstClassModules.res.txt b/tests/syntax_tests/data/parsing/grammar/modexpr/expected/firstClassModules.res.txt
index 668b92e718..1c09739d12 100644
--- a/tests/syntax_tests/data/parsing/grammar/modexpr/expected/firstClassModules.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/modexpr/expected/firstClassModules.res.txt
@@ -3,17 +3,15 @@ module Device = (val
      try Hashtbl.find devices deviceName with | Not_found -> exit 2)
   [@res.braces ]) : (module Device)))
 let draw_using_device =
-  ((Function$
-      (fun device_name ->
-         fun picture ->
-           ((let module Device = (val
-               (Hashtbl.find devices device_name : (module DEVICE))) in
-               Device.draw picture)
-           [@res.braces ])))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]device_name ->
+       fun [arity:1]picture ->
+         ((let module Device = (val
+             (Hashtbl.find devices device_name : (module DEVICE))) in
+             Device.draw picture)
+         [@res.braces ]))
 module New_three = (val (three : (module X_int)))
 let to_int =
-  ((Function$
-      (fun m -> ((let module M = (val (m : (module X_int))) in M.x)
-         [@res.braces ])))
-  [@res.arity 1])
\ No newline at end of file
+  Function$
+    (fun [arity:1]m -> ((let module M = (val (m : (module X_int))) in M.x)
+       [@res.braces ]))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/modexpr/expected/functor.res.txt b/tests/syntax_tests/data/parsing/grammar/modexpr/expected/functor.res.txt
index ef100a32bf..7f898aba98 100644
--- a/tests/syntax_tests/data/parsing/grammar/modexpr/expected/functor.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/modexpr/expected/functor.res.txt
@@ -30,27 +30,26 @@ module Make(Cmp:sig
     type nonrec coll = key list
     let empty = []
     let add =
-      ((Function$
-          (fun (y : coll) ->
-             fun (e : key) ->
-               if List.exists ((Function$ (fun x -> eq x e))[@res.arity 1]) y
-               then y
-               else e :: y))
-      [@res.arity 2])
+      Function$
+        (fun [arity:2](y : coll) ->
+           fun [arity:1](e : key) ->
+             if List.exists (Function$ (fun [arity:1]x -> eq x e)) y
+             then y
+             else e :: y)
   end 
 module Gen1(P:Primitive)() =
   struct
     type nonrec t = P.t
     type nonrec internal = P.t
-    let inject = ((Function$ (fun t -> t))[@res.arity 1])
+    let inject = Function$ (fun [arity:1]t -> t)
   end
 module DistinctString() : StringBased =
   struct
     type nonrec t = string
-    let inject = ((Function$ (fun t -> t))[@res.arity 1])
+    let inject = Function$ (fun [arity:1]t -> t)
   end 
 module DistinctString() : StringBased =
   struct
     type nonrec t = string
-    let inject = ((Function$ (fun t -> t))[@res.arity 1])
+    let inject = Function$ (fun [arity:1]t -> t)
   end 
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/any.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/any.res.txt
index a0d068e67d..3123064d47 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/any.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/any.res.txt
@@ -17,13 +17,13 @@ let _ as _y|_ as _x = 1
   | _ as _x|_ as _x -> ()
   | (_ : unit) -> ()
   | (_ : unit)|(_ : unit) -> ()
-let f = ((Function$ (fun _ -> ()))[@res.arity 1])
-let f = ((Function$ (fun (_ as _x) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (_ : unit) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (_ : unit) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ((_ : unit) as _x) -> ()))[@res.arity 1])
-let g = ((Function$ (fun a -> fun _ -> ()))[@res.arity 2])
-let g = ((Function$ (fun _ -> fun a -> ()))[@res.arity 2])
+let f = Function$ (fun [arity:1]_ -> ())
+let f = Function$ (fun [arity:1](_ as _x) -> ())
+let f = Function$ (fun [arity:1](_ : unit) -> ())
+let f = Function$ (fun [arity:1](_ : unit) -> ())
+let f = Function$ (fun [arity:1]((_ : unit) as _x) -> ())
+let g = Function$ (fun [arity:2]a -> fun [arity:1]_ -> ())
+let g = Function$ (fun [arity:2]_ -> fun [arity:1]a -> ())
 ;;for _ = 0 to 10 do () done
 ;;for _ as _x = 0 to 10 do () done
 ;;for _ = 0 to 10 do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/array.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/array.res.txt
index 94ad52cb04..32b3c803d0 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/array.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/array.res.txt
@@ -9,11 +9,11 @@ let ([|(1 : int);(2 : int)|] : int array) = ()
   | [|1;2|] -> ()
   | [|(1 : int);(2 : int)|] -> ()
   | ([|(1 : int);(2 : int)|] : int) -> ()
-let f = ((Function$ (fun [||] -> ()))[@res.arity 1])
-let f = ((Function$ (fun [|x|] -> ()))[@res.arity 1])
-let f = ((Function$ (fun [|x;y|] -> x + y))[@res.arity 1])
-let f = ((Function$ (fun ([|x|] : int) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ([|x|] : int) -> ()))[@res.arity 1])
+let f = Function$ (fun [arity:1][||] -> ())
+let f = Function$ (fun [arity:1][|x|] -> ())
+let f = Function$ (fun [arity:1][|x;y|] -> x + y)
+let f = Function$ (fun [arity:1]([|x|] : int) -> ())
+let f = Function$ (fun [arity:1]([|x|] : int) -> ())
 ;;for [||] = 0 to 10 do () done
 ;;for [||] = 0 to 10 do () done
 ;;for [||] = 0 to 10 do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/constant.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/constant.res.txt
index add745ee57..713dc033fc 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/constant.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/constant.res.txt
@@ -15,15 +15,13 @@ let ({js|stringPattern|js} : string) as s = ()
 ;;for {js|stringPattern|js} = 0 to 10 do () done
 ;;for {js|stringPattern|js} as s = 0 to 10 do () done
 ;;for {js|stringPattern|js} as s = 0 to 10 do () done
-let f = ((Function$ (fun {js|stringPattern|js} -> ()))[@res.arity 1])
-let f = ((Function$ (fun ({js|stringPattern|js} as s) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ({js|stringPattern|js} as s) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ({js|stringPattern|js} : string) -> ()))
-  [@res.arity 1])
-let f = ((Function$ (fun (({js|stringPattern|js} : string) as s) -> ()))
-  [@res.arity 1])
-let f = ((Function$ (fun ({js|stringPattern|js} : string) -> ()))
-  [@res.arity 1])
+let f = Function$ (fun [arity:1]{js|stringPattern|js} -> ())
+let f = Function$ (fun [arity:1]({js|stringPattern|js} as s) -> ())
+let f = Function$ (fun [arity:1]({js|stringPattern|js} as s) -> ())
+let f = Function$ (fun [arity:1]({js|stringPattern|js} : string) -> ())
+let f =
+  Function$ (fun [arity:1](({js|stringPattern|js} : string) as s) -> ())
+let f = Function$ (fun [arity:1]({js|stringPattern|js} : string) -> ())
 let 1 = ()
 let 1 as x = ()
 let (1 : int) = ()
@@ -33,11 +31,11 @@ let (1 : int) as x = ()
   | 1 as x -> ()
   | (1 : int) -> ()
   | (1 : int) as x -> ()
-let f = ((Function$ (fun 1 -> ()))[@res.arity 1])
-let f = ((Function$ (fun (1 as x) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (1 : int) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ((1 : int) as x) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (1 : int) -> ()))[@res.arity 1])
+let f = Function$ (fun [arity:1]1 -> ())
+let f = Function$ (fun [arity:1](1 as x) -> ())
+let f = Function$ (fun [arity:1](1 : int) -> ())
+let f = Function$ (fun [arity:1]((1 : int) as x) -> ())
+let f = Function$ (fun [arity:1](1 : int) -> ())
 ;;for i = 0 to 10 do () done
 ;;for i as x = 0 to 10 do () done
 ;;for i = 0 to 10 do () done
@@ -52,16 +50,15 @@ let f = ((Function$ (fun (1 : int) -> ()))[@res.arity 1])
       makeListPattern loc patterns None
 let _0 = 0x9A
 let print =
-  ((Function$
-      (fun ppf ->
-         fun i ->
-           match i.stamp with
-           | 0 -> fprintf ppf {js|%s!|js} i.name
-           | (-1) -> fprintf ppf {js|%s#|js} i.name
-           | 1 -> fprintf ppf {js|%s#|js} i.name
-           | (-1.) -> fprintf ppf {js|%s#|js} i.name
-           | 1. -> fprintf ppf {js|%s#|js} i.name))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]ppf ->
+       fun [arity:1]i ->
+         match i.stamp with
+         | 0 -> fprintf ppf {js|%s!|js} i.name
+         | (-1) -> fprintf ppf {js|%s#|js} i.name
+         | 1 -> fprintf ppf {js|%s#|js} i.name
+         | (-1.) -> fprintf ppf {js|%s#|js} i.name
+         | 1. -> fprintf ppf {js|%s#|js} i.name)
 let (-1)..(-1.) = x
 ;;match science with
   | (1.12, (-3.13)) -> true
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/constructor.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/constructor.res.txt
index 591a6bfb5c..cf5a42dd94 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/constructor.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/constructor.res.txt
@@ -34,25 +34,24 @@ let ((Instance (component : comp)) : React.t) = i
   | Instance (comp, tree) -> ()
   | React.Instance (comp, tree) -> ()
   | (Instance (comp : Component.t) : React.t) -> ()
-let f = ((Function$ (fun (Instance) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance as i) -> i))[@res.arity 1])
-let f = ((Function$ (fun (React.Instance) -> i))[@res.arity 1])
-let f = ((Function$ (fun (React.Instance as x) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance component) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance component) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance { render; subtree }) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance ({ render; subtree }, inst)) -> i))
-  [@res.arity 1])
-let f = ((Function$ (fun (Instance ({ render; subtree } : Instance.t)) -> i))
-  [@res.arity 1])
-let f = ((Function$ (fun (Instance ({ render; subtree } : Instance.t)) -> i))
-  [@res.arity 1])
-let f = ((Function$ (fun (Instance (component, tree)) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance (component, tree)) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance : React.t) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance : React.t) -> i))[@res.arity 1])
-let f = ((Function$ (fun (Instance (comp : Component.t) : React.t) -> ()))
-  [@res.arity 1])
+let f = Function$ (fun [arity:1](Instance) -> i)
+let f = Function$ (fun [arity:1](Instance as i) -> i)
+let f = Function$ (fun [arity:1](React.Instance) -> i)
+let f = Function$ (fun [arity:1](React.Instance as x) -> i)
+let f = Function$ (fun [arity:1](Instance component) -> i)
+let f = Function$ (fun [arity:1](Instance component) -> i)
+let f = Function$ (fun [arity:1](Instance { render; subtree }) -> i)
+let f = Function$ (fun [arity:1](Instance ({ render; subtree }, inst)) -> i)
+let f =
+  Function$ (fun [arity:1](Instance ({ render; subtree } : Instance.t)) -> i)
+let f =
+  Function$ (fun [arity:1](Instance ({ render; subtree } : Instance.t)) -> i)
+let f = Function$ (fun [arity:1](Instance (component, tree)) -> i)
+let f = Function$ (fun [arity:1](Instance (component, tree)) -> i)
+let f = Function$ (fun [arity:1](Instance : React.t) -> i)
+let f = Function$ (fun [arity:1](Instance : React.t) -> i)
+let f =
+  Function$ (fun [arity:1](Instance (comp : Component.t) : React.t) -> ())
 ;;for Blue = x to y do () done
 ;;for Blue as c = x to y do () done
 ;;for Blue = x to y do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/dict.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/dict.res.txt
index 0da786cd5b..3061adaee6 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/dict.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/dict.res.txt
@@ -1,13 +1,11 @@
 let someDict = Primitive_dict.make [|("one", {js|one|js})|]
 let (({ one?;_})[@res.dictPattern ]) = someDict
 let foo =
-  ((Function$
-      (fun () ->
-         match someDict with
-         | (({ one = {js|one|js};_})[@res.dictPattern ]) ->
-             Js.log {js|one|js}
-         | _ -> Js.log {js|not one|js}))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]() ->
+       match someDict with
+       | (({ one = {js|one|js};_})[@res.dictPattern ]) -> Js.log {js|one|js}
+       | _ -> Js.log {js|not one|js})
 type json =
   | Boolean of bool 
   | Null [@as null]
@@ -19,23 +17,22 @@ type nonrec user = {
   name: string ;
   age?: float }
 let decodeUser =
-  ((Function$
-      (fun (json : json) ->
-         (((match json with
-            | Object
-                (({ name = String name; age = ageJson;_})[@res.dictPattern ])
-                ->
-                Some
-                  {
-                    name;
-                    age =
-                      ?((match ageJson with
-                         | Number age -> Some age
-                         | _ -> None))
-                  }
-            | _ -> (Js.log {js|Not an object.|js}; None))
-         [@res.braces ]) : user option)))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](json : json) ->
+       (((match json with
+          | Object
+              (({ name = String name; age = ageJson;_})[@res.dictPattern ])
+              ->
+              Some
+                {
+                  name;
+                  age =
+                    ?((match ageJson with
+                       | Number age -> Some age
+                       | _ -> None))
+                }
+          | _ -> (Js.log {js|Not an object.|js}; None))
+       [@res.braces ]) : user option))
 ;;Js.log
     (decodeUser
        (jsonParse (({js|{"name": "John", "age": 30}|js})[@res.template ])))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/exception.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/exception.res.txt
index 725cb04362..e3d9e96b47 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/exception.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/exception.res.txt
@@ -25,14 +25,14 @@ let exception ((Foo : t exc) as e) = ()
   | exception (Foo as e) -> ()
   | exception Foo (a, b) -> ()
   | (exception Foo : t exc) -> ()
-let f = ((Function$ (fun exception Foo -> ()))[@res.arity 1])
-let f = ((Function$ (fun (exception Foo as e) -> ()))[@res.arity 1])
-let f = ((Function$ (fun exception (Foo as e) -> ()))[@res.arity 1])
-let f = ((Function$ (fun exception Foo (a, b) -> ()))[@res.arity 1])
-let f = ((Function$ (fun exception Foo -> ()))[@res.arity 1])
-let f = ((Function$ (fun (exception Foo as e) -> ()))[@res.arity 1])
-let f = ((Function$ (fun exception Foo (a, b) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (exception Foo : t exc) -> ()))[@res.arity 1])
+let f = Function$ (fun [arity:1]exception Foo -> ())
+let f = Function$ (fun [arity:1](exception Foo as e) -> ())
+let f = Function$ (fun [arity:1]exception (Foo as e) -> ())
+let f = Function$ (fun [arity:1]exception Foo (a, b) -> ())
+let f = Function$ (fun [arity:1]exception Foo -> ())
+let f = Function$ (fun [arity:1](exception Foo as e) -> ())
+let f = Function$ (fun [arity:1]exception Foo (a, b) -> ())
+let f = Function$ (fun [arity:1](exception Foo : t exc) -> ())
 ;;for exception Foo = 0 to 10 do () done
 ;;for exception Foo as e = 0 to 10 do () done
 ;;for exception Foo = 0 to 10 do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/extension.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/extension.res.txt
index 107533c8ed..9c5b5db261 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/extension.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/extension.res.txt
@@ -11,14 +11,15 @@ let [%patExt1 ]|[%patExt2 ] = ()
   | [%pat.stuff test] as _x -> ()
   | ([%pat.stuff test] : unit) -> ()
   | [%patExt1 ]|[%patExt2 ] -> ()
-let f = ((Function$ (fun [%patternExtension ] -> ()))[@res.arity 1])
-let f = ((Function$ (fun [%pattern.extension ] -> ()))[@res.arity 1])
-let f = ((Function$ (fun [%raw {js|x|js}] -> ()))[@res.arity 1])
-let f = ((Function$ (fun [%raw {js|x|js}] -> fun [%raw {js|y|js}] -> ()))
-  [@res.arity 2])
-let f = ((Function$ (fun ([%raw {js|x|js}] as _y) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ([%raw {js|x|js}] : unit) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ([%patExt1 ]|[%patExt2 ]) -> ()))[@res.arity 1])
+let f = Function$ (fun [arity:1][%patternExtension ] -> ())
+let f = Function$ (fun [arity:1][%pattern.extension ] -> ())
+let f = Function$ (fun [arity:1][%raw {js|x|js}] -> ())
+let f =
+  Function$
+    (fun [arity:2][%raw {js|x|js}] -> fun [arity:1][%raw {js|y|js}] -> ())
+let f = Function$ (fun [arity:1]([%raw {js|x|js}] as _y) -> ())
+let f = Function$ (fun [arity:1]([%raw {js|x|js}] : unit) -> ())
+let f = Function$ (fun [arity:1]([%patExt1 ]|[%patExt2 ]) -> ())
 ;;for [%ext ] = x to y do () done
 ;;for [%ext1 ]|[%ext2 ] = x to y do () done
 ;;for [%ext ] = x to y do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/firstClassModules.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/firstClassModules.res.txt
index bff40ddbbe..875aa2d5a5 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/firstClassModules.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/firstClassModules.res.txt
@@ -1,22 +1,22 @@
-let sort (type s) = ((Function$ (fun (module Set)  -> fun l -> ()))
-  [@res.arity 2])
 let sort (type s) =
-  ((Function$
-      (fun ((module Set)  : (module Set.S with type elt = s)) -> fun l -> ()))
-  [@res.arity 2])
+  Function$ (fun [arity:2](module Set)  -> fun [arity:1]l -> ())
 let sort (type s) =
-  ((Function$
-      (fun
-         ((module Set)  : (module Set.S with type elt = s and type elt2 = t))
-         -> fun l -> ()))
-  [@res.arity 2])
-let foo = ((Function$ (fun (module Foo)  -> fun baz -> Foo.bar baz))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]((module Set)  : (module Set.S with type elt = s)) ->
+       fun [arity:1]l -> ())
+let sort (type s) =
+  Function$
+    (fun
+       [arity:2]((module Set)  :
+                  (module Set.S with type elt = s and type elt2 = t))
+       -> fun [arity:1]l -> ())
+let foo =
+  Function$ (fun [arity:2](module Foo)  -> fun [arity:1]baz -> Foo.bar baz)
 let bump_list (type a) =
-  ((Function$
-      (fun ((module B)  : (module Bumpable with type t = a)) ->
-         fun (l : a list) -> List.map ~f:((B.bump l)[@res.namedArgLoc ])))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]((module B)  : (module Bumpable with type t = a)) ->
+       fun [arity:1](l : a list) ->
+         List.map ~f:((B.bump l)[@res.namedArgLoc ]))
 ;;match x with
   | (module Set)  -> ()
   | ((module Set)  : (module Set.S with type elt = s)) -> ()
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/list.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/list.res.txt
index 83fe31d17f..67522b4a55 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/list.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/list.res.txt
@@ -33,24 +33,24 @@ let ((x::xs : int list) : int list) = ()
   | x::(y::ys)::xs -> ()
   | (x as p1)::((y as p2)::(ys as tail1))::(xs as tail2) as l -> ()
   | (x::xs : int list) -> ()
-let f = ((Function$ (fun [] -> ()))[@res.arity 1])
-let f = ((Function$ (fun ([] as p) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::[]) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (((x : int) as p)::[]) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ((x as p)::[] as p2) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::xs) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::(xs as tail)) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::y::tail) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::y::[]) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::y::[]) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::xs) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::y::tail) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x::(y::ys)::xs) -> ()))[@res.arity 1])
+let f = Function$ (fun [arity:1][] -> ())
+let f = Function$ (fun [arity:1]([] as p) -> ())
+let f = Function$ (fun [arity:1](x::[]) -> ())
+let f = Function$ (fun [arity:1](((x : int) as p)::[]) -> ())
+let f = Function$ (fun [arity:1]((x as p)::[] as p2) -> ())
+let f = Function$ (fun [arity:1](x::xs) -> ())
+let f = Function$ (fun [arity:1](x::(xs as tail)) -> ())
+let f = Function$ (fun [arity:1](x::y::tail) -> ())
+let f = Function$ (fun [arity:1](x::y::[]) -> ())
+let f = Function$ (fun [arity:1](x::y::[]) -> ())
+let f = Function$ (fun [arity:1](x::xs) -> ())
+let f = Function$ (fun [arity:1](x::y::tail) -> ())
+let f = Function$ (fun [arity:1](x::(y::ys)::xs) -> ())
 let f =
-  ((Function$
-      (fun ((x as p1)::((y as p2)::(ys as tail1))::(xs as tail2) as l) -> ()))
-  [@res.arity 1])
-let f = ((Function$ (fun (x::xs : int list) -> ()))[@res.arity 1])
+  Function$
+    (fun [arity:1]((x as p1)::((y as p2)::(ys as tail1))::(xs as tail2) as l)
+       -> ())
+let f = Function$ (fun [arity:1](x::xs : int list) -> ())
 ;;for [] = x to y do () done
 ;;for [] as l = x to y do () done
 ;;for [] = x to y do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/polyvariants.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/polyvariants.res.txt
index 717f79bcca..8b18583c4b 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/polyvariants.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/polyvariants.res.txt
@@ -31,26 +31,24 @@ let ((`Instance (component : comp)) : React.t) = i
   | `Instance ({ render; subtree } : Instance.t) -> ()
   | `Instance (comp, tree) -> ()
   | (`Instance (comp : Component.t) : React.t) -> ()
-let f = ((Function$ (fun `Instance -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance as i) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance component) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance component) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance { render; subtree }) -> i))
-  [@res.arity 1])
-let f = ((Function$ (fun (`Instance ({ render; subtree }, inst)) -> i))
-  [@res.arity 1])
+let f = Function$ (fun [arity:1]`Instance -> i)
+let f = Function$ (fun [arity:1](`Instance as i) -> i)
+let f = Function$ (fun [arity:1](`Instance component) -> i)
+let f = Function$ (fun [arity:1](`Instance component) -> i)
+let f = Function$ (fun [arity:1](`Instance { render; subtree }) -> i)
+let f = Function$ (fun [arity:1](`Instance ({ render; subtree }, inst)) -> i)
 let f =
-  ((Function$ (fun (`Instance ({ render; subtree } : Instance.t)) -> i))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](`Instance ({ render; subtree } : Instance.t)) -> i)
 let f =
-  ((Function$ (fun (`Instance ({ render; subtree } : Instance.t)) -> i))
-  [@res.arity 1])
-let f = ((Function$ (fun (`Instance (component, tree)) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance (component, tree)) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance : React.t) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance : React.t) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance (comp : Component.t) : React.t) -> ()))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](`Instance ({ render; subtree } : Instance.t)) -> i)
+let f = Function$ (fun [arity:1](`Instance (component, tree)) -> i)
+let f = Function$ (fun [arity:1](`Instance (component, tree)) -> i)
+let f = Function$ (fun [arity:1](`Instance : React.t) -> i)
+let f = Function$ (fun [arity:1](`Instance : React.t) -> i)
+let f =
+  Function$ (fun [arity:1](`Instance (comp : Component.t) : React.t) -> ())
 ;;for `Blue = x to y do () done
 ;;for `Blue as c = x to y do () done
 ;;for `Blue = x to y do () done
@@ -70,22 +68,21 @@ let f = ((Function$ (fun (`Instance (comp : Component.t) : React.t) -> ()))
 ;;for `Point { x; y; z } as p = x to y do () done
 ;;match x with | #typeVar -> () | `lowercase -> ()
 let cmp =
-  ((Function$
-      (fun selectedChoice ->
-         fun value ->
-           match (selectedChoice, value) with
-           | (#a, #a) -> true
-           | [|#b;#b|] -> true
-           | #b::#b::[] -> true
-           | { x = #c; y = #c } -> true
-           | Constructor (#a, #a) -> true
-           | `Constuctor (#a, #a) -> true
-           | #a as x -> true
-           | #a|#b -> true
-           | (#a : typ) -> true
-           | exception #a -> true
-           | _ -> false))
-  [@res.arity 2])
+  Function$
+    (fun [arity:2]selectedChoice ->
+       fun [arity:1]value ->
+         match (selectedChoice, value) with
+         | (#a, #a) -> true
+         | [|#b;#b|] -> true
+         | #b::#b::[] -> true
+         | { x = #c; y = #c } -> true
+         | Constructor (#a, #a) -> true
+         | `Constuctor (#a, #a) -> true
+         | #a as x -> true
+         | #a|#b -> true
+         | (#a : typ) -> true
+         | exception #a -> true
+         | _ -> false)
 ;;match polyVar with
   | `ease-in -> ()
   | `ease-out⛰ -> ()
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/record.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/record.res.txt
index 85b7386cb7..2cb8f470d8 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/record.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/record.res.txt
@@ -32,27 +32,25 @@ let ({ a } : myRecord) = x
   | { a;_} -> ()
   | { a;_} -> ()
   | ({ a } : myRecord) -> ()
-let f = ((Function$ (fun { a } -> ()))[@res.arity 1])
-let f = ((Function$ (fun ({ a } as r) -> ()))[@res.arity 1])
-let f = ((Function$ (fun { a } -> ()))[@res.arity 1])
-let f = ((Function$ (fun { a; b } -> ()))[@res.arity 1])
-let f = ((Function$ (fun { a; b } -> ()))[@res.arity 1])
-let f = ((Function$ (fun { ReasonReact.state = state } -> ()))[@res.arity 1])
-let f = ((Function$ (fun { ReasonReact.state = (state as prevState) } -> ()))
-  [@res.arity 1])
-let f = ((Function$ (fun { ReasonReact.state = theState } -> ()))
-  [@res.arity 1])
-let f = ((Function$ (fun { a = u } -> ()))[@res.arity 1])
-let f = ((Function$ (fun { a = (u : int) } -> ()))[@res.arity 1])
-let f = ((Function$ (fun { a = (((u as x) : int) as r) } -> ()))
-  [@res.arity 1])
-let f = ((Function$ (fun { a = { x; y } } -> ()))[@res.arity 1])
-let f = ((Function$ (fun { a = { x = r; y = r2 } } -> ()))[@res.arity 1])
-let f = ((Function$ (fun { a;_} -> ()))[@res.arity 1])
-let f = ((Function$ (fun { a;_} -> ()))[@res.arity 1])
-let f = ((Function$ (fun ({ a } : myRecord) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ({ a } : myRecord) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (({ a } : myRecord) as p) -> ()))[@res.arity 1])
+let f = Function$ (fun [arity:1]{ a } -> ())
+let f = Function$ (fun [arity:1]({ a } as r) -> ())
+let f = Function$ (fun [arity:1]{ a } -> ())
+let f = Function$ (fun [arity:1]{ a; b } -> ())
+let f = Function$ (fun [arity:1]{ a; b } -> ())
+let f = Function$ (fun [arity:1]{ ReasonReact.state = state } -> ())
+let f =
+  Function$ (fun [arity:1]{ ReasonReact.state = (state as prevState) } -> ())
+let f = Function$ (fun [arity:1]{ ReasonReact.state = theState } -> ())
+let f = Function$ (fun [arity:1]{ a = u } -> ())
+let f = Function$ (fun [arity:1]{ a = (u : int) } -> ())
+let f = Function$ (fun [arity:1]{ a = (((u as x) : int) as r) } -> ())
+let f = Function$ (fun [arity:1]{ a = { x; y } } -> ())
+let f = Function$ (fun [arity:1]{ a = { x = r; y = r2 } } -> ())
+let f = Function$ (fun [arity:1]{ a;_} -> ())
+let f = Function$ (fun [arity:1]{ a;_} -> ())
+let f = Function$ (fun [arity:1]({ a } : myRecord) -> ())
+let f = Function$ (fun [arity:1]({ a } : myRecord) -> ())
+let f = Function$ (fun [arity:1](({ a } : myRecord) as p) -> ())
 ;;for { a } = 0 to 10 do () done
 ;;for { a } as p = 0 to 10 do () done
 ;;for { a } = 0 to 10 do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/tuple.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/tuple.res.txt
index 076cb61d98..04a2d31947 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/tuple.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/tuple.res.txt
@@ -15,15 +15,14 @@ let (((1 : int), (2 : int)) : (int * int)) as tup = ()
   | (((1 as p1) : int), ((2 as p2) : int)) as tup -> ()
   | (((1 : int), (2 : int)) : (int * int)) -> ()
   | (((1 : int), (2 : int)) : (int * int)) as tup -> ()
-let f = ((Function$ (fun x -> ()))[@res.arity 1])
-let f = ((Function$ (fun x -> ()))[@res.arity 1])
-let f = ((Function$ (fun (x, y) -> x + y))[@res.arity 1])
-let f = ((Function$ (fun (((x as p1), (y as p2)) as tup) -> x + y))
-  [@res.arity 1])
-let f = ((Function$ (fun ((x, y) : (int * int)) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ((x, y) : (int * int)) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ((((x, y) as tup1) : (int * int)) as tup) -> ()))
-  [@res.arity 1])
+let f = Function$ (fun [arity:1]x -> ())
+let f = Function$ (fun [arity:1]x -> ())
+let f = Function$ (fun [arity:1](x, y) -> x + y)
+let f = Function$ (fun [arity:1](((x as p1), (y as p2)) as tup) -> x + y)
+let f = Function$ (fun [arity:1]((x, y) : (int * int)) -> ())
+let f = Function$ (fun [arity:1]((x, y) : (int * int)) -> ())
+let f =
+  Function$ (fun [arity:1]((((x, y) as tup1) : (int * int)) as tup) -> ())
 ;;for (x, y) = 0 to 10 do () done
 ;;for (x, y) as tup = 0 to 10 do () done
 ;;for (x, y) = 0 to 10 do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/unit.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/unit.res.txt
index 2ecd0e444a..f1d3f48961 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/unit.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/unit.res.txt
@@ -26,13 +26,13 @@ let (() : unit) as x = ()
 ;;for (() : unit) as _u = () to () do () done
 ;;for ((() as _u) : unit) = () to () do () done
 ;;for (() : unit) as _u = () to () do () done
-let f = ((Function$ (fun () -> ()))[@res.arity 1])
-let f = ((Function$ (fun () -> ()))[@res.arity 1])
-let f = ((Function$ (fun (() as _u) -> ()))[@res.arity 1])
-let f = ((Function$ (fun () -> fun () -> ()))[@res.arity 2])
-let f = ((Function$ (fun (() as _u) -> fun (() as _u) -> ()))[@res.arity 2])
-let f = ((Function$ (fun (() : unit) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ((() as _u) : unit) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ((() : unit) as _u) -> ()))[@res.arity 1])
-let f = ((Function$ (fun (() : unit) -> ()))[@res.arity 1])
-let f = ((Function$ (fun ((() : unit) as _u) -> ()))[@res.arity 1])
\ No newline at end of file
+let f = Function$ (fun [arity:1]() -> ())
+let f = Function$ (fun [arity:1]() -> ())
+let f = Function$ (fun [arity:1](() as _u) -> ())
+let f = Function$ (fun [arity:2]() -> fun [arity:1]() -> ())
+let f = Function$ (fun [arity:2](() as _u) -> fun [arity:1](() as _u) -> ())
+let f = Function$ (fun [arity:1](() : unit) -> ())
+let f = Function$ (fun [arity:1]((() as _u) : unit) -> ())
+let f = Function$ (fun [arity:1]((() : unit) as _u) -> ())
+let f = Function$ (fun [arity:1](() : unit) -> ())
+let f = Function$ (fun [arity:1]((() : unit) as _u) -> ())
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/variantSpreads.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/variantSpreads.res.txt
index 70323d3ecf..39f0b14340 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/variantSpreads.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/variantSpreads.res.txt
@@ -13,47 +13,42 @@ type nonrec d =
   | ... of b 
   | ... of c 
 let doWithA =
-  ((Function$
-      (fun (a : a) ->
-         ((match a with
-           | One -> Js.log {js|aaa|js}
-           | Two -> Js.log {js|twwwoooo|js}
-           | Three -> Js.log {js|threeeee|js})
-         [@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](a : a) ->
+       ((match a with
+         | One -> Js.log {js|aaa|js}
+         | Two -> Js.log {js|twwwoooo|js}
+         | Three -> Js.log {js|threeeee|js})
+       [@res.braces ]))
 let doWithB =
-  ((Function$
-      (fun (b : b) ->
-         ((match b with
-           | One -> Js.log {js|aaa|js}
-           | _ -> Js.log {js|twwwoooo|js})
-         [@res.braces ])))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](b : b) ->
+       ((match b with
+         | One -> Js.log {js|aaa|js}
+         | _ -> Js.log {js|twwwoooo|js})
+       [@res.braces ]))
 let lookup =
-  ((Function$
-      (fun (b : b) ->
-         match b with
-         | ((#a)[@res.patVariantSpread ]) as a -> doWithA a
-         | Four -> Js.log {js|four|js}
-         | Five -> Js.log {js|five|js}))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](b : b) ->
+       match b with
+       | ((#a)[@res.patVariantSpread ]) as a -> doWithA a
+       | Four -> Js.log {js|four|js}
+       | Five -> Js.log {js|five|js})
 let lookup2 =
-  ((Function$
-      (fun (d : d) ->
-         match d with
-         | ((#a)[@res.patVariantSpread ]) as a -> doWithA a
-         | ((#b)[@res.patVariantSpread ]) as b -> doWithB b
-         | Six|Seven -> Js.log {js|Got rest of d|js}))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](d : d) ->
+       match d with
+       | ((#a)[@res.patVariantSpread ]) as a -> doWithA a
+       | ((#b)[@res.patVariantSpread ]) as b -> doWithB b
+       | Six|Seven -> Js.log {js|Got rest of d|js})
 let lookupOpt =
-  ((Function$
-      (fun (b : b option) ->
-         match b with
-         | Some (((#a)[@res.patVariantSpread ]) as a) -> doWithA a
-         | Some (Four) -> Js.log {js|four|js}
-         | Some (Five) -> Js.log {js|five|js}
-         | None -> Js.log {js|None|js}))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](b : b option) ->
+       match b with
+       | Some (((#a)[@res.patVariantSpread ]) as a) -> doWithA a
+       | Some (Four) -> Js.log {js|four|js}
+       | Some (Five) -> Js.log {js|five|js}
+       | None -> Js.log {js|None|js})
 module Foo =
   struct
     type nonrec zz =
@@ -64,16 +59,14 @@ module Foo =
       | Third 
   end
 let doWithZ =
-  ((Function$
-      (fun (z : Foo.zz) ->
-         match z with
-         | First -> Js.log {js|First|js}
-         | Second -> Js.log {js|Second|js}))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](z : Foo.zz) ->
+       match z with
+       | First -> Js.log {js|First|js}
+       | Second -> Js.log {js|Second|js})
 let lookup3 =
-  ((Function$
-      (fun (d : Foo.xx) ->
-         match d with
-         | ((#Foo.zz)[@res.patVariantSpread ]) as z -> Js.log z
-         | Third -> Js.log {js|Third|js}))
-  [@res.arity 1])
\ No newline at end of file
+  Function$
+    (fun [arity:1](d : Foo.xx) ->
+       match d with
+       | ((#Foo.zz)[@res.patVariantSpread ]) as z -> Js.log z
+       | Third -> Js.log {js|Third|js})
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/pattern/expected/variants.res.txt b/tests/syntax_tests/data/parsing/grammar/pattern/expected/variants.res.txt
index 3ec7ed09e4..432975df14 100644
--- a/tests/syntax_tests/data/parsing/grammar/pattern/expected/variants.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/pattern/expected/variants.res.txt
@@ -31,26 +31,24 @@ let ((`Instance (component : comp)) : React.t) = i
   | `Instance ({ render; subtree } : Instance.t) -> ()
   | `Instance (comp, tree) -> ()
   | (`Instance (comp : Component.t) : React.t) -> ()
-let f = ((Function$ (fun `Instance -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance as i) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance component) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance component) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance { render; subtree }) -> i))
-  [@res.arity 1])
-let f = ((Function$ (fun (`Instance ({ render; subtree }, inst)) -> i))
-  [@res.arity 1])
+let f = Function$ (fun [arity:1]`Instance -> i)
+let f = Function$ (fun [arity:1](`Instance as i) -> i)
+let f = Function$ (fun [arity:1](`Instance component) -> i)
+let f = Function$ (fun [arity:1](`Instance component) -> i)
+let f = Function$ (fun [arity:1](`Instance { render; subtree }) -> i)
+let f = Function$ (fun [arity:1](`Instance ({ render; subtree }, inst)) -> i)
 let f =
-  ((Function$ (fun (`Instance ({ render; subtree } : Instance.t)) -> i))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](`Instance ({ render; subtree } : Instance.t)) -> i)
 let f =
-  ((Function$ (fun (`Instance ({ render; subtree } : Instance.t)) -> i))
-  [@res.arity 1])
-let f = ((Function$ (fun (`Instance (component, tree)) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance (component, tree)) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance : React.t) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance : React.t) -> i))[@res.arity 1])
-let f = ((Function$ (fun (`Instance (comp : Component.t) : React.t) -> ()))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](`Instance ({ render; subtree } : Instance.t)) -> i)
+let f = Function$ (fun [arity:1](`Instance (component, tree)) -> i)
+let f = Function$ (fun [arity:1](`Instance (component, tree)) -> i)
+let f = Function$ (fun [arity:1](`Instance : React.t) -> i)
+let f = Function$ (fun [arity:1](`Instance : React.t) -> i)
+let f =
+  Function$ (fun [arity:1](`Instance (comp : Component.t) : React.t) -> ())
 ;;for `Blue = x to y do () done
 ;;for `Blue as c = x to y do () done
 ;;for `Blue = x to y do () done
diff --git a/tests/syntax_tests/data/parsing/grammar/signature/expected/itemExtension.res.txt b/tests/syntax_tests/data/parsing/grammar/signature/expected/itemExtension.res.txt
index bd9926db75..93b79be024 100644
--- a/tests/syntax_tests/data/parsing/grammar/signature/expected/itemExtension.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/signature/expected/itemExtension.res.txt
@@ -2,6 +2,6 @@ module type Ext  =
   sig
     [%%item.extension ]
     [%%item.extension.with.args {js|argument|js}]
-    [%%item.extension.with.args ((Function$ (fun x -> f x))[@res.arity 1])]
+    [%%item.extension.with.args Function$ (fun [arity:1]x -> f x)]
     [%%item.extension ][@@withAttr ]
   end
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/signature/expected/standAloneAttribute.res.txt b/tests/syntax_tests/data/parsing/grammar/signature/expected/standAloneAttribute.res.txt
index edf70877ed..1cbdcbe530 100644
--- a/tests/syntax_tests/data/parsing/grammar/signature/expected/standAloneAttribute.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/signature/expected/standAloneAttribute.res.txt
@@ -2,5 +2,5 @@ module type StandaloneAttribute  =
   sig
     [@@@standaloneAttribute ]
     [@@@standaloneAttribute withPayload]
-    [@@@standaloneAttribute ((Function$ (fun x -> x))[@res.arity 1])]
+    [@@@standaloneAttribute Function$ (fun [arity:1]x -> x)]
   end
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/structure/expected/itemExtension.res.txt b/tests/syntax_tests/data/parsing/grammar/structure/expected/itemExtension.res.txt
index 79c8084e6a..433e21c0b2 100644
--- a/tests/syntax_tests/data/parsing/grammar/structure/expected/itemExtension.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/structure/expected/itemExtension.res.txt
@@ -1,5 +1,5 @@
 [%%itemExtension ]
 [%%item.extension ]
 [%%item.extension.with.args {js|argument|js}]
-[%%item.extension.with.args ((Function$ (fun x -> f x))[@res.arity 1])]
+[%%item.extension.with.args Function$ (fun [arity:1]x -> f x)]
 [%%itemExtension ][@@attrOnExtension ]
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/structure/expected/letBinding.res.txt b/tests/syntax_tests/data/parsing/grammar/structure/expected/letBinding.res.txt
index 0d8bf07a45..e48fba501b 100644
--- a/tests/syntax_tests/data/parsing/grammar/structure/expected/letBinding.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/structure/expected/letBinding.res.txt
@@ -3,11 +3,9 @@ let a = 1[@@onFirstBinding ]
 let a = 1[@@onFirstBinding ]
 and b = 2[@@onSecondBinding ]
 let f : type t. t foo =
-  ((Function$
-      (fun sideEffect ->
-         ((let module M = struct exception E of t  end in
-             sideEffect (); ((Function$ ((fun x -> M.E x)))[@res.arity 1]))
-         [@res.braces ])))
-  [@res.arity 1])
-let f : type t x u. (t * x * y) list = ((Function$ (fun l -> f l))
-  [@res.arity 1])
\ No newline at end of file
+  Function$
+    (fun [arity:1]sideEffect ->
+       ((let module M = struct exception E of t  end in
+           sideEffect (); Function$ ((fun [arity:1]x -> M.E x)))
+       [@res.braces ]))
+let f : type t x u. (t * x * y) list = Function$ (fun [arity:1]l -> f l)
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/structure/expected/modExprExtension.res.txt b/tests/syntax_tests/data/parsing/grammar/structure/expected/modExprExtension.res.txt
index 4d4b2404ac..c4af1f86b1 100644
--- a/tests/syntax_tests/data/parsing/grammar/structure/expected/modExprExtension.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/structure/expected/modExprExtension.res.txt
@@ -1,5 +1,4 @@
 module A = [%modExprExtension ]
 module B = [%mod.expr.extension ]
 module C = [%mod.expr.extension.with.args {js|argument|js}]
-module D =
-  [%mod.expr.extension.with.args ((Function$ (fun x -> f x))[@res.arity 1])]
\ No newline at end of file
+module D = [%mod.expr.extension.with.args Function$ (fun [arity:1]x -> f x)]
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/structure/expected/moduleTypeExtension.res.txt b/tests/syntax_tests/data/parsing/grammar/structure/expected/moduleTypeExtension.res.txt
index b3d4e58075..1d7d7f81e0 100644
--- a/tests/syntax_tests/data/parsing/grammar/structure/expected/moduleTypeExtension.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/structure/expected/moduleTypeExtension.res.txt
@@ -2,4 +2,4 @@ module type A  = [%modTypeExtension ]
 module type B  = [%mod.type.extension ]
 module type C  = [%mod.type.extension.with.args {js|argument|js}]
 module type D  =
-  [%mod.type.extension.with.args ((Function$ (fun x -> f x))[@res.arity 1])]
\ No newline at end of file
+  [%mod.type.extension.with.args Function$ (fun [arity:1]x -> f x)]
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/structure/expected/standaloneAttribute.res.txt b/tests/syntax_tests/data/parsing/grammar/structure/expected/standaloneAttribute.res.txt
index b14fa69434..54269ef167 100644
--- a/tests/syntax_tests/data/parsing/grammar/structure/expected/standaloneAttribute.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/structure/expected/standaloneAttribute.res.txt
@@ -1,3 +1,3 @@
 [@@@standaloneAttribute ]
 [@@@standaloneAttribute {js|with payload|js}]
-[@@@standaloneAttribute ((Function$ (fun x -> x + 1))[@res.arity 1])]
\ No newline at end of file
+[@@@standaloneAttribute Function$ (fun [arity:1]x -> x + 1)]
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/typedefinition/expected/polyvariant.res.txt b/tests/syntax_tests/data/parsing/grammar/typedefinition/expected/polyvariant.res.txt
index 105e2a5dfe..500268180a 100644
--- a/tests/syntax_tests/data/parsing/grammar/typedefinition/expected/polyvariant.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/typedefinition/expected/polyvariant.res.txt
@@ -55,4 +55,4 @@ type nonrec t = [< `x [@a ]]
 type nonrec t =
   [< `a of ((int * int) * int) [@one ] | ((int)[@two ])
   | `b of string [@three ]> `w `x `y]
-let f = ((Function$ (fun (x : [ `b ]) -> x))[@res.arity 1])
\ No newline at end of file
+let f = Function$ (fun [arity:1](x : [ `b ]) -> x)
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/typexpr/expected/objectTypeSpreading.res.txt b/tests/syntax_tests/data/parsing/grammar/typexpr/expected/objectTypeSpreading.res.txt
index daf3a17b05..ce8c44639a 100644
--- a/tests/syntax_tests/data/parsing/grammar/typexpr/expected/objectTypeSpreading.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/typexpr/expected/objectTypeSpreading.res.txt
@@ -16,28 +16,23 @@ let steve =
   ((([%obj { name = {js|Steve|js}; age = 30 }] : < user ;age: int   > ))
   [@res.braces ])
 let printFullUser =
-  ((Function$ (fun (steve : < user ;age: int   > ) -> Js.log steve))
-  [@res.arity 1])
+  Function$ (fun [arity:1](steve : < user ;age: int   > ) -> Js.log steve)
 let printFullUser =
-  ((Function$
-      (fun ~user:(((user : < user ;age: int   > ))[@res.namedArgLoc ]) ->
-         Js.log steve))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]~user:(((user : < user ;age: int   > ))[@res.namedArgLoc ])
+       -> Js.log steve)
 let printFullUser =
-  ((Function$
-      (fun ~user:(((user : < user ;age: int   > ))[@res.namedArgLoc ]) ->
-         Js.log steve))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]~user:(((user : < user ;age: int   > ))[@res.namedArgLoc ])
+       -> Js.log steve)
 let printFullUser =
-  ((Function$
-      (fun ?user:(((user)[@res.namedArgLoc ])=
-         (steve : < user ;age: int   > )) -> Js.log steve))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]?user:(((user)[@res.namedArgLoc ])=
+       (steve : < user ;age: int   > )) -> Js.log steve)
 external steve : < user ;age: int   >  = "steve"[@@val ]
 let makeCeoOf30yearsOld =
-  ((Function$
-      (fun name -> ([%obj { name; age = 30 }] : < user ;age: int   > )))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1]name -> ([%obj { name; age = 30 }] : < user ;age: int   > ))
 type nonrec optionalUser = < user ;age: int   >  option
 type nonrec optionalTupleUser =
   (< user ;age: int   >  * < user ;age: int   > ) option
diff --git a/tests/syntax_tests/data/parsing/grammar/typexpr/expected/typeconstr.res.txt b/tests/syntax_tests/data/parsing/grammar/typexpr/expected/typeconstr.res.txt
index 7df7e5a953..6235a20fde 100644
--- a/tests/syntax_tests/data/parsing/grammar/typexpr/expected/typeconstr.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/typexpr/expected/typeconstr.res.txt
@@ -60,5 +60,5 @@ type nonrec ('T, 'E) id_6 =
   | Ok of 'T 
   | Err of {
   payload: 'E } 
-let foo = ((Function$ (fun (x : int as 'X) -> x))[@res.arity 1])
+let foo = Function$ (fun [arity:1](x : int as 'X) -> x)
 module type A  = (Foo with type  t =  'X constraint 'X = int)
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/grammar/typexpr/expected/unit.res.txt b/tests/syntax_tests/data/parsing/grammar/typexpr/expected/unit.res.txt
index de2e44b365..64bf9102df 100644
--- a/tests/syntax_tests/data/parsing/grammar/typexpr/expected/unit.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/typexpr/expected/unit.res.txt
@@ -3,14 +3,14 @@ type nonrec t = (unit -> unit, [ `Has_arity1 ]) function$
 type nonrec t = (unit -> unit -> unit, [ `Has_arity2 ]) function$
 type nonrec t = (unit -> unit, [ `Has_arity1 ]) function$
 let f =
-  ((Function$ (fun (f : (unit -> unit, [ `Has_arity1 ]) function$) -> f ()))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](f : (unit -> unit, [ `Has_arity1 ]) function$) -> f ())
 let f =
-  ((Function$ (fun (f : (unit -> unit, [ `Has_arity1 ]) function$) -> f ()))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](f : (unit -> unit, [ `Has_arity1 ]) function$) -> f ())
 let f =
-  ((Function$
-      (fun (f : (unit -> unit -> unit, [ `Has_arity2 ]) function$) -> f () ()))
-  [@res.arity 1])
+  Function$
+    (fun [arity:1](f : (unit -> unit -> unit, [ `Has_arity2 ]) function$) ->
+       f () ())
 external svg : (unit -> React.element, [ `Has_arity1 ]) function$ = "svg"
 external thing : (unit -> unit, [ `Has_arity1 ]) function$ = "svg"
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/infiniteLoops/expected/equalAfterBinaryExpr.res.txt b/tests/syntax_tests/data/parsing/infiniteLoops/expected/equalAfterBinaryExpr.res.txt
index 7478916b11..6e2dbf6a62 100644
--- a/tests/syntax_tests/data/parsing/infiniteLoops/expected/equalAfterBinaryExpr.res.txt
+++ b/tests/syntax_tests/data/parsing/infiniteLoops/expected/equalAfterBinaryExpr.res.txt
@@ -11,93 +11,106 @@
   Did you mean `==` here?
 
 let rec _addLoop =
-  ((Function$
-      (fun rbt ->
-         fun currentNode ->
-           ((if (Some currentNode) == (rbt |.u root)
-             then currentNode.color <- Black
+  Function$
+    (fun [arity:2]rbt ->
+       fun [arity:1]currentNode ->
+         ((if (Some currentNode) == (rbt |.u root)
+           then currentNode.color <- Black
+           else
+             if (currentNode.parent |.u castNotOption).color == Black
+             then ()
              else
-               if (currentNode.parent |.u castNotOption).color == Black
-               then ()
+               if
+                 (((let uncle = uncleOf currentNode in
+                    (uncle != None) &&
+                      ((uncle |.u castNotOption).color == Red)))
+                 [@res.braces ])
+               then
+                 ((currentNode.parent |.u castNotOption).color <- Black;
+                  ((uncleOf currentNode) |.u castNotOption).color <- Black;
+                  ((grandParentOf currentNode) |.u castNotOption).color <-
+                    Red;
+                  _addLoop rbt
+                    ((grandParentOf currentNode) |.u castNotOption))
                else
-                 if
-                   (((let uncle = uncleOf currentNode in
-                      (uncle != None) &&
-                        ((uncle |.u castNotOption).color == Red)))
-                   [@res.braces ])
-                 then
-                   ((currentNode.parent |.u castNotOption).color <- Black;
-                    ((uncleOf currentNode) |.u castNotOption).color <- Black;
-                    ((grandParentOf currentNode) |.u castNotOption).color <-
-                      Red;
-                    _addLoop rbt
-                      ((grandParentOf currentNode) |.u castNotOption))
-                 else
-                   (let currentNode =
+                 (let currentNode =
+                    if
+                      (not (isLeft currentNode)) &&
+                        (isLeft (currentNode.parent |.u castNotOption))
+                    then
+                      (rotateLeft rbt (currentNode.parent |.u castNotOption);
+                       currentNode.left |.u castNotOption)
+                    else
                       if
-                        (not (isLeft currentNode)) &&
-                          (isLeft (currentNode.parent |.u castNotOption))
+                        (isLeft currentNode) &&
+                          (not
+                             (isLeft (currentNode.parent |.u castNotOption)))
                       then
-                        (rotateLeft rbt
+                        (rotateRight rbt
                            (currentNode.parent |.u castNotOption);
-                         currentNode.left |.u castNotOption)
-                      else
-                        if
-                          (isLeft currentNode) &&
-                            (not
-                               (isLeft (currentNode.parent |.u castNotOption)))
-                        then
-                          (rotateRight rbt
-                             (currentNode.parent |.u castNotOption);
-                           currentNode.right |.u castNotOption)
-                        else currentNode in
-                    (currentNode.parent |.u castNotOption).color <- Black;
-                    ((grandParentOf currentNode) |.u castNotOption).color <-
-                      Red;
-                    if isLeft currentNode
-                    then
-                      rotateRight rbt
-                        ((grandParentOf currentNode) |.u castNotOption)
-                    else
-                      rotateLeft rbt
-                        ((grandParentOf currentNode) |.u castNotOption)))
-           [@res.braces ])))
-  [@res.arity 2])
+                         currentNode.right |.u castNotOption)
+                      else currentNode in
+                  (currentNode.parent |.u castNotOption).color <- Black;
+                  ((grandParentOf currentNode) |.u castNotOption).color <-
+                    Red;
+                  if isLeft currentNode
+                  then
+                    rotateRight rbt
+                      ((grandParentOf currentNode) |.u castNotOption)
+                  else
+                    rotateLeft rbt
+                      ((grandParentOf currentNode) |.u castNotOption)))
+         [@res.braces ]))
 let removeNode =
-  ((Function$
-      (fun rbt ->
-         fun node ->
-           ((if nodeToRemove.color == Black
-             then
-               (if successor.color == Red
-                then
-                  (successor.color <- Black;
-                   if successor.parent == None
-                   then rbt |.u (rootSet (Some successor)))
-                else
-                  (let break = ref false in
-                   let successorRef = ref successor in
-                   while not break.contents do
-                     let successor = successorRef.contents in
-                     match successor.parent with
-                     | None ->
-                         (rbt |.u (rootSet (Some successor));
-                          break.contents <- true)
-                     | Some successorParent ->
-                         let sibling = siblingOf successor in
-                         (if
-                            (sibling != None) &&
-                              ((sibling |.u castNotOption).color == Red)
-                          then
-                            (successorParent.color <- Red;
-                             (sibling |.u castNotOption).color <- Black;
-                             if isLeft successor
-                             then rotateLeft rbt successorParent
-                             else rotateRight rbt successorParent);
-                          (let sibling = siblingOf successor in
-                           let siblingNN = sibling |.u castNotOption in
+  Function$
+    (fun [arity:2]rbt ->
+       fun [arity:1]node ->
+         ((if nodeToRemove.color == Black
+           then
+             (if successor.color == Red
+              then
+                (successor.color <- Black;
+                 if successor.parent == None
+                 then rbt |.u (rootSet (Some successor)))
+              else
+                (let break = ref false in
+                 let successorRef = ref successor in
+                 while not break.contents do
+                   let successor = successorRef.contents in
+                   match successor.parent with
+                   | None ->
+                       (rbt |.u (rootSet (Some successor));
+                        break.contents <- true)
+                   | Some successorParent ->
+                       let sibling = siblingOf successor in
+                       (if
+                          (sibling != None) &&
+                            ((sibling |.u castNotOption).color == Red)
+                        then
+                          (successorParent.color <- Red;
+                           (sibling |.u castNotOption).color <- Black;
+                           if isLeft successor
+                           then rotateLeft rbt successorParent
+                           else rotateRight rbt successorParent);
+                        (let sibling = siblingOf successor in
+                         let siblingNN = sibling |.u castNotOption in
+                         if
+                           (successorParent.color == Black) &&
+                             ((sibling == None) ||
+                                (((siblingNN.color == Black) &&
+                                    ((siblingNN.left == None) ||
+                                       ((siblingNN.left |.u castNotOption).color
+                                          == Black)))
+                                   &&
+                                   ((siblingNN.right == None) ||
+                                      ((siblingNN.right |.u castNotOption).color
+                                         == Black))))
+                         then
+                           (if sibling != None then siblingNN.color <- Red;
+                            successorRef.contents <- successorParent)
+                         else
                            if
-                             (successorParent.color == Black) &&
+                             (successorParent.color == Red) &&
                                ((sibling == None) ||
                                   (((siblingNN.color == Black) &&
                                       ((siblingNN.left == None) ||
@@ -109,78 +122,60 @@ let removeNode =
                                            == Black))))
                            then
                              (if sibling != None then siblingNN.color <- Red;
-                              successorRef.contents <- successorParent)
+                              successorParent.color <- Black;
+                              break.contents <- true)
                            else
                              if
-                               (successorParent.color == Red) &&
-                                 ((sibling == None) ||
-                                    (((siblingNN.color == Black) &&
-                                        ((siblingNN.left == None) ||
-                                           ((siblingNN.left |.u castNotOption).color
-                                              == Black)))
-                                       &&
-                                       ((siblingNN.right == None) ||
-                                          ((siblingNN.right |.u castNotOption).color
-                                             == Black))))
+                               (sibling != None) &&
+                                 ((sibling |.u castNotOption).color == Black)
                              then
-                               (if sibling != None
-                                then siblingNN.color <- Red;
-                                successorParent.color <- Black;
-                                break.contents <- true)
-                             else
-                               if
-                                 (sibling != None) &&
-                                   ((sibling |.u castNotOption).color ==
-                                      Black)
-                               then
-                                 (let sibling = sibling |.u castNotOption in
+                               (let sibling = sibling |.u castNotOption in
+                                if
+                                  (((isLeft successor) &&
+                                      ((sibling.right == None) ||
+                                         ((sibling.right |.u castNotOption).color
+                                            == Black)))
+                                     && (sibling.left != None))
+                                    &&
+                                    ((sibling.left |.u castNotOption).color
+                                       == Red)
+                                then
+                                  (sibling.color <- Red;
+                                   (sibling.left |.u castNotOption).color <-
+                                     Black;
+                                   rotateRight rbt sibling)
+                                else
                                   if
-                                    (((isLeft successor) &&
-                                        ((sibling.right == None) ||
-                                           ((sibling.right |.u castNotOption).color
+                                    (((not (isLeft successor)) &&
+                                        ((sibling.left == None) ||
+                                           ((sibling.left |.u castNotOption).color
                                               == Black)))
-                                       && (sibling.left != None))
+                                       && (sibling.right != None))
                                       &&
-                                      ((sibling.left |.u castNotOption).color
+                                      ((sibling.right |.u castNotOption).color
                                          == Red)
                                   then
                                     (sibling.color <- Red;
-                                     (sibling.left |.u castNotOption).color
-                                       <- Black;
-                                     rotateRight rbt sibling)
-                                  else
-                                    if
-                                      (((not (isLeft successor)) &&
-                                          ((sibling.left == None) ||
-                                             ((sibling.left |.u castNotOption).color
-                                                == Black)))
-                                         && (sibling.right != None))
-                                        &&
-                                        ((sibling.right |.u castNotOption).color
-                                           == Red)
-                                    then
-                                      (sibling.color <- Red;
-                                       (sibling.right |.u castNotOption).color
-                                         <- Black;
-                                       rotateLeft rbt sibling);
-                                  break.contents <- true)
-                               else
-                                 (let sibling = siblingOf successor in
-                                  let sibling = sibling |.u castNotOption in
-                                  sibling.color <- (successorParent.color);
-                                  if isLeft successor
-                                  then
-                                    ((sibling.right |.u castNotOption).color
+                                     (sibling.right |.u castNotOption).color
                                        <- Black;
-                                     rotateRight rbt successorParent)
-                                  else
-                                    ((sibling.left |.u castNotOption).color
-                                       <- Black;
-                                     rotateLeft rbt successorParent))))
-                     done));
-             if isLeaf successor
-             then
-               (if (rbt |.u root) == (Some successor)
-                then (rbt |.u root) = None))
-           [@res.braces ])))
-  [@res.arity 2])
\ No newline at end of file
+                                     rotateLeft rbt sibling);
+                                break.contents <- true)
+                             else
+                               (let sibling = siblingOf successor in
+                                let sibling = sibling |.u castNotOption in
+                                sibling.color <- (successorParent.color);
+                                if isLeft successor
+                                then
+                                  ((sibling.right |.u castNotOption).color <-
+                                     Black;
+                                   rotateRight rbt successorParent)
+                                else
+                                  ((sibling.left |.u castNotOption).color <-
+                                     Black;
+                                   rotateLeft rbt successorParent))))
+                   done));
+           if isLeaf successor
+           then
+             (if (rbt |.u root) == (Some successor)
+              then (rbt |.u root) = None))
+         [@res.braces ]))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/infiniteLoops/expected/nonRecTypes.res.txt b/tests/syntax_tests/data/parsing/infiniteLoops/expected/nonRecTypes.res.txt
index 4e272f04fb..b45c9d086d 100644
--- a/tests/syntax_tests/data/parsing/infiniteLoops/expected/nonRecTypes.res.txt
+++ b/tests/syntax_tests/data/parsing/infiniteLoops/expected/nonRecTypes.res.txt
@@ -126,86 +126,102 @@ include
         ;;{js|BS:6.0.1\x84\x95\xa6\xbe\0\0\0\x13\0\0\0\x07\0\0\0\x14\0\0\0\x13\xb0\xa0\xa0A\x91@@A\x98\xa0'compare@|js}
       end
     let has =
-      ((Function$
-          (fun rbt ->
-             fun value -> (_findNode rbt (rootGet rbt) value) != None))
-      [@res.arity 2])
-    let rec minNode = ((Function$ (fun node -> [%rescript.exprhole ]))
-      [@res.arity 1])
-    let findMin = ((Function$ (fun rbt -> [%rescript.exprhole ]))
-      [@res.arity 1])
+      Function$
+        (fun [arity:2]rbt ->
+           fun [arity:1]value -> (_findNode rbt (rootGet rbt) value) != None)
+    let rec minNode = Function$ (fun [arity:1]node -> [%rescript.exprhole ])
+    let findMin = Function$ (fun [arity:1]rbt -> [%rescript.exprhole ])
     let removeNode =
-      ((Function$
-          (fun rbt ->
-             fun node ->
-               ((let nodeToRemove =
-                   match ((leftGet node), (rightGet node)) with
-                   | (Some _, Some _) ->
-                       let successor =
-                         castNotOption (minNode (rightGet node)) in
-                       (valueSet node (valueGet successor);
-                        heightSet node (heightGet successor);
-                        successor)
-                   | _ -> node in
-                 let successor =
-                   match leftGet nodeToRemove with
-                   | None -> rightGet nodeToRemove
-                   | left -> left in
-                 let (successor, isLeaf) =
-                   match successor with
-                   | None ->
-                       let leaf =
-                         createNode ~value:((Js.Internal.raw_expr {js|0|js})
-                           [@res.namedArgLoc ]) ~color:((Black)
-                           [@res.namedArgLoc ]) ~height:((0.)
-                           [@res.namedArgLoc ]) in
-                       let isLeaf =
-                         Js.Internal.fn_mk1 ((Function$ (fun x -> x == leaf))
-                           [@res.arity 1]) in
-                       (leaf, isLeaf)
-                   | Some successor ->
-                       (successor,
-                         (Js.Internal.fn_mk1 ((Function$ (fun _ -> false))
-                            [@res.arity 1]))) in
-                 let nodeParent = parentGet nodeToRemove in
-                 parentSet successor nodeParent;
-                 (match nodeParent with
-                  | None -> ()
-                  | Some parent ->
-                      leftOrRightSet parent ~node:((nodeToRemove)
-                        [@res.namedArgLoc ]) (Some successor));
-                 updateSumRecursive rbt successor;
-                 if (colorGet nodeToRemove) == Black
-                 then
-                   (if (colorGet successor) == Red
-                    then
-                      (colorSet successor Black;
-                       if (parentGet successor) == None
-                       then rootSet rbt (Some successor))
-                    else
-                      (let break = ref false in
-                       let successorRef = ref successor in
-                       while not break.contents do
-                         let successor = successorRef.contents in
-                         match parentGet successor with
-                         | None ->
-                             (rootSet rbt (Some successor);
-                              break.contents <- true)
-                         | Some successorParent ->
-                             let sibling = siblingOf successor in
-                             (if
-                                (sibling != None) &&
-                                  ((colorGet (castNotOption sibling)) == Red)
-                              then
-                                (colorSet successorParent Red;
-                                 colorSet (castNotOption sibling) Black;
-                                 if isLeft successor
-                                 then rotateLeft rbt successorParent
-                                 else rotateRight rbt successorParent);
-                              (let sibling = siblingOf successor in
-                               let siblingNN = castNotOption sibling in
+      Function$
+        (fun [arity:2]rbt ->
+           fun [arity:1]node ->
+             ((let nodeToRemove =
+                 match ((leftGet node), (rightGet node)) with
+                 | (Some _, Some _) ->
+                     let successor = castNotOption (minNode (rightGet node)) in
+                     (valueSet node (valueGet successor);
+                      heightSet node (heightGet successor);
+                      successor)
+                 | _ -> node in
+               let successor =
+                 match leftGet nodeToRemove with
+                 | None -> rightGet nodeToRemove
+                 | left -> left in
+               let (successor, isLeaf) =
+                 match successor with
+                 | None ->
+                     let leaf =
+                       createNode ~value:((Js.Internal.raw_expr {js|0|js})
+                         [@res.namedArgLoc ]) ~color:((Black)
+                         [@res.namedArgLoc ]) ~height:((0.)
+                         [@res.namedArgLoc ]) in
+                     let isLeaf =
+                       Js.Internal.fn_mk1
+                         (Function$ (fun [arity:1]x -> x == leaf)) in
+                     (leaf, isLeaf)
+                 | Some successor ->
+                     (successor,
+                       (Js.Internal.fn_mk1
+                          (Function$ (fun [arity:1]_ -> false)))) in
+               let nodeParent = parentGet nodeToRemove in
+               parentSet successor nodeParent;
+               (match nodeParent with
+                | None -> ()
+                | Some parent ->
+                    leftOrRightSet parent ~node:((nodeToRemove)
+                      [@res.namedArgLoc ]) (Some successor));
+               updateSumRecursive rbt successor;
+               if (colorGet nodeToRemove) == Black
+               then
+                 (if (colorGet successor) == Red
+                  then
+                    (colorSet successor Black;
+                     if (parentGet successor) == None
+                     then rootSet rbt (Some successor))
+                  else
+                    (let break = ref false in
+                     let successorRef = ref successor in
+                     while not break.contents do
+                       let successor = successorRef.contents in
+                       match parentGet successor with
+                       | None ->
+                           (rootSet rbt (Some successor);
+                            break.contents <- true)
+                       | Some successorParent ->
+                           let sibling = siblingOf successor in
+                           (if
+                              (sibling != None) &&
+                                ((colorGet (castNotOption sibling)) == Red)
+                            then
+                              (colorSet successorParent Red;
+                               colorSet (castNotOption sibling) Black;
+                               if isLeft successor
+                               then rotateLeft rbt successorParent
+                               else rotateRight rbt successorParent);
+                            (let sibling = siblingOf successor in
+                             let siblingNN = castNotOption sibling in
+                             if
+                               ((colorGet successorParent) == Black) &&
+                                 ((sibling == None) ||
+                                    ((((colorGet siblingNN) == Black) &&
+                                        (((leftGet siblingNN) == None) ||
+                                           ((colorGet
+                                               (castNotOption
+                                                  (leftGet siblingNN)))
+                                              == Black)))
+                                       &&
+                                       (((rightGet siblingNN) == None) ||
+                                          ((colorGet
+                                              (castNotOption
+                                                 (rightGet siblingNN)))
+                                             == Black))))
+                             then
+                               (if sibling != None
+                                then colorSet siblingNN Red;
+                                successorRef.contents <- successorParent)
+                             else
                                if
-                                 ((colorGet successorParent) == Black) &&
+                                 ((colorGet successorParent) == Red) &&
                                    ((sibling == None) ||
                                       ((((colorGet siblingNN) == Black) &&
                                           (((leftGet siblingNN) == None) ||
@@ -222,359 +238,312 @@ include
                                then
                                  (if sibling != None
                                   then colorSet siblingNN Red;
-                                  successorRef.contents <- successorParent)
+                                  colorSet successorParent Black;
+                                  break.contents <- true)
                                else
                                  if
-                                   ((colorGet successorParent) == Red) &&
-                                     ((sibling == None) ||
-                                        ((((colorGet siblingNN) == Black) &&
-                                            (((leftGet siblingNN) == None) ||
-                                               ((colorGet
-                                                   (castNotOption
-                                                      (leftGet siblingNN)))
-                                                  == Black)))
-                                           &&
-                                           (((rightGet siblingNN) == None) ||
-                                              ((colorGet
-                                                  (castNotOption
-                                                     (rightGet siblingNN)))
-                                                 == Black))))
+                                   (sibling != None) &&
+                                     ((colorGet (castNotOption sibling)) ==
+                                        Black)
                                  then
-                                   (if sibling != None
-                                    then colorSet siblingNN Red;
-                                    colorSet successorParent Black;
-                                    break.contents <- true)
-                                 else
-                                   if
-                                     (sibling != None) &&
-                                       ((colorGet (castNotOption sibling)) ==
-                                          Black)
-                                   then
-                                     (let sibling = castNotOption sibling in
+                                   (let sibling = castNotOption sibling in
+                                    if
+                                      (((isLeft successor) &&
+                                          (((rightGet sibling) == None) ||
+                                             ((colorGet
+                                                 (castNotOption
+                                                    (rightGet sibling)))
+                                                == Black)))
+                                         && ((leftGet sibling) != None))
+                                        &&
+                                        ((colorGet
+                                            (castNotOption (leftGet sibling)))
+                                           == Red)
+                                    then
+                                      (colorSet sibling Red;
+                                       colorSet
+                                         (castNotOption (leftGet sibling))
+                                         Black;
+                                       rotateRight rbt sibling)
+                                    else
                                       if
-                                        (((isLeft successor) &&
-                                            (((rightGet sibling) == None) ||
+                                        (((not (isLeft successor)) &&
+                                            (((leftGet sibling) == None) ||
                                                ((colorGet
                                                    (castNotOption
-                                                      (rightGet sibling)))
+                                                      (leftGet sibling)))
                                                   == Black)))
-                                           && ((leftGet sibling) != None))
+                                           && ((rightGet sibling) != None))
                                           &&
                                           ((colorGet
                                               (castNotOption
-                                                 (leftGet sibling)))
+                                                 (rightGet sibling)))
                                              == Red)
                                       then
                                         (colorSet sibling Red;
                                          colorSet
-                                           (castNotOption (leftGet sibling))
-                                           Black;
-                                         rotateRight rbt sibling)
-                                      else
-                                        if
-                                          (((not (isLeft successor)) &&
-                                              (((leftGet sibling) == None) ||
-                                                 ((colorGet
-                                                     (castNotOption
-                                                        (leftGet sibling)))
-                                                    == Black)))
-                                             && ((rightGet sibling) != None))
-                                            &&
-                                            ((colorGet
-                                                (castNotOption
-                                                   (rightGet sibling)))
-                                               == Red)
-                                        then
-                                          (colorSet sibling Red;
-                                           colorSet
-                                             (castNotOption
-                                                (rightGet sibling)) Black;
-                                           rotateLeft rbt sibling);
-                                      break.contents <- true)
-                                   else
-                                     (let sibling = siblingOf successor in
-                                      let sibling = castNotOption sibling in
-                                      colorSet sibling
-                                        (colorGet successorParent);
-                                      if isLeft successor
-                                      then
-                                        (colorSet
                                            (castNotOption (rightGet sibling))
                                            Black;
-                                         rotateRight rbt successorParent)
-                                      else
-                                        (colorSet
-                                           (castNotOption (leftGet sibling))
-                                           Black;
-                                         rotateLeft rbt successorParent))))
-                         done));
-                 if Js.Internal.fn_run1 isLeaf successor
-                 then
-                   (if (rootGet rbt) == (Some successor)
-                    then rootSet rbt None;
-                    (match parentGet successor with
-                     | None -> ()
-                     | Some parent ->
-                         leftOrRightSet parent ~node:((successor)
-                           [@res.namedArgLoc ]) None)))
-               [@res.braces ])))
-      [@res.arity 2])
-    let remove =
-      ((Function$
-          (fun rbt ->
-             fun value ->
-               match _findNode rbt (rootGet rbt) value with
-               | Some node ->
-                   (removeNode rbt node;
-                    sizeSet rbt ((sizeGet rbt) - 1);
-                    Some (heightGet node))
-               | None -> None))
-      [@res.arity 2])
-    let findThroughCallback =
-      ((Function$
-          (fun rbt ->
-             fun cb ->
-               ((let rec findThroughCallback =
-                   ((Function$
-                       (fun rbt ->
-                          fun node ->
-                            fun cb ->
-                              match node with
-                              | None -> None
-                              | Some node ->
-                                  let cmp =
-                                    Js.Internal.fn_run1 cb (valueGet node) in
-                                  if cmp == 0
-                                  then Some node
-                                  else
-                                    if cmp < 0
+                                         rotateLeft rbt sibling);
+                                    break.contents <- true)
+                                 else
+                                   (let sibling = siblingOf successor in
+                                    let sibling = castNotOption sibling in
+                                    colorSet sibling
+                                      (colorGet successorParent);
+                                    if isLeft successor
                                     then
-                                      findThroughCallback rbt (leftGet node)
-                                        cb
+                                      (colorSet
+                                         (castNotOption (rightGet sibling))
+                                         Black;
+                                       rotateRight rbt successorParent)
                                     else
-                                      findThroughCallback rbt (rightGet node)
-                                        cb))
-                   [@res.arity 3]) in
-                 match findThroughCallback rbt (rootGet rbt) cb with
-                 | None -> None
-                 | Some node -> Some (valueGet node))
-               [@res.braces ])))
-      [@res.arity 2])
+                                      (colorSet
+                                         (castNotOption (leftGet sibling))
+                                         Black;
+                                       rotateLeft rbt successorParent))))
+                       done));
+               if Js.Internal.fn_run1 isLeaf successor
+               then
+                 (if (rootGet rbt) == (Some successor) then rootSet rbt None;
+                  (match parentGet successor with
+                   | None -> ()
+                   | Some parent ->
+                       leftOrRightSet parent ~node:((successor)
+                         [@res.namedArgLoc ]) None)))
+             [@res.braces ]))
+    let remove =
+      Function$
+        (fun [arity:2]rbt ->
+           fun [arity:1]value ->
+             match _findNode rbt (rootGet rbt) value with
+             | Some node ->
+                 (removeNode rbt node;
+                  sizeSet rbt ((sizeGet rbt) - 1);
+                  Some (heightGet node))
+             | None -> None)
+    let findThroughCallback =
+      Function$
+        (fun [arity:2]rbt ->
+           fun [arity:1]cb ->
+             ((let rec findThroughCallback =
+                 Function$
+                   (fun [arity:3]rbt ->
+                      fun [arity:2]node ->
+                        fun [arity:1]cb ->
+                          match node with
+                          | None -> None
+                          | Some node ->
+                              let cmp =
+                                Js.Internal.fn_run1 cb (valueGet node) in
+                              if cmp == 0
+                              then Some node
+                              else
+                                if cmp < 0
+                                then
+                                  findThroughCallback rbt (leftGet node) cb
+                                else
+                                  findThroughCallback rbt (rightGet node) cb) in
+               match findThroughCallback rbt (rootGet rbt) cb with
+               | None -> None
+               | Some node -> Some (valueGet node))
+             [@res.braces ]))
     let make =
-      ((Function$
-          (fun ~compare:((compare)[@res.namedArgLoc ]) ->
-             t ~size:((0)[@res.namedArgLoc ]) ~root:((None)
-               [@res.namedArgLoc ]) ~compare:((compare)[@res.namedArgLoc ])))
-      [@res.arity 1])
+      Function$
+        (fun [arity:1]~compare:((compare)[@res.namedArgLoc ]) ->
+           t ~size:((0)[@res.namedArgLoc ]) ~root:((None)[@res.namedArgLoc ])
+             ~compare:((compare)[@res.namedArgLoc ]))
     let rec heightOfInterval =
-      ((Function$
-          (fun rbt ->
-             fun node ->
-               fun lhs ->
-                 fun rhs ->
-                   match node with
-                   | None -> 0.
-                   | Some n ->
-                       if (lhs == None) && (rhs == None)
-                       then sumGet n
+      Function$
+        (fun [arity:4]rbt ->
+           fun [arity:3]node ->
+             fun [arity:2]lhs ->
+               fun [arity:1]rhs ->
+                 match node with
+                 | None -> 0.
+                 | Some n ->
+                     if (lhs == None) && (rhs == None)
+                     then sumGet n
+                     else
+                       if
+                         (lhs != None) &&
+                           ((Js.Internal.fn_run2 (compareGet rbt)
+                               (valueGet n) (castNotOption lhs))
+                              < 0)
+                       then heightOfInterval rbt (rightGet n) lhs rhs
                        else
                          if
-                           (lhs != None) &&
+                           (rhs != None) &&
                              ((Js.Internal.fn_run2 (compareGet rbt)
-                                 (valueGet n) (castNotOption lhs))
-                                < 0)
-                         then heightOfInterval rbt (rightGet n) lhs rhs
+                                 (valueGet n) (castNotOption rhs))
+                                > 0)
+                         then heightOfInterval rbt (leftGet n) lhs rhs
                          else
-                           if
-                             (rhs != None) &&
-                               ((Js.Internal.fn_run2 (compareGet rbt)
-                                   (valueGet n) (castNotOption rhs))
-                                  > 0)
-                           then heightOfInterval rbt (leftGet n) lhs rhs
-                           else
-                             ((heightGet n) +.
-                                (heightOfInterval rbt (leftGet n) lhs None))
-                               +.
-                               (heightOfInterval rbt (rightGet n) None rhs)))
-      [@res.arity 4])
+                           ((heightGet n) +.
+                              (heightOfInterval rbt (leftGet n) lhs None))
+                             +. (heightOfInterval rbt (rightGet n) None rhs))
     let heightOfInterval =
-      ((Function$
-          (fun rbt ->
-             fun lhs -> fun rhs -> heightOfInterval rbt (rootGet rbt) lhs rhs))
-      [@res.arity 3])
+      Function$
+        (fun [arity:3]rbt ->
+           fun [arity:2]lhs ->
+             fun [arity:1]rhs -> heightOfInterval rbt (rootGet rbt) lhs rhs)
     let rec firstVisibleNode =
-      ((Function$
-          (fun node ->
-             fun offset ->
-               match node with
-               | None -> None
-               | Some node ->
-                   if (sumGet node) <= offset
-                   then None
-                   else
-                     (let nodeHeight = heightGet node in
-                      let sumLeft =
-                        match leftGet node with
-                        | None -> 0.0
-                        | Some left -> sumGet left in
-                      if sumLeft > offset
-                      then firstVisibleNode (leftGet node) offset
+      Function$
+        (fun [arity:2]node ->
+           fun [arity:1]offset ->
+             match node with
+             | None -> None
+             | Some node ->
+                 if (sumGet node) <= offset
+                 then None
+                 else
+                   (let nodeHeight = heightGet node in
+                    let sumLeft =
+                      match leftGet node with
+                      | None -> 0.0
+                      | Some left -> sumGet left in
+                    if sumLeft > offset
+                    then firstVisibleNode (leftGet node) offset
+                    else
+                      if (sumLeft +. nodeHeight) > offset
+                      then Some node
                       else
-                        if (sumLeft +. nodeHeight) > offset
-                        then Some node
-                        else
-                          firstVisibleNode (rightGet node)
-                            (offset -. (sumLeft +. nodeHeight)))))
-      [@res.arity 2])
+                        firstVisibleNode (rightGet node)
+                          (offset -. (sumLeft +. nodeHeight))))
     let lastVisibleNode =
-      ((Function$
-          (fun node ->
-             fun offset ->
-               match firstVisibleNode node offset with
-               | None -> maxNode node
-               | first -> first))
-      [@res.arity 2])
+      Function$
+        (fun [arity:2]node ->
+           fun [arity:1]offset ->
+             match firstVisibleNode node offset with
+             | None -> maxNode node
+             | first -> first)
     let firstVisible =
-      ((Function$
-          (fun rbt ->
-             fun ~offset:((offset)[@res.namedArgLoc ]) ->
-               match firstVisibleNode (rootGet rbt) offset with
-               | None -> None
-               | Some node -> Some (valueGet node)))
-      [@res.arity 2])
+      Function$
+        (fun [arity:2]rbt ->
+           fun [arity:1]~offset:((offset)[@res.namedArgLoc ]) ->
+             match firstVisibleNode (rootGet rbt) offset with
+             | None -> None
+             | Some node -> Some (valueGet node))
     let rec leftmost =
-      ((Function$
-          (fun node ->
-             match leftGet node with
-             | None -> node
-             | Some node -> leftmost node))
-      [@res.arity 1])
+      Function$
+        (fun [arity:1]node ->
+           match leftGet node with
+           | None -> node
+           | Some node -> leftmost node)
     let rec firstRightParent =
-      ((Function$
-          (fun node ->
-             match parentGet node with
-             | None -> None
-             | Some parent ->
-                 if isLeft node then Some parent else firstRightParent parent))
-      [@res.arity 1])
+      Function$
+        (fun [arity:1]node ->
+           match parentGet node with
+           | None -> None
+           | Some parent ->
+               if isLeft node then Some parent else firstRightParent parent)
     let nextNode =
-      ((Function$
-          (fun node ->
-             match rightGet node with
-             | None -> firstRightParent node
-             | Some right -> Some (leftmost right)))
-      [@res.arity 1])
+      Function$
+        (fun [arity:1]node ->
+           match rightGet node with
+           | None -> firstRightParent node
+           | Some right -> Some (leftmost right))
     let rec sumLeftSpine =
-      ((Function$
-          (fun node ->
-             fun ~fromRightChild:((fromRightChild)[@res.namedArgLoc ]) ->
-               ((let leftSpine =
-                   match leftGet node with
-                   | None -> heightGet node
-                   | Some left ->
-                       if fromRightChild
-                       then (heightGet node) +. (sumGet left)
-                       else 0.0 in
-                 match parentGet node with
-                 | None -> leftSpine
-                 | Some parent ->
-                     leftSpine +.
-                       (sumLeftSpine parent
-                          ~fromRightChild:(((rightGet parent) == (Some node))
-                          [@res.namedArgLoc ])))
-               [@res.braces ])))
-      [@res.arity 2])
+      Function$
+        (fun [arity:2]node ->
+           fun [arity:1]~fromRightChild:((fromRightChild)[@res.namedArgLoc ])
+             ->
+             ((let leftSpine =
+                 match leftGet node with
+                 | None -> heightGet node
+                 | Some left ->
+                     if fromRightChild
+                     then (heightGet node) +. (sumGet left)
+                     else 0.0 in
+               match parentGet node with
+               | None -> leftSpine
+               | Some parent ->
+                   leftSpine +.
+                     (sumLeftSpine parent
+                        ~fromRightChild:(((rightGet parent) == (Some node))
+                        [@res.namedArgLoc ])))
+             [@res.braces ]))
     let getY =
-      ((Function$
-          (fun node ->
-             (sumLeftSpine node ~fromRightChild:((true)[@res.namedArgLoc ]))
-               -. (heightGet node)))
-      [@res.arity 1])
+      Function$
+        (fun [arity:1]node ->
+           (sumLeftSpine node ~fromRightChild:((true)[@res.namedArgLoc ])) -.
+             (heightGet node))
     let linearSearch =
-      ((Function$
-          (fun rbt ->
-             fun callback ->
-               ((let rec find =
-                   ((Function$
-                       (fun node ->
-                          fun callback ->
-                            if Js.Internal.fn_run1 callback (valueGet node)
-                            then Some (valueGet node)
-                            else
-                              (match nextNode node with
-                               | None -> None
-                               | Some node -> find node callback)))
-                   [@res.arity 2]) in
-                 match minNode (rootGet rbt) with
-                 | None -> None
-                 | Some node -> find node callback)
-               [@res.braces ])))
-      [@res.arity 2])
+      Function$
+        (fun [arity:2]rbt ->
+           fun [arity:1]callback ->
+             ((let rec find =
+                 Function$
+                   (fun [arity:2]node ->
+                      fun [arity:1]callback ->
+                        if Js.Internal.fn_run1 callback (valueGet node)
+                        then Some (valueGet node)
+                        else
+                          (match nextNode node with
+                           | None -> None
+                           | Some node -> find node callback)) in
+               match minNode (rootGet rbt) with
+               | None -> None
+               | Some node -> find node callback)
+             [@res.braces ]))
     let rec iterate =
-      ((Function$
-          (fun ~inclusive:((inclusive)[@res.namedArgLoc ]) ->
-             fun firstNode ->
-               fun lastNode ->
-                 fun ~callback:((callback)[@res.namedArgLoc ]) ->
+      Function$
+        (fun [arity:4]~inclusive:((inclusive)[@res.namedArgLoc ]) ->
+           fun [arity:3]firstNode ->
+             fun [arity:2]lastNode ->
+               fun [arity:1]~callback:((callback)[@res.namedArgLoc ]) ->
+                 match firstNode with
+                 | None -> ()
+                 | Some node ->
+                     (if inclusive then Js.Internal.fn_run1 callback node;
+                      if firstNode != lastNode
+                      then
+                        (if not inclusive
+                         then Js.Internal.fn_run1 callback node;
+                         iterate ~inclusive:((inclusive)[@res.namedArgLoc ])
+                           (nextNode node) lastNode ~callback:((callback)
+                           [@res.namedArgLoc ]))))
+    let rec iterateWithY =
+      Function$
+        (fun [arity:5]?y:((y)[@res.namedArgLoc ]) ->
+           fun [arity:4]~inclusive:((inclusive)[@res.namedArgLoc ]) ->
+             fun [arity:3]firstNode ->
+               fun [arity:2]lastNode ->
+                 fun [arity:1]~callback:((callback)[@res.namedArgLoc ]) ->
                    match firstNode with
                    | None -> ()
                    | Some node ->
-                       (if inclusive then Js.Internal.fn_run1 callback node;
+                       let y = match y with | None -> getY node | Some y -> y in
+                       (if inclusive then Js.Internal.fn_run2 callback node y;
                         if firstNode != lastNode
                         then
                           (if not inclusive
-                           then Js.Internal.fn_run1 callback node;
-                           iterate ~inclusive:((inclusive)
+                           then Js.Internal.fn_run2 callback node y;
+                           iterateWithY ~y:((y +. (heightGet node))
+                             [@res.namedArgLoc ]) ~inclusive:((inclusive)
                              [@res.namedArgLoc ]) (nextNode node) lastNode
-                             ~callback:((callback)[@res.namedArgLoc ])))))
-      [@res.arity 4])
-    let rec iterateWithY =
-      ((Function$
-          (fun ?y:((y)[@res.namedArgLoc ]) ->
-             fun ~inclusive:((inclusive)[@res.namedArgLoc ]) ->
-               fun firstNode ->
-                 fun lastNode ->
-                   fun ~callback:((callback)[@res.namedArgLoc ]) ->
-                     match firstNode with
-                     | None -> ()
-                     | Some node ->
-                         let y =
-                           match y with | None -> getY node | Some y -> y in
-                         (if inclusive
-                          then Js.Internal.fn_run2 callback node y;
-                          if firstNode != lastNode
-                          then
-                            (if not inclusive
-                             then Js.Internal.fn_run2 callback node y;
-                             iterateWithY ~y:((y +. (heightGet node))
-                               [@res.namedArgLoc ]) ~inclusive:((inclusive)
-                               [@res.namedArgLoc ]) (nextNode node) lastNode
-                               ~callback:((callback)[@res.namedArgLoc ])))))
-      [@res.arity 5])
+                             ~callback:((callback)[@res.namedArgLoc ]))))
     let rec updateSum =
-      ((Function$
-          (fun node ->
-             fun ~delta:((delta)[@res.namedArgLoc ]) ->
-               match node with
+      Function$
+        (fun [arity:2]node ->
+           fun [arity:1]~delta:((delta)[@res.namedArgLoc ]) ->
+             match node with
+             | None -> ()
+             | Some node ->
+                 (sumSet node ((sumGet node) +. delta);
+                  updateSum (parentGet node) ~delta:((delta)
+                    [@res.namedArgLoc ])))
+    let setHeight =
+      Function$
+        (fun [arity:3]rbt ->
+           fun [arity:2]value ->
+             fun [arity:1]~height:((height)[@res.namedArgLoc ]) ->
+               match _findNode rbt (rootGet rbt) value with
                | None -> ()
                | Some node ->
-                   (sumSet node ((sumGet node) +. delta);
-                    updateSum (parentGet node) ~delta:((delta)
-                      [@res.namedArgLoc ]))))
-      [@res.arity 2])
-    let setHeight =
-      ((Function$
-          (fun rbt ->
-             fun value ->
-               fun ~height:((height)[@res.namedArgLoc ]) ->
-                 match _findNode rbt (rootGet rbt) value with
-                 | None -> ()
-                 | Some node ->
-                     let delta = height -. (heightGet node) in
-                     (heightSet node height;
-                      updateSum (Some node) ~delta:((delta)
-                        [@res.namedArgLoc ]))))
-      [@res.arity 3])
+                   let delta = height -. (heightGet node) in
+                   (heightSet node height;
+                    updateSum (Some node) ~delta:((delta)[@res.namedArgLoc ])))
     type nonrec 'value oldNewVisibleNodes =
       {
       mutable old: 'value array ;
diff --git a/tests/syntax_tests/data/parsing/infiniteLoops/expected/templateEof.res.txt b/tests/syntax_tests/data/parsing/infiniteLoops/expected/templateEof.res.txt
index bd5be8ca2e..e32f3b1321 100644
--- a/tests/syntax_tests/data/parsing/infiniteLoops/expected/templateEof.res.txt
+++ b/tests/syntax_tests/data/parsing/infiniteLoops/expected/templateEof.res.txt
@@ -20,7 +20,6 @@
 
 ;;et
 ;;foo =
-    ((Function$
-        (fun x ->
-           match x with | (("")[@res.template ]) -> [%rescript.exprhole ]))
-    [@res.arity 1])
\ No newline at end of file
+    (Function$
+       (fun [arity:1]x ->
+          match x with | (("")[@res.template ]) -> [%rescript.exprhole ]))
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/recovery/expression/expected/emptyBlock.res.txt b/tests/syntax_tests/data/parsing/recovery/expression/expected/emptyBlock.res.txt
index 545f257b92..b08fdd7412 100644
--- a/tests/syntax_tests/data/parsing/recovery/expression/expected/emptyBlock.res.txt
+++ b/tests/syntax_tests/data/parsing/recovery/expression/expected/emptyBlock.res.txt
@@ -1,2 +1,2 @@
 let x = {  }
-let f = ((Function$ (fun a -> fun b -> {  }))[@res.arity 2])
\ No newline at end of file
+let f = Function$ (fun [arity:2]a -> fun [arity:1]b -> {  })
\ No newline at end of file
diff --git a/tests/syntax_tests/data/parsing/recovery/expression/expected/list.res.txt b/tests/syntax_tests/data/parsing/recovery/expression/expected/list.res.txt
index 511169c1c6..3be57506a5 100644
--- a/tests/syntax_tests/data/parsing/recovery/expression/expected/list.res.txt
+++ b/tests/syntax_tests/data/parsing/recovery/expression/expected/list.res.txt
@@ -31,16 +31,15 @@ let flags =
     then
       ((let parts = Utils.split_on_char ' ' flags in
         let rec loop =
-          ((Function$
-              (fun items ->
-                 ((match items with
-                   | [|{js|-pp|js};_ppFlag;rest|] -> loop rest
-                   | [|x;rest|] ->
-                       ((Belt.Array.concatMany)[@res.spread ])
-                         [|[|x|];(loop rest)|]
-                   | [||] -> [||])
-                 [@res.braces ])))
-          [@res.arity 1]) in
+          Function$
+            (fun [arity:1]items ->
+               ((match items with
+                 | [|{js|-pp|js};_ppFlag;rest|] -> loop rest
+                 | [|x;rest|] ->
+                     ((Belt.Array.concatMany)[@res.spread ])
+                       [|[|x|];(loop rest)|]
+                 | [||] -> [||])
+               [@res.braces ])) in
         String.concat {js| |js} (loop parts))
       [@res.braces ])
     else flags)
diff --git a/tests/syntax_tests/data/printer/expr/expected/underscoreApply.res.txt b/tests/syntax_tests/data/printer/expr/expected/underscoreApply.res.txt
index dc58ba4594..a5ef46cc9e 100644
--- a/tests/syntax_tests/data/printer/expr/expected/underscoreApply.res.txt
+++ b/tests/syntax_tests/data/printer/expr/expected/underscoreApply.res.txt
@@ -44,7 +44,7 @@ getDirector(a, b, _).name
 
 f(a, b, _) ? g(x, y, _) : h(alpha, beta, _)
 
-<div onClick={f(a, b, _)} />
+<div onClick=f(a, b, _) />
 <div> {f(a, b, _)} </div>
 
 f(a, b, _)[ix]
diff --git a/tests/tools_tests/ppx/TestPpx.res b/tests/tools_tests/ppx/TestPpx.res
index afbca8e037..5ac14dbddb 100644
--- a/tests/tools_tests/ppx/TestPpx.res
+++ b/tests/tools_tests/ppx/TestPpx.res
@@ -18,3 +18,9 @@ module OptionalFields = {
 
   let r = {y: 1.0}
 }
+
+module Arity = {
+  let one = x => x
+  let two = (x, y) => x + y
+  let n = two(one(1), 5)
+}
diff --git a/tests/tools_tests/src/expected/TestPpx.res.jsout b/tests/tools_tests/src/expected/TestPpx.res.jsout
index 9aa7b06165..f1abca17ba 100644
--- a/tests/tools_tests/src/expected/TestPpx.res.jsout
+++ b/tests/tools_tests/src/expected/TestPpx.res.jsout
@@ -16,6 +16,22 @@ let OptionalFields = {
   }
 };
 
+function one(x) {
+  return x;
+}
+
+function two(x, y) {
+  return x + y | 0;
+}
+
+let n = 6;
+
+let Arity = {
+  one: one,
+  two: two,
+  n: n
+};
+
 let a = "A";
 
 let b = "B";
@@ -27,4 +43,5 @@ exports.b = b;
 exports.M = M;
 exports.vv = vv;
 exports.OptionalFields = OptionalFields;
+exports.Arity = Arity;
 /*  Not a pure module */

From ca652d855f1e2099265adf8872bdd6b0046a8f78 Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Tue, 10 Dec 2024 09:33:23 +0100
Subject: [PATCH 2/6] Use uncurried type for `@deriving(jsConverter)`

This goes towards making all arities explicit.
---
 compiler/frontend/ast_derive_js_mapper.ml | 44 ++++++++++++++---------
 tests/tests/src/ast_abstract_test.mjs     |  8 ++---
 2 files changed, 31 insertions(+), 21 deletions(-)

diff --git a/compiler/frontend/ast_derive_js_mapper.ml b/compiler/frontend/ast_derive_js_mapper.ml
index 70f4babb77..ca10a03da1 100644
--- a/compiler/frontend/ast_derive_js_mapper.ml
+++ b/compiler/frontend/ast_derive_js_mapper.ml
@@ -128,7 +128,9 @@ let app1 = Ast_compatible.app1
 
 let app2 = Ast_compatible.app2
 
-let ( ->~ ) a b = Ast_compatible.arrow a b
+let ( ->~ ) a b =
+  Ast_uncurried.uncurried_type ~loc:Location.none ~arity:1
+    (Ast_compatible.arrow a b)
 
 let raise_when_not_found_ident =
   Longident.Ldot (Lident Primitive_modules.util, "raiseWhenNotFound")
@@ -167,9 +169,10 @@ let init () =
               in
               let to_js_body body =
                 Ast_comb.single_non_rec_value pat_to_js
-                  (Ast_compatible.fun_ ~arity:None
-                     (Pat.constraint_ (Pat.var pat_param) core_type)
-                     body)
+                  (Ast_uncurried.uncurried_fun ~loc:Location.none ~arity:1
+                     (Ast_compatible.fun_ ~arity:None
+                        (Pat.constraint_ (Pat.var pat_param) core_type)
+                        body))
               in
               let ( +> ) a ty = Exp.constraint_ (erase_type a) ty in
               let ( +: ) a ty = erase_type (Exp.constraint_ a ty) in
@@ -211,12 +214,16 @@ let init () =
                 in
                 let from_js =
                   Ast_comb.single_non_rec_value pat_from_js
-                    (Ast_compatible.fun_ ~arity:None (Pat.var pat_param)
-                       (if create_type then
-                          Exp.let_ Nonrecursive
-                            [Vb.mk (Pat.var pat_param) (exp_param +: new_type)]
-                            (Exp.constraint_ obj_exp core_type)
-                        else Exp.constraint_ obj_exp core_type))
+                    (Ast_uncurried.uncurried_fun ~loc:Location.none ~arity:1
+                       (Ast_compatible.fun_ ~arity:(Some 1) (Pat.var pat_param)
+                          (if create_type then
+                             Exp.let_ Nonrecursive
+                               [
+                                 Vb.mk (Pat.var pat_param)
+                                   (exp_param +: new_type);
+                               ]
+                               (Exp.constraint_ obj_exp core_type)
+                           else Exp.constraint_ obj_exp core_type)))
                 in
                 let rest = [to_js; from_js] in
                 if create_type then erase_type_str :: new_type_str :: rest
@@ -253,12 +260,14 @@ let init () =
                            app2 unsafe_index_get_exp exp_map exp_param
                          else app1 erase_type_exp exp_param);
                       Ast_comb.single_non_rec_value pat_from_js
-                        (Ast_compatible.fun_ ~arity:None (Pat.var pat_param)
-                           (let result =
-                              app2 unsafe_index_get_exp rev_exp_map exp_param
-                            in
-                            if create_type then raise_when_not_found result
-                            else result));
+                        (Ast_uncurried.uncurried_fun ~loc:Location.none ~arity:1
+                           (Ast_compatible.fun_ ~arity:(Some 1)
+                              (Pat.var pat_param)
+                              (let result =
+                                 app2 unsafe_index_get_exp rev_exp_map exp_param
+                               in
+                               if create_type then raise_when_not_found result
+                               else result)));
                     ]
                   in
                   if create_type then new_type_str :: v else v
@@ -285,7 +294,8 @@ let init () =
               let pat_from_js = {Asttypes.loc; txt = from_js} in
               let to_js_type result =
                 Ast_comb.single_non_rec_val pat_to_js
-                  (Ast_compatible.arrow core_type result)
+                  (Ast_uncurried.uncurried_type ~loc:Location.none ~arity:1
+                     (Ast_compatible.arrow core_type result))
               in
               let new_type, new_tdcl =
                 U.new_type_of_type_declaration tdcl ("abs_" ^ name)
diff --git a/tests/tests/src/ast_abstract_test.mjs b/tests/tests/src/ast_abstract_test.mjs
index 4044c5b37f..82fee77942 100644
--- a/tests/tests/src/ast_abstract_test.mjs
+++ b/tests/tests/src/ast_abstract_test.mjs
@@ -68,6 +68,10 @@ function idx(v) {
   eq("File \"ast_abstract_test.res\", line 29, characters 18-25", xFromJs(v), v);
 }
 
+let x0 = "a";
+
+let x1 = "b";
+
 idx("a");
 
 idx("b");
@@ -76,10 +80,6 @@ idx("c");
 
 Mt.from_pair_suites("Ast_abstract_test", suites.contents);
 
-let x0 = "a";
-
-let x1 = "b";
-
 export {
   suites,
   test_id,

From d1d40c266f4ea3263addc2609565c4c50b4ca500 Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Tue, 10 Dec 2024 09:42:34 +0100
Subject: [PATCH 3/6] Clean up arity in derive projector.

The arity is later overridden anyway, but being explicit can help cleaning up later on if `Function$` is removed entirely.
---
 compiler/frontend/ast_derive_projector.ml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/compiler/frontend/ast_derive_projector.ml b/compiler/frontend/ast_derive_projector.ml
index 6884dbe497..86514c4ef0 100644
--- a/compiler/frontend/ast_derive_projector.ml
+++ b/compiler/frontend/ast_derive_projector.ml
@@ -46,7 +46,7 @@ let init () =
                   ->
                     let txt = "param" in
                     Ast_comb.single_non_rec_value ?attrs:gentype_attrs pld_name
-                      (Ast_compatible.fun_ ~arity:None
+                      (Ast_compatible.fun_ ~arity:(Some 1)
                          (Pat.constraint_ (Pat.var {txt; loc}) core_type)
                          (Exp.field
                             (Exp.ident {txt = Lident txt; loc})
@@ -108,7 +108,7 @@ let init () =
                              annotate_type
                          in
                          Ext_list.fold_right vars exp (fun var b ->
-                             Ast_compatible.fun_ ~arity:None
+                             Ast_compatible.fun_ ~arity:(Some 1)
                                (Pat.var {loc; txt = var})
                                b)
                          |> handle_uncurried_accessor_tranform ~loc ~arity))

From c1c8e11253de665ff7f50ea08ccc66b72bb34560 Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Tue, 10 Dec 2024 09:53:24 +0100
Subject: [PATCH 4/6] Most of Untypeast is never used.

---
 compiler/ml/untypeast.ml  | 561 --------------------------------------
 compiler/ml/untypeast.mli |  53 ----
 2 files changed, 614 deletions(-)

diff --git a/compiler/ml/untypeast.ml b/compiler/ml/untypeast.ml
index da064d8d77..1ae24b62dc 100644
--- a/compiler/ml/untypeast.ml
+++ b/compiler/ml/untypeast.ml
@@ -13,95 +13,8 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open Longident
 open Asttypes
 open Parsetree
-open Ast_helper
-
-module T = Typedtree
-
-type mapper = {
-  attribute: mapper -> T.attribute -> attribute;
-  attributes: mapper -> T.attribute list -> attribute list;
-  case: mapper -> T.case -> case;
-  cases: mapper -> T.case list -> case list;
-  constructor_declaration:
-    mapper -> T.constructor_declaration -> constructor_declaration;
-  expr: mapper -> T.expression -> expression;
-  extension_constructor:
-    mapper -> T.extension_constructor -> extension_constructor;
-  include_declaration: mapper -> T.include_declaration -> include_declaration;
-  include_description: mapper -> T.include_description -> include_description;
-  label_declaration: mapper -> T.label_declaration -> label_declaration;
-  location: mapper -> Location.t -> Location.t;
-  module_binding: mapper -> T.module_binding -> module_binding;
-  module_declaration: mapper -> T.module_declaration -> module_declaration;
-  module_expr: mapper -> T.module_expr -> module_expr;
-  module_type: mapper -> T.module_type -> module_type;
-  module_type_declaration:
-    mapper -> T.module_type_declaration -> module_type_declaration;
-  package_type: mapper -> T.package_type -> package_type;
-  open_description: mapper -> T.open_description -> open_description;
-  pat: mapper -> T.pattern -> pattern;
-  row_field: mapper -> T.row_field -> row_field;
-  object_field: mapper -> T.object_field -> object_field;
-  signature: mapper -> T.signature -> signature;
-  signature_item: mapper -> T.signature_item -> signature_item;
-  structure: mapper -> T.structure -> structure;
-  structure_item: mapper -> T.structure_item -> structure_item;
-  typ: mapper -> T.core_type -> core_type;
-  type_declaration: mapper -> T.type_declaration -> type_declaration;
-  type_extension: mapper -> T.type_extension -> type_extension;
-  type_kind: mapper -> T.type_kind -> type_kind;
-  value_binding: mapper -> T.value_binding -> value_binding;
-  value_description: mapper -> T.value_description -> value_description;
-  with_constraint:
-    mapper ->
-    Path.t * Longident.t Location.loc * T.with_constraint ->
-    with_constraint;
-}
-
-open T
-
-(*
-Some notes:
-
-   * For Pexp_function, we cannot go back to the exact original version
-   when there is a default argument, because the default argument is
-   translated in the typer. The code, if printed, will not be parsable because
-   new generated identifiers are not correct.
-
-   * For Pexp_apply, it is unclear whether arguments are reordered, especially
-    when there are optional arguments.
-
-*)
-
-(** Utility functions. *)
-
-let map_opt f = function
-  | None -> None
-  | Some e -> Some (f e)
-
-let rec lident_of_path = function
-  | Path.Pident id -> Longident.Lident (Ident.name id)
-  | Path.Pdot (p, s, _) -> Longident.Ldot (lident_of_path p, s)
-  | Path.Papply (p1, p2) ->
-    Longident.Lapply (lident_of_path p1, lident_of_path p2)
-
-let map_loc sub {loc; txt} = {loc = sub.location sub loc; txt}
-
-(** Try a name [$name$0], check if it's free, if not, increment and repeat. *)
-let fresh_name s env =
-  let rec aux i =
-    let name = s ^ string_of_int i in
-    try
-      let _ = Env.lookup_value (Lident name) env in
-      name
-    with Not_found -> aux (i + 1)
-  in
-  aux 0
-
-(** Mapping functions. *)
 
 let constant = function
   | Const_char c -> Pconst_char c
@@ -112,477 +25,3 @@ let constant = function
   | Const_bigint (sign, i) ->
     Pconst_integer (Bigint_utils.to_string sign i, Some 'n')
   | Const_float f -> Pconst_float (f, None)
-
-let attribute sub (s, p) = (map_loc sub s, p)
-let attributes sub l = List.map (sub.attribute sub) l
-
-let structure sub str = List.map (sub.structure_item sub) str.str_items
-
-let open_description sub od =
-  let loc = sub.location sub od.open_loc in
-  let attrs = sub.attributes sub od.open_attributes in
-  Opn.mk ~loc ~attrs ~override:od.open_override (map_loc sub od.open_txt)
-
-let structure_item sub item =
-  let loc = sub.location sub item.str_loc in
-  let desc =
-    match item.str_desc with
-    | Tstr_eval (exp, attrs) -> Pstr_eval (sub.expr sub exp, attrs)
-    | Tstr_value (rec_flag, list) ->
-      Pstr_value (rec_flag, List.map (sub.value_binding sub) list)
-    | Tstr_primitive vd -> Pstr_primitive (sub.value_description sub vd)
-    | Tstr_type (rec_flag, list) ->
-      Pstr_type (rec_flag, List.map (sub.type_declaration sub) list)
-    | Tstr_typext tyext -> Pstr_typext (sub.type_extension sub tyext)
-    | Tstr_exception ext -> Pstr_exception (sub.extension_constructor sub ext)
-    | Tstr_module mb -> Pstr_module (sub.module_binding sub mb)
-    | Tstr_recmodule list ->
-      Pstr_recmodule (List.map (sub.module_binding sub) list)
-    | Tstr_modtype mtd -> Pstr_modtype (sub.module_type_declaration sub mtd)
-    | Tstr_open od -> Pstr_open (sub.open_description sub od)
-    | Tstr_class () -> Pstr_class ()
-    | Tstr_class_type () -> Pstr_class_type ()
-    | Tstr_include incl -> Pstr_include (sub.include_declaration sub incl)
-    | Tstr_attribute x -> Pstr_attribute x
-  in
-  Str.mk ~loc desc
-
-let value_description sub v =
-  let loc = sub.location sub v.val_loc in
-  let attrs = sub.attributes sub v.val_attributes in
-  Val.mk ~loc ~attrs ~prim:v.val_prim (map_loc sub v.val_name)
-    (sub.typ sub v.val_desc)
-
-let module_binding sub mb =
-  let loc = sub.location sub mb.mb_loc in
-  let attrs = sub.attributes sub mb.mb_attributes in
-  Mb.mk ~loc ~attrs (map_loc sub mb.mb_name) (sub.module_expr sub mb.mb_expr)
-
-let type_parameter sub (ct, v) = (sub.typ sub ct, v)
-
-let type_declaration sub decl =
-  let loc = sub.location sub decl.typ_loc in
-  let attrs = sub.attributes sub decl.typ_attributes in
-  Type.mk ~loc ~attrs
-    ~params:(List.map (type_parameter sub) decl.typ_params)
-    ~cstrs:
-      (List.map
-         (fun (ct1, ct2, loc) ->
-           (sub.typ sub ct1, sub.typ sub ct2, sub.location sub loc))
-         decl.typ_cstrs)
-    ~kind:(sub.type_kind sub decl.typ_kind)
-    ~priv:decl.typ_private
-    ?manifest:(map_opt (sub.typ sub) decl.typ_manifest)
-    (map_loc sub decl.typ_name)
-
-let type_kind sub tk =
-  match tk with
-  | Ttype_abstract -> Ptype_abstract
-  | Ttype_variant list ->
-    Ptype_variant (List.map (sub.constructor_declaration sub) list)
-  | Ttype_record list ->
-    Ptype_record (List.map (sub.label_declaration sub) list)
-  | Ttype_open -> Ptype_open
-
-let constructor_arguments sub = function
-  | Cstr_tuple l -> Pcstr_tuple (List.map (sub.typ sub) l)
-  | Cstr_record l -> Pcstr_record (List.map (sub.label_declaration sub) l)
-
-let constructor_declaration sub cd =
-  let loc = sub.location sub cd.cd_loc in
-  let attrs = sub.attributes sub cd.cd_attributes in
-  Type.constructor ~loc ~attrs
-    ~args:(constructor_arguments sub cd.cd_args)
-    ?res:(map_opt (sub.typ sub) cd.cd_res)
-    (map_loc sub cd.cd_name)
-
-let label_declaration sub ld =
-  let loc = sub.location sub ld.ld_loc in
-  let attrs = sub.attributes sub ld.ld_attributes in
-  Type.field ~loc ~attrs ~mut:ld.ld_mutable ~optional:ld.ld_optional
-    (map_loc sub ld.ld_name) (sub.typ sub ld.ld_type)
-
-let type_extension sub tyext =
-  let attrs = sub.attributes sub tyext.tyext_attributes in
-  Te.mk ~attrs
-    ~params:(List.map (type_parameter sub) tyext.tyext_params)
-    ~priv:tyext.tyext_private
-    (map_loc sub tyext.tyext_txt)
-    (List.map (sub.extension_constructor sub) tyext.tyext_constructors)
-
-let extension_constructor sub ext =
-  let loc = sub.location sub ext.ext_loc in
-  let attrs = sub.attributes sub ext.ext_attributes in
-  Te.constructor ~loc ~attrs (map_loc sub ext.ext_name)
-    (match ext.ext_kind with
-    | Text_decl (args, ret) ->
-      Pext_decl (constructor_arguments sub args, map_opt (sub.typ sub) ret)
-    | Text_rebind (_p, lid) -> Pext_rebind (map_loc sub lid))
-
-let pattern sub pat =
-  let loc = sub.location sub pat.pat_loc in
-  (* todo: fix attributes on extras *)
-  let attrs = sub.attributes sub pat.pat_attributes in
-  let desc =
-    match pat with
-    | {pat_extra = [(Tpat_unpack, _, _attrs)]; pat_desc = Tpat_var (_, name); _}
-      ->
-      Ppat_unpack name
-    | {pat_extra = [(Tpat_type (_path, lid), _, _attrs)]; _} ->
-      Ppat_type (map_loc sub lid)
-    | {pat_extra = (Tpat_constraint ct, _, _attrs) :: rem; _} ->
-      Ppat_constraint (sub.pat sub {pat with pat_extra = rem}, sub.typ sub ct)
-    | _ -> (
-      match pat.pat_desc with
-      | Tpat_any -> Ppat_any
-      | Tpat_var (id, name) -> (
-        match (Ident.name id).[0] with
-        | 'A' .. 'Z' -> Ppat_unpack name
-        | _ -> Ppat_var name)
-      (* We transform (_ as x) in x if _ and x have the same location.
-         The compiler transforms (x:t) into (_ as x : t).
-         This avoids transforming a warning 27 into a 26.
-      *)
-      | Tpat_alias ({pat_desc = Tpat_any; pat_loc}, _id, name)
-        when pat_loc = pat.pat_loc ->
-        Ppat_var name
-      | Tpat_alias (pat, _id, name) -> Ppat_alias (sub.pat sub pat, name)
-      | Tpat_constant cst -> Ppat_constant (constant cst)
-      | Tpat_tuple list -> Ppat_tuple (List.map (sub.pat sub) list)
-      | Tpat_construct (lid, _, args) ->
-        Ppat_construct
-          ( map_loc sub lid,
-            match args with
-            | [] -> None
-            | [arg] -> Some (sub.pat sub arg)
-            | args -> Some (Pat.tuple ~loc (List.map (sub.pat sub) args)) )
-      | Tpat_variant (label, pato, _) ->
-        Ppat_variant (label, map_opt (sub.pat sub) pato)
-      | Tpat_record (list, closed) ->
-        Ppat_record
-          ( List.map
-              (fun (lid, _, pat, opt) ->
-                (map_loc sub lid, sub.pat sub pat, opt))
-              list,
-            closed )
-      | Tpat_array list -> Ppat_array (List.map (sub.pat sub) list)
-      | Tpat_or (p1, p2, _) -> Ppat_or (sub.pat sub p1, sub.pat sub p2)
-      | Tpat_lazy p -> Ppat_lazy (sub.pat sub p))
-  in
-  Pat.mk ~loc ~attrs desc
-
-let exp_extra sub (extra, loc, attrs) sexp =
-  let loc = sub.location sub loc in
-  let attrs = sub.attributes sub attrs in
-  let desc =
-    match extra with
-    | Texp_coerce ((), cty2) -> Pexp_coerce (sexp, (), sub.typ sub cty2)
-    | Texp_constraint cty -> Pexp_constraint (sexp, sub.typ sub cty)
-    | Texp_open (ovf, _path, lid, _) -> Pexp_open (ovf, map_loc sub lid, sexp)
-    | Texp_poly cto -> Pexp_poly (sexp, map_opt (sub.typ sub) cto)
-    | Texp_newtype s -> Pexp_newtype (mkloc s loc, sexp)
-  in
-  Exp.mk ~loc ~attrs desc
-
-let cases sub l = List.map (sub.case sub) l
-
-let case sub {c_lhs; c_guard; c_rhs} =
-  {
-    pc_lhs = sub.pat sub c_lhs;
-    pc_guard = map_opt (sub.expr sub) c_guard;
-    pc_rhs = sub.expr sub c_rhs;
-  }
-
-let value_binding sub vb =
-  let loc = sub.location sub vb.vb_loc in
-  let attrs = sub.attributes sub vb.vb_attributes in
-  Vb.mk ~loc ~attrs (sub.pat sub vb.vb_pat) (sub.expr sub vb.vb_expr)
-
-let expression sub exp =
-  let loc = sub.location sub exp.exp_loc in
-  let attrs = sub.attributes sub exp.exp_attributes in
-  let desc =
-    match exp.exp_desc with
-    | Texp_ident (_path, lid, _) -> Pexp_ident (map_loc sub lid)
-    | Texp_constant cst -> Pexp_constant (constant cst)
-    | Texp_let (rec_flag, list, exp) ->
-      Pexp_let
-        (rec_flag, List.map (sub.value_binding sub) list, sub.expr sub exp)
-    (* Pexp_function can't have a label, so we split in 3 cases. *)
-    (* One case, no guard: It's a fun. *)
-    | Texp_function
-        {arg_label; cases = [{c_lhs = p; c_guard = None; c_rhs = e}]; _} ->
-      let arity = assert false in
-      Pexp_fun (arg_label, None, sub.pat sub p, sub.expr sub e, arity)
-    (* No label: it's a function. *)
-    | Texp_function {arg_label = Nolabel; cases; _} ->
-      Pexp_function (sub.cases sub cases)
-    (* Mix of both, we generate `fun ~label:$name$ -> match $name$ with ...` *)
-    | Texp_function {arg_label = (Labelled s | Optional s) as label; cases; _}
-      ->
-      let name = fresh_name s exp.exp_env in
-      let arity = assert false in
-      Pexp_fun
-        ( label,
-          None,
-          Pat.var ~loc {loc; txt = name},
-          Exp.match_ ~loc
-            (Exp.ident ~loc {loc; txt = Lident name})
-            (sub.cases sub cases),
-          arity )
-    | Texp_apply (exp, list) ->
-      Pexp_apply
-        ( sub.expr sub exp,
-          List.fold_right
-            (fun (label, expo) list ->
-              match expo with
-              | None -> list
-              | Some exp -> (label, sub.expr sub exp) :: list)
-            list [] )
-    | Texp_match (exp, cases, exn_cases, _) ->
-      let merged_cases =
-        sub.cases sub cases
-        @ List.map
-            (fun c ->
-              let uc = sub.case sub c in
-              let pat = {uc.pc_lhs with ppat_desc = Ppat_exception uc.pc_lhs} in
-              {uc with pc_lhs = pat})
-            exn_cases
-      in
-      Pexp_match (sub.expr sub exp, merged_cases)
-    | Texp_try (exp, cases) -> Pexp_try (sub.expr sub exp, sub.cases sub cases)
-    | Texp_tuple list -> Pexp_tuple (List.map (sub.expr sub) list)
-    | Texp_construct (lid, _, args) ->
-      Pexp_construct
-        ( map_loc sub lid,
-          match args with
-          | [] -> None
-          | [arg] -> Some (sub.expr sub arg)
-          | args -> Some (Exp.tuple ~loc (List.map (sub.expr sub) args)) )
-    | Texp_variant (label, expo) ->
-      Pexp_variant (label, map_opt (sub.expr sub) expo)
-    | Texp_record {fields; extended_expression; _} ->
-      let list =
-        Array.fold_left
-          (fun l -> function
-            | _, Kept _, _ -> l
-            | _, Overridden (lid, exp), opt -> (lid, sub.expr sub exp, opt) :: l)
-          [] fields
-      in
-      Pexp_record (list, map_opt (sub.expr sub) extended_expression)
-    | Texp_field (exp, lid, _label) ->
-      Pexp_field (sub.expr sub exp, map_loc sub lid)
-    | Texp_setfield (exp1, lid, _label, exp2) ->
-      Pexp_setfield (sub.expr sub exp1, map_loc sub lid, sub.expr sub exp2)
-    | Texp_array list -> Pexp_array (List.map (sub.expr sub) list)
-    | Texp_ifthenelse (exp1, exp2, expo) ->
-      Pexp_ifthenelse
-        (sub.expr sub exp1, sub.expr sub exp2, map_opt (sub.expr sub) expo)
-    | Texp_sequence (exp1, exp2) ->
-      Pexp_sequence (sub.expr sub exp1, sub.expr sub exp2)
-    | Texp_while (exp1, exp2) ->
-      Pexp_while (sub.expr sub exp1, sub.expr sub exp2)
-    | Texp_for (_id, name, exp1, exp2, dir, exp3) ->
-      Pexp_for
-        (name, sub.expr sub exp1, sub.expr sub exp2, dir, sub.expr sub exp3)
-    | Texp_send (exp, meth, _) ->
-      Pexp_send
-        ( sub.expr sub exp,
-          match meth with
-          | Tmeth_name name -> mkloc name loc )
-    | Texp_new _ | Texp_instvar _ | Texp_setinstvar _ | Texp_override _ ->
-      assert false
-    | Texp_letmodule (_id, name, mexpr, exp) ->
-      Pexp_letmodule (name, sub.module_expr sub mexpr, sub.expr sub exp)
-    | Texp_letexception (ext, exp) ->
-      Pexp_letexception (sub.extension_constructor sub ext, sub.expr sub exp)
-    | Texp_assert exp -> Pexp_assert (sub.expr sub exp)
-    | Texp_lazy exp -> Pexp_lazy (sub.expr sub exp)
-    | Texp_pack mexpr -> Pexp_pack (sub.module_expr sub mexpr)
-    | Texp_extension_constructor (lid, _) ->
-      Pexp_extension
-        ( {txt = "ocaml.extension_constructor"; loc},
-          PStr [Str.eval ~loc (Exp.construct ~loc (map_loc sub lid) None)] )
-  in
-  List.fold_right (exp_extra sub) exp.exp_extra (Exp.mk ~loc ~attrs desc)
-
-let package_type sub pack =
-  ( map_loc sub pack.pack_txt,
-    List.map (fun (s, ct) -> (s, sub.typ sub ct)) pack.pack_fields )
-
-let module_type_declaration sub mtd =
-  let loc = sub.location sub mtd.mtd_loc in
-  let attrs = sub.attributes sub mtd.mtd_attributes in
-  Mtd.mk ~loc ~attrs
-    ?typ:(map_opt (sub.module_type sub) mtd.mtd_type)
-    (map_loc sub mtd.mtd_name)
-
-let signature sub sg = List.map (sub.signature_item sub) sg.sig_items
-
-let signature_item sub item =
-  let loc = sub.location sub item.sig_loc in
-  let desc =
-    match item.sig_desc with
-    | Tsig_value v -> Psig_value (sub.value_description sub v)
-    | Tsig_type (rec_flag, list) ->
-      Psig_type (rec_flag, List.map (sub.type_declaration sub) list)
-    | Tsig_typext tyext -> Psig_typext (sub.type_extension sub tyext)
-    | Tsig_exception ext -> Psig_exception (sub.extension_constructor sub ext)
-    | Tsig_module md -> Psig_module (sub.module_declaration sub md)
-    | Tsig_recmodule list ->
-      Psig_recmodule (List.map (sub.module_declaration sub) list)
-    | Tsig_modtype mtd -> Psig_modtype (sub.module_type_declaration sub mtd)
-    | Tsig_open od -> Psig_open (sub.open_description sub od)
-    | Tsig_include incl -> Psig_include (sub.include_description sub incl)
-    | Tsig_class () -> Psig_class ()
-    | Tsig_class_type () -> Psig_class_type ()
-    | Tsig_attribute x -> Psig_attribute x
-  in
-  Sig.mk ~loc desc
-
-let module_declaration sub md =
-  let loc = sub.location sub md.md_loc in
-  let attrs = sub.attributes sub md.md_attributes in
-  Md.mk ~loc ~attrs (map_loc sub md.md_name) (sub.module_type sub md.md_type)
-
-let include_infos f sub incl =
-  let loc = sub.location sub incl.incl_loc in
-  let attrs = sub.attributes sub incl.incl_attributes in
-  Incl.mk ~loc ~attrs (f sub incl.incl_mod)
-
-let include_declaration sub = include_infos sub.module_expr sub
-let include_description sub = include_infos sub.module_type sub
-
-let module_type sub mty =
-  let loc = sub.location sub mty.mty_loc in
-  let attrs = sub.attributes sub mty.mty_attributes in
-  let desc =
-    match mty.mty_desc with
-    | Tmty_ident (_path, lid) -> Pmty_ident (map_loc sub lid)
-    | Tmty_alias (_path, lid) -> Pmty_alias (map_loc sub lid)
-    | Tmty_signature sg -> Pmty_signature (sub.signature sub sg)
-    | Tmty_functor (_id, name, mtype1, mtype2) ->
-      Pmty_functor
-        (name, map_opt (sub.module_type sub) mtype1, sub.module_type sub mtype2)
-    | Tmty_with (mtype, list) ->
-      Pmty_with
-        (sub.module_type sub mtype, List.map (sub.with_constraint sub) list)
-    | Tmty_typeof mexpr -> Pmty_typeof (sub.module_expr sub mexpr)
-  in
-  Mty.mk ~loc ~attrs desc
-
-let with_constraint sub (_path, lid, cstr) =
-  match cstr with
-  | Twith_type decl ->
-    Pwith_type (map_loc sub lid, sub.type_declaration sub decl)
-  | Twith_module (_path, lid2) ->
-    Pwith_module (map_loc sub lid, map_loc sub lid2)
-  | Twith_typesubst decl ->
-    Pwith_typesubst (map_loc sub lid, sub.type_declaration sub decl)
-  | Twith_modsubst (_path, lid2) ->
-    Pwith_modsubst (map_loc sub lid, map_loc sub lid2)
-
-let module_expr sub mexpr =
-  let loc = sub.location sub mexpr.mod_loc in
-  let attrs = sub.attributes sub mexpr.mod_attributes in
-  match mexpr.mod_desc with
-  | Tmod_constraint (m, _, Tmodtype_implicit, _) -> sub.module_expr sub m
-  | _ ->
-    let desc =
-      match mexpr.mod_desc with
-      | Tmod_ident (_p, lid) -> Pmod_ident (map_loc sub lid)
-      | Tmod_structure st -> Pmod_structure (sub.structure sub st)
-      | Tmod_functor (_id, name, mtype, mexpr) ->
-        Pmod_functor
-          ( name,
-            Misc.may_map (sub.module_type sub) mtype,
-            sub.module_expr sub mexpr )
-      | Tmod_apply (mexp1, mexp2, _) ->
-        Pmod_apply (sub.module_expr sub mexp1, sub.module_expr sub mexp2)
-      | Tmod_constraint (mexpr, _, Tmodtype_explicit mtype, _) ->
-        Pmod_constraint (sub.module_expr sub mexpr, sub.module_type sub mtype)
-      | Tmod_constraint (_mexpr, _, Tmodtype_implicit, _) -> assert false
-      | Tmod_unpack (exp, _pack) -> Pmod_unpack (sub.expr sub exp)
-      (* TODO , sub.package_type sub pack) *)
-    in
-    Mod.mk ~loc ~attrs desc
-
-let core_type sub ct =
-  let loc = sub.location sub ct.ctyp_loc in
-  let attrs = sub.attributes sub ct.ctyp_attributes in
-  let desc =
-    match ct.ctyp_desc with
-    | Ttyp_any -> Ptyp_any
-    | Ttyp_var s -> Ptyp_var s
-    | Ttyp_arrow (label, ct1, ct2) ->
-      Ptyp_arrow (label, sub.typ sub ct1, sub.typ sub ct2)
-    | Ttyp_tuple list -> Ptyp_tuple (List.map (sub.typ sub) list)
-    | Ttyp_constr (_path, lid, list) ->
-      Ptyp_constr (map_loc sub lid, List.map (sub.typ sub) list)
-    | Ttyp_object (list, o) ->
-      Ptyp_object (List.map (sub.object_field sub) list, o)
-    | Ttyp_alias (ct, s) -> Ptyp_alias (sub.typ sub ct, s)
-    | Ttyp_variant (list, bool, labels) ->
-      Ptyp_variant (List.map (sub.row_field sub) list, bool, labels)
-    | Ttyp_poly (list, ct) ->
-      let list = List.map (fun v -> mkloc v loc) list in
-      Ptyp_poly (list, sub.typ sub ct)
-    | Ttyp_package pack -> Ptyp_package (sub.package_type sub pack)
-  in
-  Typ.mk ~loc ~attrs desc
-
-let row_field sub rf =
-  match rf with
-  | Ttag (label, attrs, bool, list) ->
-    Rtag (label, sub.attributes sub attrs, bool, List.map (sub.typ sub) list)
-  | Tinherit ct -> Rinherit (sub.typ sub ct)
-
-let object_field sub ofield =
-  match ofield with
-  | OTtag (label, attrs, ct) ->
-    Otag (label, sub.attributes sub attrs, sub.typ sub ct)
-  | OTinherit ct -> Oinherit (sub.typ sub ct)
-
-let location _sub l = l
-
-let default_mapper =
-  {
-    attribute;
-    attributes;
-    structure;
-    structure_item;
-    module_expr;
-    signature;
-    signature_item;
-    module_type;
-    with_constraint;
-    type_declaration;
-    type_kind;
-    typ = core_type;
-    type_extension;
-    extension_constructor;
-    value_description;
-    pat = pattern;
-    expr = expression;
-    module_declaration;
-    module_type_declaration;
-    module_binding;
-    package_type;
-    open_description;
-    include_description;
-    include_declaration;
-    value_binding;
-    constructor_declaration;
-    label_declaration;
-    cases;
-    case;
-    location;
-    row_field;
-    object_field;
-  }
-
-let untype_structure ?(mapper = default_mapper) structure =
-  mapper.structure mapper structure
-
-let untype_signature ?(mapper = default_mapper) signature =
-  mapper.signature mapper signature
diff --git a/compiler/ml/untypeast.mli b/compiler/ml/untypeast.mli
index a86e69be65..b4471a1ac6 100644
--- a/compiler/ml/untypeast.mli
+++ b/compiler/ml/untypeast.mli
@@ -13,57 +13,4 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open Parsetree
-
-val lident_of_path : Path.t -> Longident.t
-
-type mapper = {
-  attribute: mapper -> Typedtree.attribute -> attribute;
-  attributes: mapper -> Typedtree.attribute list -> attribute list;
-  case: mapper -> Typedtree.case -> case;
-  cases: mapper -> Typedtree.case list -> case list;
-  constructor_declaration:
-    mapper -> Typedtree.constructor_declaration -> constructor_declaration;
-  expr: mapper -> Typedtree.expression -> expression;
-  extension_constructor:
-    mapper -> Typedtree.extension_constructor -> extension_constructor;
-  include_declaration:
-    mapper -> Typedtree.include_declaration -> include_declaration;
-  include_description:
-    mapper -> Typedtree.include_description -> include_description;
-  label_declaration: mapper -> Typedtree.label_declaration -> label_declaration;
-  location: mapper -> Location.t -> Location.t;
-  module_binding: mapper -> Typedtree.module_binding -> module_binding;
-  module_declaration:
-    mapper -> Typedtree.module_declaration -> module_declaration;
-  module_expr: mapper -> Typedtree.module_expr -> module_expr;
-  module_type: mapper -> Typedtree.module_type -> module_type;
-  module_type_declaration:
-    mapper -> Typedtree.module_type_declaration -> module_type_declaration;
-  package_type: mapper -> Typedtree.package_type -> package_type;
-  open_description: mapper -> Typedtree.open_description -> open_description;
-  pat: mapper -> Typedtree.pattern -> pattern;
-  row_field: mapper -> Typedtree.row_field -> row_field;
-  object_field: mapper -> Typedtree.object_field -> object_field;
-  signature: mapper -> Typedtree.signature -> signature;
-  signature_item: mapper -> Typedtree.signature_item -> signature_item;
-  structure: mapper -> Typedtree.structure -> structure;
-  structure_item: mapper -> Typedtree.structure_item -> structure_item;
-  typ: mapper -> Typedtree.core_type -> core_type;
-  type_declaration: mapper -> Typedtree.type_declaration -> type_declaration;
-  type_extension: mapper -> Typedtree.type_extension -> type_extension;
-  type_kind: mapper -> Typedtree.type_kind -> type_kind;
-  value_binding: mapper -> Typedtree.value_binding -> value_binding;
-  value_description: mapper -> Typedtree.value_description -> value_description;
-  with_constraint:
-    mapper ->
-    Path.t * Longident.t Location.loc * Typedtree.with_constraint ->
-    with_constraint;
-}
-
-val default_mapper : mapper
-
-val untype_structure : ?mapper:mapper -> Typedtree.structure -> structure
-val untype_signature : ?mapper:mapper -> Typedtree.signature -> signature
-
 val constant : Asttypes.constant -> Parsetree.constant

From dfb4b2e4ec5c268e8993246738e6f81e9c403799 Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Tue, 10 Dec 2024 10:16:41 +0100
Subject: [PATCH 5/6] More explicit translation for `Function$`.

Cosmetic change that will be needed later to remove `Function$`.
---
 compiler/ml/translcore.ml | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/compiler/ml/translcore.ml b/compiler/ml/translcore.ml
index 2e1afdfa29..1fdb458a0a 100644
--- a/compiler/ml/translcore.ml
+++ b/compiler/ml/translcore.ml
@@ -823,7 +823,9 @@ and transl_exp0 (e : Typedtree.expression) : Lambda.lambda =
     with Not_constant -> Lprim (Pmakeblock Blk_tuple, ll, e.exp_loc))
   | Texp_construct ({txt = Lident "false"}, _, []) -> Lconst Const_false
   | Texp_construct ({txt = Lident "true"}, _, []) -> Lconst Const_true
-  | Texp_construct ({txt = Lident "Function$"}, _, [expr]) ->
+  | Texp_construct
+      ({txt = Lident "Function$"}, _, [({exp_desc = Texp_function _} as expr)])
+    ->
     (* ReScript uncurried encoding *)
     let loc = expr.exp_loc in
     let lambda = transl_exp expr in

From 0ba5b76e86962a5ad176577cf91844b3436e245b Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Tue, 10 Dec 2024 16:36:14 +0100
Subject: [PATCH 6/6] Update CHANGELOG.md

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2e6e4f8f00..e9bbc2ec47 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,7 @@
 - Ast cleanup: remove exp object and exp unreachable. https://github.com/rescript-lang/rescript/pull/7189
 - Ast cleanup: explicit representation for optional record fields in types. https://github.com/rescript-lang/rescript/pull/7190 https://github.com/rescript-lang/rescript/pull/7191
 - AST cleanup: first-class expression and patterns for records with optional fields. https://github.com/rescript-lang/rescript/pull/7192
+- AST cleanup: Represent the arity of uncurried function definitions directly in the AST. https://github.com/rescript-lang/rescript/pull/7197
 
 
 # 12.0.0-alpha.5