Merge pull request #4207 from armanbilge/issue/3687

Make `whenA`/`unlessA` syntax by-name lazy

Author: armanbilge

core/src/main/scala/cats/syntax/applicative.scala

@@ -25,16 +25,31 @@ package syntax
 trait ApplicativeSyntax {
   implicit final def catsSyntaxApplicativeId[A](a: A): ApplicativeIdOps[A] =
     new ApplicativeIdOps[A](a)
-  implicit final def catsSyntaxApplicative[F[_], A](fa: F[A]): ApplicativeOps[F, A] =
+  implicit final def catsSyntaxApplicativeByName[F[_], A](fa: => F[A]): ApplicativeByNameOps[F, A] =
+    new ApplicativeByNameOps[F, A](() => fa)
+  implicit final def catsSyntaxApplicativeByValue[F[_], A](fa: F[A]): ApplicativeByValueOps[F, A] =
+    new ApplicativeByValueOps[F, A](fa)
+  @deprecated("Use by-value or by-name version", "2.8.0")
+  final def catsSyntaxApplicative[F[_], A](fa: F[A]): ApplicativeOps[F, A] =
     new ApplicativeOps[F, A](fa)
 }
 
 final class ApplicativeIdOps[A](private val a: A) extends AnyVal {
   def pure[F[_]](implicit F: Applicative[F]): F[A] = F.pure(a)
 }
 
+@deprecated("Use by-value or by-name version", "2.8.0")
 final class ApplicativeOps[F[_], A](private val fa: F[A]) extends AnyVal {
   def replicateA(n: Int)(implicit F: Applicative[F]): F[List[A]] = F.replicateA(n, fa)
   def unlessA(cond: Boolean)(implicit F: Applicative[F]): F[Unit] = F.unlessA(cond)(fa)
   def whenA(cond: Boolean)(implicit F: Applicative[F]): F[Unit] = F.whenA(cond)(fa)
 }
+
+final class ApplicativeByValueOps[F[_], A](private val fa: F[A]) extends AnyVal {
+  def replicateA(n: Int)(implicit F: Applicative[F]): F[List[A]] = F.replicateA(n, fa)
+}
+
+final class ApplicativeByNameOps[F[_], A](private val fa: () => F[A]) extends AnyVal {
+  def unlessA(cond: Boolean)(implicit F: Applicative[F]): F[Unit] = F.unlessA(cond)(fa())
+  def whenA(cond: Boolean)(implicit F: Applicative[F]): F[Unit] = F.whenA(cond)(fa())
+}

tests/shared/src/test/scala/cats/tests/ApplicativeSuite.scala

@@ -63,6 +63,16 @@ class ApplicativeSuite extends CatsSuite {
     }
   }
 
+  test("by-name ops are lazy") {
+    var i = 0
+    Option(i += 1).whenA(false)
+    assertEquals(i, 0)
+
+    var j = 0
+    Option(j += 1).unlessA(true)
+    assertEquals(j, 0)
+  }
+
   {
     implicit val optionMonoid: Monoid[Option[Int]] = Applicative.monoid[Option, Int]
     checkAll("Applicative[Option].monoid", MonoidTests[Option[Int]](optionMonoid).monoid)