Union types are approximated too often, especially in the REPL

[XuefengWu]

union type works well in container type.

scala> Seq[Int|Double](1)
res25: ([+X0] -> scala.collection.Seq[X0])[Int | Double] = List(1)
scala> Seq[Int|Double](1)(0)
res26: Int | Double = 1
scala> if(true) then Seq[Int|Double](1)(0) else Seq[Int|Double](1.1)(0)
res27: Int | Double = 1

but if union type value is not in container, it's result type is Any

scala> 1: Int | String
res0: Any = 1

and it also doest work for method.

  def f(x: Int): String | Int | Double = {
  if(x == 1) x.toString 
  else if (x == 2) x.toInt  
  else x.toDouble }

f: (x: Int)String | Int | Double
scala> f(1)
res1: Any = 1

the result type should be union type.

[smarter]

Yes, we widen union types too much as discussed in #1495

For now, you can run the compiler with -language:keepUnions (though there's a reason this is not the default, see http://guillaume.martres.me/talks/dotty-tutorial/#/1/13 (use the ↓ arrow key to see the full slide)), you can run the repl with arguments like this: dotc -repl -language:keepUnions

[XuefengWu]

Thank you for explanation.

On Thu, Sep 22, 2016 at 11:05 PM, Guillaume Martres <
[email protected]> wrote:

Yes, we widen union types too much as discussed in #1495
#1495

For now, you can run the compiler with -language:keepUnions (though
there's a reason this is not the default, see http://guillaume.martres.me/
talks/dotty-tutorial/#/1/13 (use the ↓ arrow key to see the full slide)),
you can run the repl with arguments like this: dotc -repl
-language:keepUnions

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[smarter]

Closing since this is unlikely to change for now. As a workaround you can define an identity type and use Identity[Foo | Bar] instead of Foo | Bar.

scala> type Identity[T] = T 
// defined alias type Identity = [T] => T
scala> 1: Identity[Int | String] 
val res1: Identity[Int | String] = 1