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Path-dep types in case class constructor #7930

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scabug opened this issue Oct 23, 2013 · 2 comments
Closed

Path-dep types in case class constructor #7930

scabug opened this issue Oct 23, 2013 · 2 comments
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scabug commented Oct 23, 2013

Given:

trait A { type B }

I don't see why this shouldn't compile:

case class DoesntWork(a: A)(b: a.type#B)

Instead I have to do the same thing manually a la:

class Blah(val a: A, rawb: Any) { def b = rawb.asInstanceOf[a.type#B] }
object Blah { def apply(a: A)(b: a.type#B) = new Blah(a, b) }

// Give it a try
val x = new A { type B=Int }
Blah(x)(123) // should work
Blah(x)("should fail")
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scabug commented Oct 23, 2013

Imported From: https://issues.scala-lang.org/browse/SI-7930?orig=1
Reporter: David Barri (japgolly)
Affected Versions: 2.10.3
Duplicates #5712

@scabug scabug closed this as completed Oct 23, 2013
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scabug commented Oct 23, 2013

@retronym said:
Duplicate of #5712.

So far we only have dependent method types; constructors are not supported. It's surprisingly tricky to implement this unfortunately.

Here's a pattern to workaround:

scala> class A { class B }
defined class A

scala> object O1 extends A; object O2 extends A
defined module O1
defined module O2

scala> object Test { class Blah[AA <: A](val a: AA, b: AA#B); object Blah { def apply(a: A)(b: a.B) = new Blah[a.type](a, b) } }
defined module Test

scala> Test.Blah(O1)(new O1.B)
res2: Test.Blah[O1.type] = Test$Blah@54becf73

scala> Test.Blah(O1)(new O2.B)
<console>:15: error: type mismatch;
 found   : O2.B
 required: O1.B
              Test.Blah(O1)(new O2.B)
                            ^

Could I ask that you please check for duplicate tickets, either by searching JIRA or asking on the mailing list, before lodging new ones? Thanks.

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