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Bug when pattern matching on existential values #9247

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scabug opened this issue Mar 24, 2015 · 4 comments
Open

Bug when pattern matching on existential values #9247

scabug opened this issue Mar 24, 2015 · 4 comments
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fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) patmat
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Backlog

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@scabug
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scabug commented Mar 24, 2015 •
edited by adriaanm

Existentials as arguments to unapply result in a compilation error:

Minimal example:

  trait Match {
    type X
    def unapply(x: X): Option[String]
  }

  def foo(m: Match): m.X => String = {
    case m(s) => s
  }

yields

an unapply result must have a member `def isEmpty: Boolean
[error]     case m(s) => s
[error]          ^
[error] one error found
[error] (compile:compile) Compilation failed

In contrast, a variant using universal types works just fine:

  trait Match[X] {
    def unapply(x: X): Option[String]
  }

  def foo[X](m: Match[X]): X => String = {
    case m(s) => s
  }

Might be related to the following issues:
@scabug
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scabug commented Mar 24, 2015

Imported From: https://issues.scala-lang.org/browse/SI-9247?orig=1
Reporter: Jonathan Brachthäuser (b-studios)
Affected Versions: 2.11.3, 2.11.4, 2.11.5, 2.11.6

@scabug
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scabug commented Aug 11, 2016

Julien Richard-Foy (julienrf) said (edited on Aug 11, 2016 11:21:42 AM UTC):
Is there a workaround for this issue?

@scabug
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scabug commented Aug 11, 2016

@paulp said:
Workaround:

  def foo[A](m: Match { type X = A }): A => String = {
    case m(s) => s
  }

@scabug
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scabug commented Aug 12, 2016

Jonathan Brachthäuser (b-studios) said:
Workaround 2: Use dotty.

Just tried with 0.1-SNAPSHOT and it works perfectly fine.

@scabug scabug added the patmat label Apr 7, 2017
@scabug scabug added this to the Backlog milestone Apr 7, 2017
@adriaanm adriaanm removed their assignment Sep 28, 2018
@smarter smarter added the fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) label Oct 19, 2018
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