Unable to resolve unapply

Question

object micro extends App {

  sealed trait FuncExpr

  sealed trait FuncSpecialize

  sealed case class Func(i:Int) extends FuncExpr
  sealed case class Cube(b:Boolean) extends FuncSpecialize

  object Cube {
    def unapply(arg:Func) : Option[Boolean] = arg match {
      case Func(i) if i == 42 => Some(true)
      case _ => None
    }
  }

  val x : FuncSpecialize = Cube(true)
  x match {
    case Cube(b) => println("Success")
  }
}

This is a minimized example in which i try to implement a custom unapply method for case class Cube together with the default unapply method. However if try to pattern match x i get the following error message which i would not expect to happen because Func and FuncSpezialize are completely different types:

Error:(19, 10) cannot resolve overloaded unapply
    case Cube(b) => println("Success")
Error:(19, 10) overloaded method value unapply with alternatives:
  (x$0: micro.Cube)Option[Boolean] <and>
  (arg: micro.Func)Option[Boolean]
 cannot be applied to (micro.FuncSpecialize)
    case Cube(b) => println("Success")

Any idea, why this is the case?

score 1 · Accepted Answer · answered May 09 '19 at 15:19

1

Since Cube is a case class, compiler generated unapply for it

  object Cube {
    // auto-generated
    def unapply(arg: Cube): Option[Boolean] = Some(arg.b)

    // custom
    def unapply(arg:Func) : Option[Boolean] = arg match {
      case Func(i) if i == 42 => Some(true)
      case _ => None
    }
  }

Now when you try to match pattern

  val x : FuncSpecialize = Cube(true)
  x match {
    case Cube(b) => println("Success")
  }

it's actually a call Cube.unapply(x). You can't apply any of unapply methods to x of type FuncSpecialize.

answered May 09 '19 at 15:19

Dmytro Mitin

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if i delete the custom unapply method, the pattern matching however works fine . Shouldn't that fail as well regarding your explanation? Thx – Tilman Zuckmantel May 09 '19 at 15:28
@TilmanZuckmantel it doesn't work fine. It compiles but fails with `MatchError` at runtime. – Dmytro Mitin May 09 '19 at 15:33
If I delete the custom unapply it compiles and prints success for me. – Tilman Zuckmantel May 09 '19 at 15:36
@TilmanZuckmantel `Cube` is still a case class. So `unapply` is still generated by compiler. – Dmytro Mitin May 09 '19 at 15:39
yes but the default implementation is this right? `def unapply(arg: Cube): Option[Boolean] = Some(arg.b)` so this should still be there if i add a custom unapply? – Tilman Zuckmantel May 09 '19 at 15:40
1

@TilmanZuckmantel https://stackoverflow.com/questions/24227037/unapply-method-of-a-case-class-is-not-used-by-the-scala-compiler-to-do-pattern-m – Dmytro Mitin May 09 '19 at 15:55
https://stackoverflow.com/questions/39004088/how-to-reproduce-case-class-behaviour-with-apply-unapply-methods – Dmytro Mitin May 09 '19 at 16:20

Unable to resolve unapply

1 Answers1