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Pattern matching and path dependent types

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Pavel Perikov

Pattern matching and path dependent types

It seem like pattern matching looses type information. I can't instantly say if this a fundamental limitation or the implementation limitation. Can anyone with better understanding of Scala's type system comment on this?

The code is on gist: https://gist.github.com/p-pavel/24e63bb9f40ea95fd988787a35ad4d80 :

trait A {
	type T
	val t : T
	}

	object A {
	def unapply(arg: A): Option[arg.T] = Some(arg.t)
	}

	def use(a : A) = a match {
	case A(t) ⇒
	val s: a.T = t // type mismatch.
	// found t.type (with underlying type <unapply-selector>.T)
	// required: a.T
	}

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martin odersky-2

Re: Pattern matching and path dependent types

It looks very reasonable to recover that type information. However, it would need a particular rule in the specification and implementation of pattern matching. A priori, there is no relationship specified between the selector `a` and the result of the unapply. This would be something worthwhile to work on, generally pattern matching is still too underspecified.

- Martin

On Thu, Feb 23, 2017 at 1:47 PM, Pavel Perikov <[hidden email]> wrote:

It seem like pattern matching looses type information. I can't instantly say if this a fundamental limitation or the implementation limitation. Can anyone with better understanding of Scala's type system comment on this?

The code is on gist: https://gist.github.com/p-pavel/24e63bb9f40ea95fd988787a35ad4d80 :

trait A {
type T
val t : T
}

object A {
def unapply(arg: A): Option[arg.T] = Some(arg.t)
}

def use(a : A) = a match {
case A(t) ⇒
val s: a.T = t // type mismatch.
// found t.type (with underlying type <unapply-selector>.T)
// required: a.T
}

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Martin Odersky
EPFL and Lightbend