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Use TypeTags when pattern-matching otherwise erased types #6517

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scabug opened this issue Oct 11, 2012 · 9 comments
Open

Use TypeTags when pattern-matching otherwise erased types #6517

scabug opened this issue Oct 11, 2012 · 9 comments

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@scabug
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scabug commented Oct 11, 2012

When the scrutinee of a match has a parametric type, and appropriate TypeTags are in scope, it should be possible to formulate a match like the following:

def f[T: TypeTag](x: T) = x match {
  case xs: List[String] => ...
  case xs: List[Int] => ...
}

It would be nice if that would also work with type parameters used in non-top-level positions:

def f[T: TypeTag](xs: List[T]) = xs match {
  case (s: String) :: tail =>

In principle the compiler should have all necessary knowledge available, but I cannot judge how difficult it would be to make this work.

Talking with Adriaan did not result in immediate dismissal, and he asked me to reference #5143 as related.

@scabug
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scabug commented Oct 11, 2012

Imported From: https://issues.scala-lang.org/browse/SI-6517?orig=1
Reporter: @rkuhn
See #5143

@scabug
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scabug commented Oct 11, 2012

@paulp said:
See also #329, which I closed as a duplicate of this, if only to be reminded about checking refinement types at runtime too.

@scabug
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scabug commented Oct 11, 2012

@paulp said:
By the way, I can't tell what you want from the second example. That doesn't need to know anything about T.

@scabug
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scabug commented Oct 12, 2012

@rkuhn said:
You’re right, I missed the mark slightly: make that

def f[T: TypeTag](xs: List[T]) = xs match {
  case (s: Set[String]) :: tail => ...
  case xs: List[Map[Int, Int]]  => ...
}

@scabug
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scabug commented Jul 10, 2013

@adriaanm said:
Unassigning and rescheduling to M6 as previous deadline was missed.

@scabug
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scabug commented Aug 28, 2016

Naftoli Gugenheim (naftoligug) said:
@roland.kuhn what do you need the first case for in your updated second example?
Also perhaps the Description should be updated

@scabug
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scabug commented Mar 14, 2017

@SethTisue said:
scala/scala#5774

@scabug
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scabug commented Mar 14, 2017

@milessabin said:
I don't think that PR does address this ticket, at least not directly.

@scabug
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scabug commented Mar 14, 2017

@SethTisue said:
yeah, I think anyone interested in this ticket will be interested in that PR, but I've removed the "has-pull-request" tag.

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