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Ordering[T] and friends should be contravariant #7179

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scabug opened this issue Feb 25, 2013 · 3 comments
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Ordering[T] and friends should be contravariant #7179

scabug opened this issue Feb 25, 2013 · 3 comments
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scabug commented Feb 25, 2013 •
edited by retronym

The Ordering[T] trait and its friends Equiv[T] and PartialOrdering[T] should be contravariant in T. This would for example allow SeqLike.sorted to be declared as:

def sorted(implicit ord: math.Ordering[A]): Repr

rather than

def sorted[B >: A](implicit ord: math.Ordering[B]): Repr

It would also provide a good didactic example of contravariance in the library!
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scabug commented Feb 25, 2013

Imported From: https://issues.scala-lang.org/browse/SI-7179?orig=1
Reporter: Dan Rosen (mergeconflict)

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scabug commented Mar 2, 2013 •
edited by retronym

Dan Rosen (mergeconflict) said:
I spent a little time tinkering with this and found two issues:

  • Ordering[T] defines methods max and min, which return T (in covariant position).
  • Ordering[T] extends java.util.Comparator[T] which, having been defined in Java, is invariant in T.

The first problem seems easy to deal with. The second, though, well... I wish Scala could somehow infer correct variance for Comparator.

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scabug commented Mar 7, 2013

@vigdorchik said:
It's not possible to have Ordering contravariant until we drop the compatibility with java.util.Comparator, which is going to be a major change that needs good reason.
It's also not possible to declare an implicit view, since we don't control the implicit scope of Comparator.

@scabug scabug closed this as completed Mar 14, 2013
@smarter smarter mentioned this issue Aug 15, 2017
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