cats.Order: Support for pattern matching on comparison results

[benhutchison]

When doing comparisons with cats.Order, would be desirable to pattern match on the cases, like eg:

a comparison b match {
  case GreaterThan => ???
  case Equivalent => ???
  case LessThan => ???
}

Order.compare returns an Int for performance and Java 1.7 compat reasons, which is fair enough, when you need that. But that requires an if (c <0) ... else if (c == 0) .. else ... construct to handle a comparison; less readable than the above alternative IMO.

Proposal is a function, implemented on top of compare: Int, perhaps named comparison: Comparison that returned a case object (GreaterThan etc) for pleasant pattern matching.

Gitter discussion: https://gitter.im/typelevel/cats?at=5758beb8064b9e7266f0c086

[julienrf]

Note that you can still get quite the same syntax without changing Order:

object GreaterThan {
  def unapply(i: Int): Boolean = i > 0
}

(a comparison b) match {
  case GreaterThan() => ???
}

[ceedubs]

@julienrf I thought the same thing at first. The problem is that while you do get the syntax, you don't get the exhaustive matching. So if you match on the GT and LT case but forget the EQ case, your code will just blow up at runtime.

[julienrf]

Fair point :)

[benhutchison]

IMO this isnt quite done, since there's no syntax to enable a comparison b.

I think (?) Ive seen some syntax in the cats project autogenerated, maybe from @typeclass; why doesnt Order use that?

[adelbertc]

@benhutchison Ah true, oops forgot about that.

I believe kernel doesn't use simulacrum to minimize dependencies