Add count as syntax to UnorderedFoldable

[leusgalvan]

Fixes #2447 .

[codecov-io]

Codecov Report

Merging #2520 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2520      +/-   ##
==========================================
+ Coverage   95.35%   95.36%   +<.01%     
==========================================
  Files         358      359       +1     
  Lines        6530     6532       +2     
  Branches      282      280       -2     
==========================================
+ Hits         6227     6229       +2     
  Misses        303      303

Impacted Files	Coverage Δ
testkit/src/main/scala/cats/tests/CatsSuite.scala	70% <ø> (ø)	⬆️
...src/main/scala/cats/syntax/unorderedFoldable.scala	100% <100%> (ø)

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[LukaJCB]

Hi @leusgalvan thanks a lot for contributing! Is there a reason you're using the underscore syntax, why not just count? :)

[leusgalvan]

@LukaJCB only because when I tried to use it in a simple way, the count method from the standard library (which returns an Int) gets called instead of the cats one (which returns Long):

import cats._
import cats.implicits._
Set("hi", "world").count(_.length > 3)
res0: Int = 1

[kubukoz]

Maybe it'd be better to have count defined in the UnorderedFoldable trait itself, and only call count here? Otherwise you can only use it with the syntax implicits, and not just F.count

[LukaJCB]

You can't do that if you want to maintain binary compat.

[kubukoz]

Aww damn :( the encoding of traits strikes again

[kubukoz]

we should be able to define extensions for typeclass instances, like:

implicit class UFOps[F[_]](val instance: UnorderedFoldable[F]) extends AnyVal {
  def count(...)
}

[LukaJCB]

@leusgalvan I think I'd prefer to just call it count, we have a convention where an underscore after a name usually means it's some kind of operation that doesn't quite do what the same function without the underscore would do.

See filter_, takeWhile_, traverse_, etc.

[kubukoz]

@LukaJCB if it's called count, using it with a concrete type that has count defined on it will make it impossible to use - similar to fold and combineAll.

Although it may be alright to keep it that way here, because there's a perfectly sane count on Lists and similar types that works the same as the one here...

[leusgalvan]

@kubukoz @LukaJCB Ok let's go with count.

[LukaJCB]

Looks great, one small thing is that I think we should add this to the cats.syntax package object :)