Add existsM and forallM to Foldable

[aryairani]

I was missing these utility functions. I've been out of the loop for a while so let me know if I didn't contrib right, and I'll be happy to update.

[codecov-io]

Codecov Report

Merging #1784 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1784      +/-   ##
==========================================
+ Coverage   94.87%   94.88%   +<.01%     
==========================================
  Files         241      241              
  Lines        4139     4147       +8     
  Branches      103      110       +7     
==========================================
+ Hits         3927     3935       +8     
  Misses        212      212

Impacted Files	Coverage Δ
core/src/main/scala/cats/Foldable.scala	100% <100%> (ø)	⬆️

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[kailuowang]

I think if you use Fordable.Source and tailRecM directly rather than foldM, you should be able to shortcircuit
e.g.

  def existsM[G[_], A](fa: F[A])(p: A => G[Boolean])(implicit G: Monad[G]): G[Boolean] = {
    val src = Foldable.Source.fromFoldable(fa)(self)
    G.tailRecM(src) { srcc => srcc.uncons match {
      case Some((a, src)) => G.map(p(a))(bb => if (bb) Right(true) else Left(src.value))
      case None => G.pure(Right(false))
    }}
  }

[kailuowang]

might be better to have a negative test case here, e.g.

   * scala> F.existsM(List(1,2,3,4))(n => if (n > 4) Option(true) else Option(false))
   * res1: Option[Boolean] = Some(false)

[aryairani]

@kailuowang Yes, but do we want it to short-circuit and skip effects? I could imagine an argument either way. For my use case, short-circuiting would probably be fine, but I didn’t want to presume that for everyone. IIRC scalaz's intentionally doesn’t shortcircuit, but I don’t know the rationale.

[kailuowang]

I think there is definitely a use case for short-circuiting implementation. If we have to pick from the two, I would argue we should pick the short-circuit one, because the other one isn't that cumbersome to write on use-site if needed. How about we ask for more opinions on gitter or something?

[aryairani]

100% agreement. I meant to go back and say that short-circuiting would probably be more efficient in my use case; since the effects typically involve extra throwaway computation.

[aryairani]

@kailuowang I was mistaken when I said scalaz doesn't short-circuit; it does:

scala> import scalaz._, Scalaz._
import scalaz._
import Scalaz._

scala> List(Some(true), None).anyM(identity)
res0: Option[Boolean] = Some(true)

scala> List(Some(true), None).allM(identity)
res1: Option[Boolean] = None

I'll redo the PR when I get a chance.

[kailuowang]

err. not sure if you need to close this PR. If you replace with the implementation I gave it shortcircuit and pass the tests.

[aryairani]

Ok, I wasn't sure if I should create a new one, or rebase or what.

[johnynek]

This isn't being used right.

[aryairani]

I meant to delete line 393, which isn't used, but do you want to elaborate?

[johnynek]

👍

[kailuowang]

awesome! thanks!

[aryairani]

I will add an example to the tut docs. Sorry I didn't think of it sooner.