Add withFilter to OptionT

[travisbrown]

-Xfuture on 2.11 doesn't allow filter methods to be used in for-comprehensions, which means e.g. this won't compile with -Xfuture on:

import cats.Eval, cats.data.OptionT

for { x <- OptionT(Eval.now(Option(1))) if x > 0 } yield x

It sounds like there's no reason not to add a withFilter method to OptionT.

[xuwei-k]

FYI: There is MonadPlusOps#withFilter in scalaz.

• https://github.com/scalaz/scalaz/blob/v7.2.2/core/src/main/scala/scalaz/syntax/MonadPlusSyntax.scala#L12

[adelbertc]

I like the idea of putting it on MonadFilter, but when defining a MonadFilter instance for OptionT:

 47 [info] - OptionT[List, Int].monadFilter.monadFilter right empty *** FAILED *** (14 milliseconds)
 48 [info]   GeneratorDrivenPropertyCheckFailedException was thrown during property evaluation.
 49 [info]    (Discipline.scala:14)
 50 [info]     Falsified after 0 successful property evaluations.
 51 [info]     Location: (Discipline.scala:14)
 52 [info]     Occurred when passed generated values (
 53 [info]       arg0 = OptionT(List())
 54 [info]     )
 55 [info]     Label of failing property:
 56 [info]       (OptionT(List()) ?== OptionT(List(None))) failed

Related: #698

[ceedubs]

I'd vote for adding withFilter to OptionT now and worrying about the MonadFilter component later as a resolution to #694 (which I'm guessing we won't get around to for a while).

[ceedubs]

Added in #1028.