Fix Foldabe#dropWhile_ bug.

[non]

In addition to fixing this bug, the commit defines several
FoldableCheck tests for various types that have foldable
instances.

Finally, the commit adds takeWhile_.

Fixes #563.

[codecov-io]

Current coverage is 76.35%

Merging #623 into master will increase coverage by +0.22% as of 3ca7a2b

@@            master    #623   diff @@
======================================
  Files          159     159       
  Stmts         2070    2081    +11
  Branches        69      70     +1
  Methods          0       0       
======================================
+ Hit           1576    1589    +13
  Partial          0       0       
+ Missed         494     492     -2

Review entire Coverage Diff as of 3ca7a2b

Powered by Codecov. Updated on successful CI builds.

[ceedubs]

Does this belong in cats core? In a recent gitter conversation, the consensus was to put IndexedSeq instances in alleycats because IndexedSeq can potentially be mutable. Wouldn't the same arguments apply to Iterable?

[ceedubs]

You didn't mention that this PR also adds a Foldable[Iterable] instance :). I've left a question about that. Other than that, these changes look great to me.

[non]

My claim is that foldable instances are different (e.g. we have one for Set), and none of the foldable methods return F[A]. But I'm fine removing the instance too, it just seemed to me to be an oversight.

[ceedubs]

@tpolecat what are your thoughts?

Are we checking FoldableLaws for the Foldable[Iterable]? It doesn't look like it to me, but I might be overlooking some inheritance or something.

[tpolecat]

Bad Cop to the rescue! So, yeah I think Foldable[Iterable] is bogus because the internal call to .iterator is observably side-effecting. That is,

val a = F.fold(xs)
val b = F.fold(xs)

is not necessarily the same program as

val a = F.fold(xs)
val b = a

even though fold is called with a pure value.

[non]

Ugh. OK, sure.

Personally I'd prefer to use correctness-preserving when dealing with something like Scala-collections but I see your point. I'll remove it. 🔨 🔥

[ceedubs]

👍

[adelbertc]

👍