Applicative/Functor instance missing for OptionT

[tbrown1979]

Missing for when F doesn't have a Monad

[yilinwei]

This is absolutely the case because a monad transfromer is not the same as a Nested.

See explanation here #1106.

[peterneyens]

It would still be possible to add something like scalaz.OptionT[F, ?].app, but it would give the same result as Nested[F, Option, ?].ap so that wouldn't be that useful probably.

I was wondering if we should add some sort of toNested on OptionT and XorT, in that way you could do OptionT(List(Some(1))).toNested.ap( ... ).
We could probably also use it to implement methods like OptionT.foldLeft for Foldable[OptionT[F, ?]] (which I just added in #1103, which also includes the Functor you mentioned)

def foldLeft[B](b: B)(f: (B, A) => B)(implicit F: Foldable[F]): B =
  toNested.foldLeft(value, b)(f)
  // F.foldLeft(value, b)((b, o) => optionInstance.foldLeft(o, b)(f))

[ceedubs]

@peterneyens I like your idea of toNested.

[ceedubs]

toNested was added in #1172, so I'm going to close this out.