[Haskell-cafe] Stacking monads

[Jake McArthur]

Andrew Coppin wrote:
> (As an aside, Control.Monad.ap is not a function I've ever heard of. 
> It seems simple enough, but what an unfortunate name...!) 
I think it makes sense. It stands for "apply," or at least that is what 
I think of when I see it. If we have a function f :: A -> B -> C -> D 
and values a :: m A, b :: m B, c :: m C, then we can do:

    f `liftM` a `ap` b `ap` c

... which is the same as (using Applicative):

    f <$> a <*> b <*> c

... both having type m D.

- Jake