How and why is ap defined as liftM2 id in Haskell
- by luke_randall
Whilst trying to better understand Applicative, I looked at the definition of <*, which tends to be defined as ap, which in turn is defined as:
ap :: (Monad m) => m (a -> b) -> m a -> m b
ap = liftM2 id
Looking at the type signatures for liftM2 and id, namely:
liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
id :: a -> a
I fail to understand how just by passing in id, the relevant part of the type signature seems to transform from (a1 -> a2 -> r) -> m a1 to m (a -> b). What am I missing here?
