:t foldl
:print foldl

:t fmap
:print fmap

:t return
:print return

:t pure
:print pure

:m + Data.Monoid
:p mempty
:p mappend

:m + Data.List
:p foldl'

:set -XRankNTypes
f :: (forall a. a -> a) -> b -> b; f g x = g x
:p f