I just meant it's not immediately clear how

foo :: forall x. (x -> x -> y)

is different from

foo :: (forall x. x -> x) -> y

It takes a bit of getting used to.

Those are different functions all together, so perhaps you meant these.

foo :: forall x y. (x -> x) -> y
bar :: forall y. (forall x . x -> x) -> y

While neither function is seemingly useful, the second says that the higher-order argument must be polymorphic. I see two options:

bar id
bar undefined

The first has these and many more:

foo (+1)
foo show
foo ($)
...

Sean