{-# LANGUAGE ImpredicativeTypes, RequiredTypeArguments, TypeAbstractions #-}

module Foo where

foo :: Functor f => f (forall a -> f a) -> forall a -> f (f a)
foo x a = fmap (\y -> y a) x

-- means ^
foo' :: Functor f => f (forall a. f a) -> forall a. f (f a)
foo' x @a = fmap (\y -> y @a) x

-- means ^
foo'' :: Functor f => f (forall a. f a) -> forall a. f (f a)
foo'' x = fmap (\y -> y) x