{-# LANGUAGE Rank2Types #-}

-- Examples showing the limits of Hindley-Milner type inference:


import Prelude hiding (id)

-- Declare this function with a polymorphic function as first argument:
funsum :: (forall a . [a] -> Int) -> [b] -> [c] -> Int
funsum f xs ys = f xs + f ys

mainfunsum = funsum length [1,2,3] "abc"

-- Self application of argument possible if it is a polymoprhic function:
fun :: (forall a. a -> a) -> a -> a
fun g = g g

id :: forall a . a -> a
id x = x

mainfun = (fun id) True