TypeFail.hs

import Data.Maybe

-- The fixed point datatype
data Y f = Y (f (Y f))

-- dummy function for illustration
maybeToInt :: Maybe a -> Int
maybeToInt = length . maybeToList


f :: Y Maybe -> Int
f (Y x) = g maybeToInt x

-- This is the type it wants to infer
-- g :: (Maybe Int -> Int) -> Maybe (Y Maybe) -> Int

-- This is the type I think it should have, note you can't force
-- with typesig without -XRelaxedPolyRec
-- g :: (Functor f) => (f Int -> b) -> f (Y Maybe) -> b
g h x = h $ fmap f x