TypeLambda.hs

{-# LANGUAGE TypeFamilies, GADTs #-}

data OfInt (f :: * -> *) where
    OfInt :: f Int -> OfInt f

type family F a where
    F a = Maybe a

runF :: F Int -> OfInt F
runF = OfInt {- Error on this line:
Couldn't match type ‘F’ with ‘Maybe’
    Expected type: F Int -> OfInt F
      Actual type: Maybe Int -> OfInt Maybe
    In the expression: OfInt
    In an equation for ‘runF’: runF = OfInt
-}

type family G a where
    G a = Either a Bool

runG :: G Int -> OfInt G
runG = OfInt {- Error on this line:
Couldn't match type ‘Bool’ with ‘Int’
    Expected type: G Int -> OfInt G
      Actual type: Either Int Int -> OfInt (Either Int)
    In the expression: OfInt
    In an equation for ‘runG’: runG = OfInt
-}

newtype D a = D (Either a Bool)

runD :: D Int -> OfInt D
runD = OfInt {- works perfectly -}