{-# LANGUAGE GADTs #-} data Func where Func :: (A -> B) -> Func data A = AInt Int | AString String deriving (Show, Read) data B = BInt Int | BString String deriving (Show, Read) f1 :: Func f1 = Func $ \ (AInt i) -> BInt $ i * 2 f2 :: Func f2 = Func $ \ (AString i) -> BString $ i ++ "!" getF :: String -> Func getF "f1" = f1 getF "f2" = f2 exec :: Func -> A -> B exec (Func f) = f main = print $ exec (getF "f1") (read "AInt 4")