Hi, can anyone please explain why in the following code evalAST compiles while evalAST2 doesn't?: Is that because the polymorphic function k is specialised in two different ways in evalAST while in evalAST2 it is constrained to be the same function? {-# LANGUAGE GADTs #-} test = evalAST (TxtA "abc") -- This is OK evalAST :: AST -> IO () evalAST (IntA i) = k $ Lit i evalAST (TxtA i) = k $ Lit i k :: (Show a) => Expr a -> IO () k e = print $ eval e -- This is the same thing, only the k function is passed as a parameter. -- But it won't compile. -- I would expect its type to be: -- evalAST2 :: (Expr a -> IO()) -> AST -> IO () -- But is actually: -- evalAST2 :: (Expr Int -> IO ()) -> AST -> IO () evalAST2 k (IntA i) = k $ Lit i -- evalAST2 k (TxtA i) = k $ Lit i -- The untyped expression data AST = IntA Int | TxtA String -- A typed expression. data Expr a where Lit :: a -> Expr a eval :: Expr a -> a eval (Lit i) = i Thanks titto