GADTs

gistfile1.hs

data Exp a where
    Lit :: Int -> Exp Int
    Add :: Exp Int -> Exp Int -> Exp Int
    Mul :: Exp Int -> Exp Int -> Exp Int
    Cmp :: Eq a => Exp a -> Exp a -> Exp Bool
    If  :: Exp Bool -> Exp a -> Exp a -> Exp a

deriving instance Show (Exp a)

eval :: Exp a -> a
eval (Lit a) = a
eval (Add a b) = eval a + eval b
eval (Mul a b) = eval a * eval b
eval (Cmp a b) = eval a == eval b
eval (If cnd yes no) = eval $ if (eval cnd) then yes else no