expr.hs

data Expr = I Int
          | B Bool
          | Add Expr Expr
          | Mul Expr Expr
          | Eq  Expr Expr
          deriving Show

eval :: Expr -> Maybe (Either Int Bool)
eval (I n)       = Just (Left n)
eval (B b)       = Just (Right b)
eval (Add e1 e2) = case (eval e1, eval e2) of
                     (Just (Left x), Just (Left y)) -> Just (Left (x + y))
                     _                              -> Nothing
eval (Mul e1 e2) = case (eval e1, eval e2) of
                     (Just (Left x), Just (Left y)) -> Just (Left (x * y))
                     _                              -> Nothing

eval (Eq  e1 e2) = case (eval e1, eval e2) of
                     (Just (Right x), Just (Right y)) -> Just (Right (x == y))
                     (Just (Left  x), Just (Left  y)) -> Just (Right (x == y))
                     _                                -> Nothing