data Expr = Constant Int | Plus Expr Expr | Div Expr Expr

safeEval :: Expr -> Maybe Int
safeEval (Constant i) = Just i
safeEval (Div a Nothing) = Nothing
safeEval (Div a b) = safeEval (Div (safeEval a) (safeEval b))
safeEval (Plus (Just a) (Just b)) = Just (a + b)
safeEval (Plus a b) = safeEval (Plus (safeEval a) + (safeEval b))