That was a poor choice. The local “a” shadows the parameter “a”. You can of course write: addM :: Maybe Int -> Maybe Int -> Maybe Int addM ma mb = do

That was a poor choice. The local “a” shadows the parameter “a”. You can of course write: addM :: Maybe Int -> Maybe Int -> Maybe Int addM ma mb = do a <- ma b <- mb return (a + b) As an aside, the inferred type would be much more general, because (+) works on any number and do/return work on any monad: addM :: (Monad m, Num a) => m a -> m a -> m a Which means you could use it like this: addM (Just 5) (Just 10) == Just 15 But also like this: addM [1, 2] [4, 8] == [1 + 4, 1 + 8, 2 + 4, 2 + 8] == [5, 9, 6, 10] Or like this: addM (Right 42) (Left "NaN") == Left "NaN" And in many more interesting ways. :)