Die Maybe-Monade

data Maybe a = Nothing | Just a
instance Monad Maybe where ...

Beispiel-Anwendung:

case ( evaluate e l ) of
    Nothing -> Nothing
    Just a  -> case ( evaluate e r ) of
        Nothing -> Nothing
        Just b  -> Just ( a + b )

mittels der Monad-Instanz von Maybe:

evaluate e l >>= \ a ->
    evaluate e r >>= \ b ->
        return ( a + b)

Ü: dasselbe mit do-Notation