data Maybe (a) where {
    Nothing : Maybe (a)
    Just : a -> Maybe (a)
    };

fmap : (a -> b) -> Maybe (a) -> Maybe (b) ;
fmap f ma = case ma of {
    Nothing => Nothing ;
    Just a => Just (f a) ;
};

pure : a -> Maybe (a) ;
pure x = Just x ;

ap mf ma = case mf of {
    Just f => case ma of {
        Nothing => Nothing;
        Just a => Just (f a);
    };
    Nothing => Nothing;
};
    
return = pure;

bind ma f = case ma of {
    Nothing => Nothing ;
    Just a => f a ;
};