On 13 Jul 2010, at 15:31, edgar klerks wrote:
You can use maybe from Data.Maybe: b -> (a -> b) -> Maybe a -> b
to create your maybe plus function:
maybePlus x y = maybe 0 id $ liftM2 (+) x y
That is somewhat cleaner.
Aah thanks Edgar - I'd meant to ask about this specifically actually, as addition is a monoid over the real numbers and therefore has an identity element, I was wondering if there was an easier way to generalise it to cope with arguments in the Maybe monad. As far as I can see, this is precisely what you describe above. Therefore, would I be right in saying that your approach can be generalised to any function which forms a monoid over a specific type? I'm imagining something like: maybeMonoid :: (a -> a -> a) -> a -> (Maybe a -> Maybe a -> a) maybeMonoid f identity = maybe identity id $ liftM2 f Thanks for the feedback! Cheers, Tim