How to use the maybe monoid and combine values with a custom operation, easily?

Question

What I\'m trying to do is trivial to define by hand, basically

maybeCombine :: (a->a->a) -> Maybe a -> Maybe a -> Maybe a
maybeCombine _ Nothing N         


        

Answer 1

import Data.Monoid
maybeCombine :: (a->a->a) -> Maybe a -> Maybe a -> Maybe a
maybeCombine f mx my = let combine = mx >>= (\x -> my >>= (\y -> Just (f x y)))
                       in getFirst $ First combine `mappend` First mx `mappend` First` my

in GHCi, this gives me

*Main> maybeCombine (+) Nothing Nothing
Nothing
*Main> maybeCombine (+) (Just 3) Nothing
Just 3
*Main> maybeCombine (+) (Just 3) (Just 5)
Just 8

Also works with getLast if you put Last combine at the end of the mappend sequence