On Sun, May 30, 2010 at 1:35 AM, Michael Vanier
I stumbled across this monadic combinator:
mcombine :: Monad m => (a -> a -> a) -> m a -> m a -> m a mcombine f mx my = do x <- mx y <- my return (f x y)
I used it to chain the outputs of two Parsec String parsers together using this operator:
(<++>) :: Monad m => m String -> m String -> m String (<++>) = mcombine (++)
mcombine seems like such a basic operation that it should be a library function, but I couldn't find one matching it on hoogle. Is there one?
It's a lift of sorts. The tip off is the type signature. Your type is equivalent to this type: Monad m => (a -> a -> a) -> (m a -> m a -> m a) Hopefully the parens added for the grouping helps make the intuition clear. Your function takes a function and makes a new one that expects monadic parameters. Look at liftM2 in Control.Monad. More generally there is a formula here: liftM<N> f x1 .. x<N> = f` `liftM` x1 `ap` ... `ap` x<N> If you have an applicative this could also be: f <$> m1 <*> m2 HTH, Jason