On 2006-08-08 at 13:37BST Chris Kuklewicz wrote:
The new one looks better to me. But the foldr is not needed:
filterM _ [] = return [] filterM p (x:xs) = do flg <- p x if flg then do ys <- filterM p xs return (x:ys) else filterM p xs
I'm curious to know how this performs:
filterM p = fmap catMaybes . mapM (predMToMaybe p)
with the subsidiary (and generally useful) predMToMaybe¹ defined something like this:
predMToMaybe p x = fmap (\b->if b then Just x else Nothing ) $ p x
since this definition of filterM clearly shouldn't hold onto anything in the second half (. mapM (predMToMaybe p)) and the first half (fmap catMaybes) will be OK provided that catMaybes and fmap behave themselves -- and if they don't they should be given what for. Jón [1] or perhaps it should be called predFToMaybe since its type is (Functor f) => (a -> f Bool) -> a -> f (Maybe a) (if someone can think of a better name, I'd be glad to hear it) -- Jón Fairbairn Jon.Fairbairn at cl.cam.ac.uk