foldM and foldr &foldl

finally_know_foldX_functions.hs

import System.Directory
import Control.Monad

selectM :: Monad m => (a -> m Bool) -> ([a], [a]) -> a -> m ([a], [a])
selectM p ~(ts,fs) x = do
        r <- p x
        if r then return (x:ts, fs) else return (ts, x:fs)

partitionM :: Monad m => (a -> m Bool) -> [a] -> m ([a],[a])
partitionM p xs = do
        -- selectM p x ([], [])
-- foldM :: Monad m => (a -> b -> m a) -> a -> [b] -> m a
        foldM (selectM p ) ([],[]) xs

test = do
  partitionM doesDirectoryExist  ["/home/david","/","/home","/tmp/test/t.hs"]