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{- monadic stuff
-
- Copyright 2010-2011 Joey Hess <joey@kitenet.net>
-
- Licensed under the GNU GPL version 3 or higher.
-}
module Utility.Monad where
import Data.Maybe
import Control.Monad (liftM)
{- Return the first value from a list, if any, satisfying the given
- predicate -}
firstM :: (Monad m) => (a -> m Bool) -> [a] -> m (Maybe a)
firstM _ [] = return Nothing
firstM p (x:xs) = do
q <- p x
if q
then return (Just x)
else firstM p xs
{- Returns true if any value in the list satisfies the predicate,
- stopping once one is found. -}
anyM :: (Monad m) => (a -> m Bool) -> [a] -> m Bool
anyM p = liftM isJust . firstM p
{- Runs an action on values from a list until it succeeds. -}
untilTrue :: (Monad m) => [a] -> (a -> m Bool) -> m Bool
untilTrue = flip anyM
{- Runs an action, passing its value to an observer before returning it. -}
observe :: (Monad m) => (a -> m b) -> m a -> m a
observe observer a = do
r <- a
_ <- observer r
return r
{- b `after` a runs first a, then b, and returns the value of a -}
after :: (Monad m) => m b -> m a -> m a
after = observe . const
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