{- monadic stuff - - Copyright 2010-2012 Joey Hess - - License: BSD-2-clause -} {-# OPTIONS_GHC -fno-warn-tabs #-} module Utility.Monad where import Data.Maybe import Control.Monad {- 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) = ifM (p x) (return \$ Just x , firstM p xs) {- Runs the action on values from the list until it succeeds, returning - its result. -} getM :: Monad m => (a -> m (Maybe b)) -> [a] -> m (Maybe b) getM _ [] = return Nothing getM p (x:xs) = maybe (getM p xs) (return . Just) =<< p x {- 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 allM :: Monad m => (a -> m Bool) -> [a] -> m Bool allM _ [] = return True allM p (x:xs) = p x <&&> allM p xs {- Runs an action on values from a list until it succeeds. -} untilTrue :: Monad m => [a] -> (a -> m Bool) -> m Bool untilTrue = flip anyM {- if with a monadic conditional. -} ifM :: Monad m => m Bool -> (m a, m a) -> m a ifM cond (thenclause, elseclause) = do c <- cond if c then thenclause else elseclause {- short-circuiting monadic || -} (<||>) :: Monad m => m Bool -> m Bool -> m Bool ma <||> mb = ifM ma ( return True , mb ) {- short-circuiting monadic && -} (<&&>) :: Monad m => m Bool -> m Bool -> m Bool ma <&&> mb = ifM ma ( mb , return False ) {- Same fixity as && and || -} infixr 3 <&&> infixr 2 <||> {- 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 {- do nothing -} noop :: Monad m => m () noop = return ()