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+{- monadic stuff
+ -
+ - Copyright 2010-2012 Joey Hess <joey@kitenet.net>
+ -
+ - License: BSD-2-clause
+ -}
+
+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 ()