69 lines
1.9 KiB
Haskell
69 lines
1.9 KiB
Haskell
{- monadic stuff
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-
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- Copyright 2010-2012 Joey Hess <joey@kitenet.net>
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-
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- License: BSD-2-clause
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-}
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module Utility.Monad where
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import Data.Maybe
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import Control.Monad
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{- Return the first value from a list, if any, satisfying the given
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- predicate -}
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firstM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a)
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firstM _ [] = return Nothing
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firstM p (x:xs) = ifM (p x) (return $ Just x , firstM p xs)
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{- Runs the action on values from the list until it succeeds, returning
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- its result. -}
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getM :: Monad m => (a -> m (Maybe b)) -> [a] -> m (Maybe b)
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getM _ [] = return Nothing
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getM p (x:xs) = maybe (getM p xs) (return . Just) =<< p x
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{- Returns true if any value in the list satisfies the predicate,
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- stopping once one is found. -}
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anyM :: Monad m => (a -> m Bool) -> [a] -> m Bool
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anyM p = liftM isJust . firstM p
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allM :: Monad m => (a -> m Bool) -> [a] -> m Bool
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allM _ [] = return True
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allM p (x:xs) = p x <&&> allM p xs
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{- Runs an action on values from a list until it succeeds. -}
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untilTrue :: Monad m => [a] -> (a -> m Bool) -> m Bool
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untilTrue = flip anyM
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{- if with a monadic conditional. -}
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ifM :: Monad m => m Bool -> (m a, m a) -> m a
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ifM cond (thenclause, elseclause) = do
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c <- cond
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if c then thenclause else elseclause
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{- short-circuiting monadic || -}
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(<||>) :: Monad m => m Bool -> m Bool -> m Bool
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ma <||> mb = ifM ma ( return True , mb )
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{- short-circuiting monadic && -}
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(<&&>) :: Monad m => m Bool -> m Bool -> m Bool
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ma <&&> mb = ifM ma ( mb , return False )
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{- Same fixity as && and || -}
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infixr 3 <&&>
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infixr 2 <||>
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{- Runs an action, passing its value to an observer before returning it. -}
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observe :: Monad m => (a -> m b) -> m a -> m a
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observe observer a = do
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r <- a
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_ <- observer r
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return r
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{- b `after` a runs first a, then b, and returns the value of a -}
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after :: Monad m => m b -> m a -> m a
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after = observe . const
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{- do nothing -}
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noop :: Monad m => m ()
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noop = return ()
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