41 lines
1.1 KiB
Haskell
41 lines
1.1 KiB
Haskell
{- monadic stuff
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-
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- Copyright 2010-2011 Joey Hess <joey@kitenet.net>
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-
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- Licensed under the GNU GPL version 3 or higher.
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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 (liftM)
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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) = do
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q <- p x
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if q
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then return (Just x)
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else firstM p xs
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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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{- 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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{- 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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