{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts, Rank2Types #-} -- | -- Module : Control.Monad.Sharing.Classes -- Copyright : Chung-chieh Shan, Oleg Kiselyov, and Sebastian Fischer -- License : PublicDomain -- Maintainer : Sebastian Fischer <mailto:sebf@informatik.uni-kiel.de> -- Stability : experimental -- -- This library provides type classes for explicit sharing of monadic -- effects. Usually you don't need to import this library as it is -- reexported by the module 'Control.Monad.Sharing'. You may want to -- do so, however, when writing your own implementation of explicit -- sharing. module Control.Monad.Sharing.Classes ( Sharing(..), Shareable(..), Convertible(..), convert ) where -- | Interface of monads that support explicit sharing. class Sharing m where -- | -- Yields an action that returns the same results as the given -- action but whose effects are only executed once. Especially, when -- the resulting action is duplicated it returns the same result at -- every occurrence. share :: Shareable m a => m a -> m (m a) -- | -- Interface of shareable nested monadic data types. The provided -- function 'shareArgs' is supposed to map the given function on every -- monadic argument. -- -- We provide instances of the 'Shareable' class for some predefined -- Haskell types. For flat types the function 'shareArgs' just returns -- its argument which has no arguments to which the given function -- could be applied. class Shareable m a where shareArgs :: Monad n => (forall b . Shareable m b => m b -> n (m b)) -> a -> n a instance Monad m => Shareable m Bool where shareArgs _ = return instance Monad m => Shareable m Int where shareArgs _ = return instance Monad m => Shareable m Char where shareArgs _ = return instance Monad m => Shareable m [Bool] where shareArgs _ = return instance Monad m => Shareable m [Int] where shareArgs _ = return instance Monad m => Shareable m [Char] where shareArgs _ = return -- | An instance for lists with monadic elements. instance (Monad m, Shareable m a) => Shareable m [m a] where shareArgs f = mapM f -- | -- Interface for convertible datatypes. The provided function -- 'convArgs' is supposed to map the given function on every argument -- of the given value and combine the results to give the converted -- value. -- -- We provide instances of the 'Convertible' class for some predefined -- Haskell types. For flat types the function 'convArgs' just returns -- its argument which has no arguments to which the given function -- could be applied. class Convertible m a b where convArgs :: (forall c d . Convertible m c d => c -> m d) -> a -> m b -- | Converts a convertible value recursively. convert :: Convertible m a b => a -> m b convert = convArgs convert instance Monad m => Convertible m Bool Bool where convArgs _ = return instance Monad m => Convertible m Int Int where convArgs _ = return instance Monad m => Convertible m Char Char where convArgs _ = return instance Monad m => Convertible m [Bool] [Bool] where convArgs _ = return instance Monad m => Convertible m [Int] [Int] where convArgs _ = return instance Monad m => Convertible m [Char] [Char] where convArgs _ = return -- | -- An instance to convert ordinary lists into lists with monadic -- elements. instance (Monad m, Convertible m a b) => Convertible m [a] [m b] where convArgs f = return . map f -- | -- An instance to convert lists with monadic elements into ordinary -- lists. instance (Monad m, Convertible m a b) => Convertible m [m a] [b] where convArgs f = mapM (>>=f)