diff --git a/Control/Monad/Codensity.hs b/Control/Monad/Codensity.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Codensity.hs @@ -0,0 +1,55 @@ +{-# OPTIONS -XRank2Types #-} + +{- | +Module : Control.Monad.Codensity +Copyright : (c) Mauro Jaskelioff 2008, +License : BSD-style (see the file libraries/base/LICENSE) + +Maintainer : mjj@cs.nott.ac.uk +Stability : experimental +Portability : non-portable (Rank-2 Types) + +[Useful for:] Algebraization of operations + +The Codensity monad (also called the Backtracking monad). +-} + + +module Control.Monad.Codensity ( + Codensity(..), + toCodensity, + fromCodensity, + module Control.Monad, + ) where + +import Control.Monad +import Control.Monad.Fix + +-- requires -XRank2Types +newtype Codensity f a = Codensity { + runCodensity :: forall b. (a -> f b) -> f b +} + +-- --------------------------------------------------------------------------- +-- Codensity instances for Functor and Monad + +instance Functor (Codensity f) where + fmap f m = Codensity (\k -> runCodensity m (k. f)) + +instance Monad (Codensity f) where + return a = Codensity (\k -> k a) + c >>= f = Codensity (\k -> runCodensity c (\a -> runCodensity (f a) k)) + +toCodensity :: Monad m => m a -> Codensity m a +toCodensity m = Codensity (m >>=) + +fromCodensity :: Monad m => Codensity m a -> m a +fromCodensity c = runCodensity c return + +-- still need to prove that MonadFix laws hold +instance MonadFix m => MonadFix (Codensity m) where + mfix f = Codensity $ \k -> mfix (fromCodensity. f) >>= k + +------------------------ + + diff --git a/Control/Monad/Cont.hs b/Control/Monad/Cont.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Cont.hs @@ -0,0 +1,243 @@ +{-# OPTIONS -fallow-undecidable-instances #-} + +{- | +Module : Control.Monad.Cont +Copyright : (c) The University of Glasgow 2001, + (c) Jeff Newbern 2003-2007, + (c) Andriy Palamarchuk 2007, + (c) Mauro Jaskelioff 2008, +License : BSD-style (see the file libraries/base/LICENSE) + +Maintainer : mjj@cs.nott.ac.uk +Stability : experimental +Portability : non-portable (multi-parameter type classes) + +[Computation type:] Computations which can be interrupted and resumed. + +[Binding strategy:] Binding a function to a monadic value creates +a new continuation which uses the function as the continuation of the monadic +computation. + +[Useful for:] Complex control structures, error handling, +and creating co-routines. + +[Zero and plus:] None. + +[Example type:] @'Cont' r a@ + +The Continuation monad represents computations in continuation-passing style +(CPS). +In continuation-passing style function result is not returned, +but instead is passed to another function, +received as a parameter (continuation). +Computations are built up from sequences +of nested continuations, terminated by a final continuation (often @id@) +which produces the final result. +Since continuations are functions which represent the future of a computation, +manipulation of the continuation functions can achieve complex manipulations +of the future of the computation, +such as interrupting a computation in the middle, aborting a portion +of a computation, restarting a computation, and interleaving execution of +computations. +The Continuation monad adapts CPS to the structure of a monad. + +Before using the Continuation monad, be sure that you have +a firm understanding of continuation-passing style +and that continuations represent the best solution to your particular +design problem. +Many algorithms which require continuations in other languages do not require +them in Haskell, due to Haskell's lazy semantics. +Abuse of the Continuation monad can produce code that is impossible +to understand and maintain. +-} + +module Control.Monad.Cont ( + module Control.Monad.Cont.Class, + Cont(..), + mapCont, + withCont, + ContT(..), + mapContT, + withContT, + module Control.Monad, + module Control.Monad.Trans, + -- * Example 1: Simple Continuation Usage + -- $simpleContExample + + -- * Example 2: Using @callCC@ + -- $callCCExample + + -- * Example 3: Using @ContT@ Monad Transformer + -- $ContTExample + ) where + +import Control.Monad +import Control.Monad.Cont.Class +import Control.Monad.Trans + +{- | +Continuation monad. +@Cont r a@ is a CPS computation that produces an intermediate result +of type @a@ within a CPS computation whose final result type is @r@. + +The @return@ function simply creates a continuation which passes the value on. + +The @>>=@ operator adds the bound function into the continuation chain. +-} +newtype Cont r a = Cont { + + {- | Runs a CPS computation, returns its result after applying + the final continuation to it. + Parameters: + + * a continuation computation (@Cont@). + + * the final continuation, which produces the final result (often @id@). + -} + runCont :: (a -> r) -> r +} + +mapCont :: (r -> r) -> Cont r a -> Cont r a +mapCont f m = Cont $ f . runCont m + +withCont :: ((b -> r) -> (a -> r)) -> Cont r a -> Cont r b +withCont f m = Cont $ runCont m . f + +instance Functor (Cont r) where + fmap f m = Cont $ \c -> runCont m (c . f) + +instance Monad (Cont r) where + return a = Cont ($ a) + m >>= k = Cont $ \c -> runCont m $ \a -> runCont (k a) c + +instance MonadCont (Cont r) where + callCC f = Cont $ \c -> runCont (f (\a -> Cont $ \_ -> c a)) c + +instance (MonadTrans t, Monad (t (Cont r))) => MonadCont (t (Cont r)) where + callCC f = join $ lift $ Cont $ \c -> runCont + (return $ f (\a -> lift $ Cont $ \_ -> c (return a))) c + +{- | +The continuation monad transformer. +Can be used to add continuation handling to other monads. +-} +newtype ContT r m a = ContT { runContT :: (a -> m r) -> m r } + +mapContT :: (m r -> m r) -> ContT r m a -> ContT r m a +mapContT f m = ContT $ f . runContT m + +withContT :: ((b -> m r) -> (a -> m r)) -> ContT r m a -> ContT r m b +withContT f m = ContT $ runContT m . f + +instance (Monad m) => Functor (ContT r m) where + fmap f m = ContT $ \c -> runContT m (c . f) + +instance (Monad m) => Monad (ContT r m) where + return a = ContT ($ a) + m >>= k = ContT $ \c -> runContT m (\a -> runContT (k a) c) + +instance (Monad m) => MonadCont (ContT r m) where + callCC f = ContT $ \c -> runContT (f (\a -> ContT $ \_ -> c a)) c + +-- --------------------------------------------------------------------------- +-- Instances for other mtl transformers + +instance (Monad m, MonadTrans t, Monad (t (ContT r m))) => + MonadCont (t (ContT r m)) where + callCC f = join $ lift $ ContT $ \c -> + runContT (return $ f (\a -> lift $ ContT $ \_ -> c (return a))) c + +instance MonadTrans (ContT r) where + lift m = ContT (m >>=) + tmap f g m = ContT $ \k -> f $ runContT m (g . k) + +instance (MonadIO m) => MonadIO (ContT r m) where + liftIO = lift . liftIO + + +{- $simpleContExample +Calculating length of a list continuation-style: + +>calculateLength :: [a] -> Cont r Int +>calculateLength l = return (length l) + +Here we use @calculateLength@ by making it to pass its result to @print@: + +>main = do +> runCont (calculateLength "123") print +> -- result: 3 + +It is possible to chain 'Cont' blocks with @>>=@. + +>double :: Int -> Cont r Int +>double n = return (n * 2) +> +>main = do +> runCont (calculateLength "123" >>= double) print +> -- result: 6 +-} + +{- $callCCExample +This example gives a taste of how escape continuations work, shows a typical +pattern for their usage. + +>-- Returns a string depending on the length of the name parameter. +>-- If the provided string is empty, returns an error. +>-- Otherwise, returns a welcome message. +>whatsYourName :: String -> String +>whatsYourName name = +> (`runCont` id) $ do -- 1 +> response <- callCC $ \exit -> do -- 2 +> validateName name exit -- 3 +> return $ "Welcome, " ++ name ++ "!" -- 4 +> return response -- 5 +> +>validateName name exit = do +> when (null name) (exit "You forgot to tell me your name!") + +Here is what this example does: + +(1) Runs an anonymous 'Cont' block and extracts value from it with +@(\`runCont\` id)@. Here @id@ is the continuation, passed to the @Cont@ block. + +(1) Binds @response@ to the result of the following 'callCC' block, +binds @exit@ to the continuation. + +(1) Validates @name@. +This approach illustrates advantage of using 'callCC' over @return@. +We pass the continuation to @validateName@, +and interrupt execution of the @Cont@ block from /inside/ of @validateName@. + +(1) Returns the welcome message from the @callCC@ block. +This line is not executed if @validateName@ fails. + +(1) Returns from the @Cont@ block. +-} + +{-$ContTExample +'ContT' can be used to add continuation handling to other monads. +Here is an example how to combine it with @IO@ monad: + +>import Control.Monad.Cont +>import System.IO +> +>main = do +> hSetBuffering stdout NoBuffering +> runContT (callCC askString) reportResult +> +>askString :: (String -> ContT () IO String) -> ContT () IO String +>askString next = do +> liftIO $ putStrLn "Please enter a string" +> s <- liftIO $ getLine +> next s +> +>reportResult :: String -> IO () +>reportResult s = do +> putStrLn ("You entered: " ++ s) + +Action @askString@ requests user to enter a string, +and passes it to the continuation. +@askString@ takes as a parameter a continuation taking a string parameter, +and returning @IO ()@. +Compare its signature to 'runContT' definition. +-} diff --git a/Control/Monad/Cont/Class.hs b/Control/Monad/Cont/Class.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Cont/Class.hs @@ -0,0 +1,78 @@ +{-# OPTIONS -fallow-undecidable-instances #-} +-- Search for -fallow-undecidable-instances to see why this is needed + +{- | +Module : Control.Monad.Cont.Class +Copyright : (c) The University of Glasgow 2001, + (c) Jeff Newbern 2003-2007, + (c) Andriy Palamarchuk 2007 +License : BSD-style (see the file libraries/base/LICENSE) + +Maintainer : libraries@haskell.org +Stability : experimental +Portability : non-portable (multi-parameter type classes) + +[Computation type:] Computations which can be interrupted and resumed. + +[Binding strategy:] Binding a function to a monadic value creates +a new continuation which uses the function as the continuation of the monadic +computation. + +[Useful for:] Complex control structures, error handling, +and creating co-routines. + +[Zero and plus:] None. + +[Example type:] @'Cont' r a@ + +The Continuation monad represents computations in continuation-passing style +(CPS). +In continuation-passing style function result is not returned, +but instead is passed to another function, +received as a parameter (continuation). +Computations are built up from sequences +of nested continuations, terminated by a final continuation (often @id@) +which produces the final result. +Since continuations are functions which represent the future of a computation, +manipulation of the continuation functions can achieve complex manipulations +of the future of the computation, +such as interrupting a computation in the middle, aborting a portion +of a computation, restarting a computation, and interleaving execution of +computations. +The Continuation monad adapts CPS to the structure of a monad. + +Before using the Continuation monad, be sure that you have +a firm understanding of continuation-passing style +and that continuations represent the best solution to your particular +design problem. +Many algorithms which require continuations in other languages do not require +them in Haskell, due to Haskell's lazy semantics. +Abuse of the Continuation monad can produce code that is impossible +to understand and maintain. +-} + +module Control.Monad.Cont.Class ( + MonadCont(..), + ) where + +class (Monad m) => MonadCont m where + {- | @callCC@ (call-with-current-continuation) + calls a function with the current continuation as its argument. + Provides an escape continuation mechanism for use with Continuation monads. + Escape continuations allow to abort the current computation and return + a value immediately. + They achieve a similar effect to 'Control.Monad.Error.throwError' + and 'Control.Monad.Error.catchError' + within an 'Control.Monad.Error.Error' monad. + Advantage of this function over calling @return@ is that it makes + the continuation explicit, + allowing more flexibility and better control + (see examples in "Control.Monad.Cont"). + + The standard idiom used with @callCC@ is to provide a lambda-expression + to name the continuation. Then calling the named continuation anywhere + within its scope will escape from the computation, + even if it is many layers deep within nested computations. + -} + callCC :: ((a -> m b) -> m a) -> m a + diff --git a/Control/Monad/Error.hs b/Control/Monad/Error.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Error.hs @@ -0,0 +1,287 @@ +{-# OPTIONS -fallow-undecidable-instances #-} + +{- | +Module : Control.Monad.Error +Copyright : (c) Michael Weber 2001, + (c) Jeff Newbern 2003-2006, + (c) Andriy Palamarchuk 2006, + (c) Mauro Jaskelioff 2008, +License : BSD-style (see the file libraries/base/LICENSE) + +Maintainer : mjj@cs.nott.ac.uk +Stability : experimental +Portability : non-portable (multi-parameter type classes) + +[Computation type:] Computations which may fail or throw exceptions. + +[Binding strategy:] Failure records information about the cause\/location +of the failure. Failure values bypass the bound function, +other values are used as inputs to the bound function. + +[Useful for:] Building computations from sequences of functions that may fail +or using exception handling to structure error handling. + +[Zero and plus:] Zero is represented by an empty error and the plus operation +executes its second argument if the first fails. + +[Example type:] @'Data.Either' String a@ + +The Error monad (also called the Exception monad). +-} + +{- + Rendered by Michael Weber , + inspired by the Haskell Monad Template Library from + Andy Gill () +-} +module Control.Monad.Error ( + module Control.Monad.Error.Class, + ErrorT(..), + mapErrorT, + module Control.Monad, + module Control.Monad.Fix, + module Control.Monad.Trans, + -- * Example 1: Custom Error Data Type + -- $customErrorExample + + -- * Example 2: Using ErrorT Monad Transformer + -- $ErrorTExample + ) where + +import Control.Monad +import Control.Monad.Error.Class +import Control.Monad.Fix +import Control.Monad.Trans +import Control.Monad.Codensity + +import Control.Monad.Instances () +import System.IO + +instance MonadPlus IO where + mzero = ioError (userError "mzero") + m `mplus` n = m `catch` \_ -> n + +instance MonadError IOError IO where + throwError = ioError + catchError = catch + +-- --------------------------------------------------------------------------- +-- Our parameterizable error monad + +instance (Error e) => Monad (Either e) where + return = Right + Left l >>= _ = Left l + Right r >>= k = k r + fail msg = Left (strMsg msg) + +instance (Error e) => MonadPlus (Either e) where + mzero = Left noMsg + Left _ `mplus` n = n + m `mplus` _ = m + +instance (Error e) => MonadFix (Either e) where + mfix f = let + a = f $ case a of + Right r -> r + _ -> error "empty mfix argument" + in a + +instance (Error e) => MonadError e (Either e) where + throwError = Left + Left l `catchError` h = h l + Right r `catchError` _ = Right r + + + +instance (Error e, MonadTrans t, Monad (t (Either e)), + Monad (t (Codensity (Either e)))) + => MonadError e (t (Either e)) where + throwError = lift . Left + catchError e h = tmap from to $ join $ lift $ + Codensity $ \k -> + case (k $ tmap to from e) of + Left l -> k (tmap to from $ h l) + Right r -> Right r + where to = toCodensity + from = fromCodensity + + +{- | +The error monad transformer. It can be used to add error handling to other +monads. + +The @ErrorT@ Monad structure is parameterized over two things: + + * e - The error type. + + * m - The inner monad. + +Here are some examples of use: + +> -- wraps IO action that can throw an error e +> type ErrorWithIO e a = ErrorT e IO a +> ==> ErrorT (IO (Either e a)) +> +> -- IO monad wrapped in StateT inside of ErrorT +> type ErrorAndStateWithIO e s a = ErrorT e (StateT s IO) a +> ==> ErrorT (StateT s IO (Either e a)) +> ==> ErrorT (StateT (s -> IO (Either e a,s))) +-} + +newtype ErrorT e m a = ErrorT { runErrorT :: m (Either e a) } + +mapErrorT :: (m (Either e a) -> n (Either e' b)) + -> ErrorT e m a + -> ErrorT e' n b +mapErrorT f m = ErrorT $ f (runErrorT m) + +instance (Monad m) => Functor (ErrorT e m) where + fmap f m = ErrorT $ do + a <- runErrorT m + case a of + Left l -> return (Left l) + Right r -> return (Right (f r)) + +instance (Monad m, Error e) => Monad (ErrorT e m) where + return a = ErrorT $ return (Right a) + m >>= k = ErrorT $ do + a <- runErrorT m + case a of + Left l -> return (Left l) + Right r -> runErrorT (k r) + fail msg = ErrorT $ return (Left (strMsg msg)) + +instance (Monad m, Error e) => MonadPlus (ErrorT e m) where + mzero = ErrorT $ return (Left noMsg) + m `mplus` n = ErrorT $ do + a <- runErrorT m + case a of + Left _ -> runErrorT n + Right r -> return (Right r) + +instance (MonadFix m, Error e) => MonadFix (ErrorT e m) where + mfix f = ErrorT $ mfix $ \a -> runErrorT $ f $ case a of + Right r -> r + _ -> error "empty mfix argument" + +instance (Monad m, Error e) => MonadError e (ErrorT e m) where + throwError l = ErrorT $ return (Left l) + m `catchError` h = ErrorT $ do + a <- runErrorT m + case a of + Left l -> runErrorT (h l) + Right r -> return (Right r) + +-- --------------------------------------------------------------------------- +-- Instances for other mtl transformers + +instance (Monad m, Error e, MonadTrans t, + Monad (t (ErrorT e m)), Monad (t (Codensity (ErrorT e m)))) + => MonadError e (t (ErrorT e m)) where + throwError l = lift $ ErrorT $ return (Left l) + catchError e h = tmap from to $ join $ lift $ + Codensity $ \k -> ErrorT $ + (runErrorT $ k $ tmap to from e) >>= \e' -> + case e' of + Left l -> runErrorT $ k (tmap to from $ h l) + Right r -> runErrorT $ return r + where to = toCodensity + from = fromCodensity + +instance (Error e) => MonadTrans (ErrorT e) where + lift m = ErrorT $ do + a <- m + return (Right a) + tmap f _ m = ErrorT $ f $ runErrorT m + +instance (Error e, MonadIO m) => MonadIO (ErrorT e m) where + liftIO = lift . liftIO + +{- $customErrorExample +Here is an example that demonstrates the use of a custom 'Error' data type with +the 'throwError' and 'catchError' exception mechanism from 'MonadError'. +The example throws an exception if the user enters an empty string +or a string longer than 5 characters. Otherwise it prints length of the string. + +>-- This is the type to represent length calculation error. +>data LengthError = EmptyString -- Entered string was empty. +> | StringTooLong Int -- A string is longer than 5 characters. +> -- Records a length of the string. +> | OtherError String -- Other error, stores the problem description. +> +>-- We make LengthError an instance of the Error class +>-- to be able to throw it as an exception. +>instance Error LengthError where +> noMsg = OtherError "A String Error!" +> strMsg s = OtherError s +> +>-- Converts LengthError to a readable message. +>instance Show LengthError where +> show EmptyString = "The string was empty!" +> show (StringTooLong len) = +> "The length of the string (" ++ (show len) ++ ") is bigger than 5!" +> show (OtherError msg) = msg +> +>-- For our monad type constructor, we use Either LengthError +>-- which represents failure using Left LengthError +>-- or a successful result of type a using Right a. +>type LengthMonad = Either LengthError +> +>main = do +> putStrLn "Please enter a string:" +> s <- getLine +> reportResult (calculateLength s) +> +>-- Wraps length calculation to catch the errors. +>-- Returns either length of the string or an error. +>calculateLength :: String -> LengthMonad Int +>calculateLength s = (calculateLengthOrFail s) `catchError` Left +> +>-- Attempts to calculate length and throws an error if the provided string is +>-- empty or longer than 5 characters. +>-- The processing is done in Either monad. +>calculateLengthOrFail :: String -> LengthMonad Int +>calculateLengthOrFail [] = throwError EmptyString +>calculateLengthOrFail s | len > 5 = throwError (StringTooLong len) +> | otherwise = return len +> where len = length s +> +>-- Prints result of the string length calculation. +>reportResult :: LengthMonad Int -> IO () +>reportResult (Right len) = putStrLn ("The length of the string is " ++ (show len)) +>reportResult (Left e) = putStrLn ("Length calculation failed with error: " ++ (show e)) +-} + +{- $ErrorTExample +@'ErrorT'@ monad transformer can be used to add error handling to another monad. +Here is an example how to combine it with an @IO@ monad: + +>import Control.Monad.Error +> +>-- An IO monad which can return String failure. +>-- It is convenient to define the monad type of the combined monad, +>-- especially if we combine more monad transformers. +>type LengthMonad = ErrorT String IO +> +>main = do +> -- runErrorT removes the ErrorT wrapper +> r <- runErrorT calculateLength +> reportResult r +> +>-- Asks user for a non-empty string and returns its length. +>-- Throws an error if user enters an empty string. +>calculateLength :: LengthMonad Int +>calculateLength = do +> -- all the IO operations have to be lifted to the IO monad in the monad stack +> liftIO $ putStrLn "Please enter a non-empty string: " +> s <- liftIO getLine +> if null s +> then throwError "The string was empty!" +> else return $ length s +> +>-- Prints result of the string length calculation. +>reportResult :: Either String Int -> IO () +>reportResult (Right len) = putStrLn ("The length of the string is " ++ (show len)) +>reportResult (Left e) = putStrLn ("Length calculation failed with error: " ++ (show e)) +-} + diff --git a/Control/Monad/Error/Class.hs b/Control/Monad/Error/Class.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Error/Class.hs @@ -0,0 +1,93 @@ +{-# OPTIONS -fallow-undecidable-instances #-} +-- Needed for the same reasons as in Reader, State etc + +{- | +Module : Control.Monad.Error.Class +Copyright : (c) Michael Weber 2001, + (c) Jeff Newbern 2003-2006, + (c) Andriy Palamarchuk 2006 +License : BSD-style (see the file libraries/base/LICENSE) + +Maintainer : libraries@haskell.org +Stability : experimental +Portability : non-portable (multi-parameter type classes) + +[Computation type:] Computations which may fail or throw exceptions. + +[Binding strategy:] Failure records information about the cause\/location +of the failure. Failure values bypass the bound function, +other values are used as inputs to the bound function. + +[Useful for:] Building computations from sequences of functions that may fail +or using exception handling to structure error handling. + +[Zero and plus:] Zero is represented by an empty error and the plus operation +executes its second argument if the first fails. + +[Example type:] @'Data.Either' String a@ + +The Error monad (also called the Exception monad). +-} + +{- + Rendered by Michael Weber , + inspired by the Haskell Monad Template Library from + Andy Gill () +-} +module Control.Monad.Error.Class ( + Error(..), + MonadError(..), + ) where + +-- | An exception to be thrown. +-- An instance must redefine at least one of 'noMsg', 'strMsg'. +class Error a where + -- | Creates an exception without a message. + -- Default implementation is @'strMsg' \"\"@. + noMsg :: a + -- | Creates an exception with a message. + -- Default implementation is 'noMsg'. + strMsg :: String -> a + + noMsg = strMsg "" + strMsg _ = noMsg + +-- | A string can be thrown as an error. +instance Error String where + noMsg = "" + strMsg = id + +instance Error IOError where + strMsg = userError + +{- | +The strategy of combining computations that can throw exceptions +by bypassing bound functions +from the point an exception is thrown to the point that it is handled. + +Is parameterized over the type of error information and +the monad type constructor. +It is common to use @'Data.Either' String@ as the monad type constructor +for an error monad in which error descriptions take the form of strings. +In that case and many other common cases the resulting monad is already defined +as an instance of the 'MonadError' class. +You can also define your own error type and\/or use a monad type constructor +other than @'Data.Either' String@ or @'Data.Either' IOError@. +In these cases you will have to explicitly define instances of the 'Error' +and\/or 'MonadError' classes. +-} +class (Monad m) => MonadError e m | m -> e where + -- | Is used within a monadic computation to begin exception processing. + throwError :: e -> m a + + {- | + A handler function to handle previous errors and return to normal execution. + A common idiom is: + + > do { action1; action2; action3 } `catchError` handler + + where the @action@ functions can call 'throwError'. + Note that @handler@ and the do-block must have the same return type. + -} + catchError :: m a -> (e -> m a) -> m a + diff --git a/Control/Monad/Identity.hs b/Control/Monad/Identity.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Identity.hs @@ -0,0 +1,95 @@ +{- | +Module : Control.Monad.Identity +Copyright : (c) Andy Gill 2001, + (c) Oregon Graduate Institute of Science and Technology 2001, + (c) Jeff Newbern 2003-2006, + (c) Andriy Palamarchuk 2006 +License : BSD-style (see the file libraries/base/LICENSE) + +Maintainer : libraries@haskell.org +Stability : experimental +Portability : portable + +[Computation type:] Simple function application. + +[Binding strategy:] The bound function is applied to the input value. +@'Identity' x >>= f == 'Identity' (f x)@ + +[Useful for:] Monads can be derived from monad transformers applied to the +'Identity' monad. + +[Zero and plus:] None. + +[Example type:] @'Identity' a@ + +The @Identity@ monad is a monad that does not embody any computational strategy. +It simply applies the bound function to its input without any modification. +Computationally, there is no reason to use the @Identity@ monad +instead of the much simpler act of simply applying functions to their arguments. +The purpose of the @Identity@ monad is its fundamental role in the theory +of monad transformers. +Any monad transformer applied to the @Identity@ monad yields a non-transformer +version of that monad. + + Inspired by the paper + /Functional Programming with Overloading and + Higher-Order Polymorphism/, + Mark P Jones () + Advanced School of Functional Programming, 1995. +-} + +module Control.Monad.Identity ( + Identity(..), + + module Control.Monad, + module Control.Monad.Fix, + ) where + +import Control.Monad +import Control.Monad.Fix + +{- | Identity wrapper. +Abstraction for wrapping up a object. +If you have an monadic function, say: + +> example :: Int -> Identity Int +> example x = return (x*x) + + you can \"run\" it, using + +> Main> runIdentity (example 42) +> 1764 :: Int + +A typical use of the Identity monad is to derive a monad +from a monad transformer. + +@ +-- derive the 'Control.Monad.State.State' monad using the 'Control.Monad.State.StateT' monad transformer +type 'Control.Monad.State.State' s a = 'Control.Monad.State.StateT' s 'Identity' a +@ + +The @'runIdentity'@ label is used in the type definition because it follows +a style of monad definition that explicitly represents monad values as +computations. In this style, a monadic computation is built up using the monadic +operators and then the value of the computation is extracted +using the @run******@ function. +Because the @Identity@ monad does not do any computation, its definition +is trivial. +For a better example of this style of monad, +see the @'Control.Monad.State.State'@ monad. +-} + +newtype Identity a = Identity { runIdentity :: a } + +-- --------------------------------------------------------------------------- +-- Identity instances for Functor and Monad + +instance Functor Identity where + fmap f m = Identity (f (runIdentity m)) + +instance Monad Identity where + return a = Identity a + m >>= k = k (runIdentity m) + +instance MonadFix Identity where + mfix f = Identity (fix (runIdentity . f)) diff --git a/Control/Monad/List.hs b/Control/Monad/List.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/List.hs @@ -0,0 +1,72 @@ +{-# OPTIONS -fallow-undecidable-instances #-} + +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.List +-- Copyright : (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology 2001, +-- (c) Mauro Jaskelioff 2008, +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : mjj@cs.nott.ac.uk +-- Stability : experimental +-- Portability : non-portable (multi-parameter type classes) +-- +-- The List monad. +-- +----------------------------------------------------------------------------- + +module Control.Monad.List ( + ListT(..), + mapListT, + module Control.Monad, + module Control.Monad.Trans, + ) where + +import Control.Monad +import Control.Monad.Trans + +-- --------------------------------------------------------------------------- +-- Our parameterizable list monad, with an inner monad + +newtype ListT m a = ListT { runListT :: m [a] } + +mapListT :: (m [a] -> n [b]) -> ListT m a -> ListT n b +mapListT f m = ListT $ f (runListT m) + +instance (Monad m) => Functor (ListT m) where + fmap f m = ListT $ do + a <- runListT m + return (map f a) + +instance (Monad m) => Monad (ListT m) where + return a = ListT $ return [a] + m >>= k = ListT $ do + a <- runListT m + b <- mapM (runListT . k) a + return (concat b) + fail _ = ListT $ return [] + +instance (Monad m) => MonadPlus (ListT m) where + mzero = ListT $ return [] + m `mplus` n = ListT $ do + a <- runListT m + b <- runListT n + return (a ++ b) + +-- --------------------------------------------------------------------------- +-- Instances for other mtl transformers + +instance (Monad m, MonadTrans t, Monad (t (ListT m))) => + MonadPlus (t (ListT m)) where + mzero = lift $ ListT $ return [] + mplus m n = join $ lift $ ListT $ return [m,n] + +instance MonadTrans ListT where + lift m = ListT $ do + a <- m + return [a] + tmap f _ = ListT . f . runListT + +instance (MonadIO m) => MonadIO (ListT m) where + liftIO = lift . liftIO diff --git a/Control/Monad/Reader.hs b/Control/Monad/Reader.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Reader.hs @@ -0,0 +1,266 @@ +{-# OPTIONS #-} +{- | +Module : Control.Monad.Reader +Copyright : (c) Mauro Jaskelioff 2008, + (c) Andy Gill 2001, + (c) Oregon Graduate Institute of Science and Technology 2001, + (c) Jeff Newbern 2003-2007, + (c) Andriy Palamarchuk 2007 +License : BSD-style (see the file libraries/base/LICENSE) + +Maintainer : mjj@cs.nott.ac.uk +Stability : experimental +Portability : non-portable (multi-param classes, functional dependencies) + +[Computation type:] Computations which read values from a shared environment. + +[Binding strategy:] Monad values are functions from the environment to a value. +The bound function is applied to the bound value, and both have access +to the shared environment. + +[Useful for:] Maintaining variable bindings, or other shared environment. + +[Zero and plus:] None. + +[Example type:] @'Reader' [(String,Value)] a@ + +The 'Reader' monad (also called the Environment monad). +Represents a computation, which can read values from +a shared environment, pass values from function to function, +and execute sub-computations in a modified environment. +Using 'Reader' monad for such computations is often clearer and easier +than using the 'Control.Monad.State.State' monad. + + Inspired by the paper + /Functional Programming with Overloading and + Higher-Order Polymorphism/, + Mark P Jones () + Advanced School of Functional Programming, 1995. +-} + +module Control.Monad.Reader ( + module Control.Monad.Reader.Class, + Reader(..), + mapReader, + withReader, + ReaderT(..), + mapReaderT, + withReaderT, + module Control.Monad, + module Control.Monad.Fix, + module Control.Monad.Trans, + -- * Example 1: Simple Reader Usage + -- $simpleReaderExample + + -- * Example 2: Modifying Reader Content With @local@ + -- $localExample + + -- * Example 3: @ReaderT@ Monad Transformer + -- $ReaderTExample + ) where + +import Control.Monad +import Control.Monad.Fix +import Control.Monad.Instances () +import Control.Monad.Reader.Class +import Control.Monad.State +import Control.Monad.Trans + + +-- ---------------------------------------------------------------------------- +-- The partially applied function type is a simple reader monad + +instance MonadReader r ((->) r) where + ask = id + local f m = m . f + +{- | +The parameterizable reader monad. + +The @return@ function creates a @Reader@ that ignores the environment, +and produces the given value. + +The binding operator @>>=@ produces a @Reader@ that uses the environment +to extract the value its left-hand side, +and then applies the bound function to that value in the same environment. +-} +newtype Reader r a = Reader { + {- | + Runs @Reader@ and extracts the final value from it. + To extract the value apply @(runReader reader)@ to an environment value. + Parameters: + + * A @Reader@ to run. + + * An initial environment. + -} + runReader :: r -> a +} + +mapReader :: (a -> b) -> Reader r a -> Reader r b +mapReader f m = Reader $ f . runReader m + +-- | A more general version of 'local'. + +withReader :: (r' -> r) -> Reader r a -> Reader r' a +withReader f m = Reader $ runReader m . f + +instance Functor (Reader r) where + fmap f m = Reader $ \r -> f (runReader m r) + +instance Monad (Reader r) where + return a = Reader $ \_ -> a + m >>= k = Reader $ \r -> runReader (k (runReader m r)) r + +instance MonadFix (Reader r) where + mfix f = Reader $ \r -> let a = runReader (f a) r in a + +instance MonadReader r (Reader r) where + ask = Reader id + local f m = Reader $ runReader m . f + +-- --------------------------------------------------------------------------- + +reader2State :: Reader r a -> State r a +reader2State m = State $ \s -> (runReader m s, s) + +state2Reader :: State r a -> Reader r a +state2Reader m = Reader $ \r -> fst (runState m r) + +instance (MonadTrans t, Monad (t (Reader r)), Monad (t (State r))) => + MonadReader r (t (Reader r)) where + ask = lift ask + local f m = tmap state2Reader reader2State $ join $ lift $ + State $ \r -> (tmap reader2State state2Reader m, f r) + +-- --------------------------------------------------------------------------- + +{- | +The reader monad transformer. +Can be used to add environment reading functionality to other monads. +-} +newtype ReaderT r m a = ReaderT { runReaderT :: r -> m a } + +mapReaderT :: (m a -> n b) -> ReaderT w m a -> ReaderT w n b +mapReaderT f m = ReaderT $ f . runReaderT m + +withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a +withReaderT f m = ReaderT $ runReaderT m . f + +instance (Monad m) => Functor (ReaderT r m) where + fmap f m = ReaderT $ \r -> do + a <- runReaderT m r + return (f a) + +instance (Monad m) => Monad (ReaderT r m) where + return a = ReaderT $ \_ -> return a + m >>= k = ReaderT $ \r -> do + a <- runReaderT m r + runReaderT (k a) r + fail msg = ReaderT $ \_ -> fail msg + +instance (MonadPlus m) => MonadPlus (ReaderT r m) where + mzero = ReaderT $ \_ -> mzero + m `mplus` n = ReaderT $ \r -> runReaderT m r `mplus` runReaderT n r + +instance (MonadFix m) => MonadFix (ReaderT r m) where + mfix f = ReaderT $ \r -> mfix $ \a -> runReaderT (f a) r + +instance (Monad m) => MonadReader r (ReaderT r m) where + ask = ReaderT return + local f m = ReaderT $ \r -> runReaderT m (f r) + + +-- --------------------------------------------------------------------------- +readerT2StateT :: Monad m => ReaderT r m a -> StateT r m a +readerT2StateT m = StateT $ \s -> liftM (\a -> (a,s)) $ runReaderT m s + +stateT2ReaderT :: Monad m => StateT r m a -> ReaderT r m a +stateT2ReaderT m = ReaderT $ \r -> liftM fst (runStateT m r) + +instance (Monad m, MonadTrans t, Monad (t (ReaderT r m)), + Monad (t (StateT r m))) => MonadReader r (t (ReaderT r m)) where + ask = lift $ ReaderT $ \r -> return r + local f m = tmap stateT2ReaderT readerT2StateT $ join $ lift $ + StateT $ \r -> return + (tmap readerT2StateT stateT2ReaderT m, f r) + +-- --------------------------------------------------------------------------- + +instance MonadTrans (ReaderT r) where + lift m = ReaderT $ \_ -> m + tmap f _ m = ReaderT $ \r -> f $ runReaderT m r + +instance (MonadIO m) => MonadIO (ReaderT r m) where + liftIO = lift . liftIO + +{- $simpleReaderExample + +In this example the @Reader@ monad provides access to variable bindings. +Bindings are a 'Map' of integer variables. +The variable @count@ contains number of variables in the bindings. +You can see how to run a Reader monad and retrieve data from it +with 'runReader', how to access the Reader data with 'ask' and 'asks'. + +> type Bindings = Map String Int; +> +>-- Returns True if the "count" variable contains correct bindings size. +>isCountCorrect :: Bindings -> Bool +>isCountCorrect bindings = runReader calc_isCountCorrect bindings +> +>-- The Reader monad, which implements this complicated check. +>calc_isCountCorrect :: Reader Bindings Bool +>calc_isCountCorrect = do +> count <- asks (lookupVar "count") +> bindings <- ask +> return (count == (Map.size bindings)) +> +>-- The selector function to use with 'asks'. +>-- Returns value of the variable with specified name. +>lookupVar :: String -> Bindings -> Int +>lookupVar name bindings = fromJust (Map.lookup name bindings) +> +>sampleBindings = Map.fromList [("count",3), ("1",1), ("b",2)] +> +>main = do +> putStr $ "Count is correct for bindings " ++ (show sampleBindings) ++ ": "; +> putStrLn $ show (isCountCorrect sampleBindings); +-} + +{- $localExample + +Shows how to modify Reader content with 'local'. + +>calculateContentLen :: Reader String Int +>calculateContentLen = do +> content <- ask +> return (length content); +> +>-- Calls calculateContentLen after adding a prefix to the Reader content. +>calculateModifiedContentLen :: Reader String Int +>calculateModifiedContentLen = local ("Prefix " ++) calculateContentLen +> +>main = do +> let s = "12345"; +> let modifiedLen = runReader calculateModifiedContentLen s +> let len = runReader calculateContentLen s +> putStrLn $ "Modified 's' length: " ++ (show modifiedLen) +> putStrLn $ "Original 's' length: " ++ (show len) +-} + +{- $ReaderTExample + +Now you are thinking: 'Wow, what a great monad! I wish I could use +Reader functionality in MyFavoriteComplexMonad!'. Don't worry. +This can be easy done with the 'ReaderT' monad transformer. +This example shows how to combine @ReaderT@ with the IO monad. + +>-- The Reader/IO combined monad, where Reader stores a string. +>printReaderContent :: ReaderT String IO () +>printReaderContent = do +> content <- ask +> liftIO $ putStrLn ("The Reader Content: " ++ content) +> +>main = do +> runReaderT printReaderContent "Some Content" +-} diff --git a/Control/Monad/Reader/Class.hs b/Control/Monad/Reader/Class.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Reader/Class.hs @@ -0,0 +1,74 @@ +{-# OPTIONS -fallow-undecidable-instances #-} +{- | +Module : Control.Monad.Reader.Class +Copyright : (c) Andy Gill 2001, + (c) Oregon Graduate Institute of Science and Technology 2001, + (c) Jeff Newbern 2003-2007, + (c) Andriy Palamarchuk 2007 +License : BSD-style (see the file libraries/base/LICENSE) + +Maintainer : libraries@haskell.org +Stability : experimental +Portability : non-portable (multi-param classes, functional dependencies) + +[Computation type:] Computations which read values from a shared environment. + +[Binding strategy:] Monad values are functions from the environment to a value. +The bound function is applied to the bound value, and both have access +to the shared environment. + +[Useful for:] Maintaining variable bindings, or other shared environment. + +[Zero and plus:] None. + +[Example type:] @'Reader' [(String,Value)] a@ + +The 'Reader' monad (also called the Environment monad). +Represents a computation, which can read values from +a shared environment, pass values from function to function, +and execute sub-computations in a modified environment. +Using 'Reader' monad for such computations is often clearer and easier +than using the 'Control.Monad.State.State' monad. + + Inspired by the paper + /Functional Programming with Overloading and + Higher-Order Polymorphism/, + Mark P Jones () + Advanced School of Functional Programming, 1995. +-} + +module Control.Monad.Reader.Class ( + MonadReader(..), + asks, + ) where + +{- | +See examples in "Control.Monad.Reader". +Note, the partially applied function type @(->) r@ is a simple reader monad. +See the @instance@ declaration below. +-} +class (Monad m) => MonadReader r m | m -> r where + -- | Retrieves the monad environment. + ask :: m r + {- | Executes a computation in a modified environment. Parameters: + + * The function to modify the environment. + + * @Reader@ to run. + + * The resulting @Reader@. + -} + local :: (r -> r) -> m a -> m a + +{- | +Retrieves a function of the current environment. Parameters: + +* The selector function to apply to the environment. + +See an example in "Control.Monad.Reader". +-} +asks :: (MonadReader r m) => (r -> a) -> m a +asks f = do + r <- ask + return (f r) + diff --git a/Control/Monad/State.hs b/Control/Monad/State.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/State.hs @@ -0,0 +1,27 @@ +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.State +-- Copyright : (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : mjj@cs.nott.ac.uk +-- Stability : experimental +-- Portability : non-portable (multi-param classes, functional dependencies) +-- +-- State monads. +-- +-- This module is inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. + +----------------------------------------------------------------------------- + +module Control.Monad.State ( + module Control.Monad.State.Lazy + ) where + +import Control.Monad.State.Lazy + diff --git a/Control/Monad/State/Class.hs b/Control/Monad/State/Class.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/State/Class.hs @@ -0,0 +1,62 @@ +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.State.Class +-- Copyright : (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : libraries@haskell.org +-- Stability : experimental +-- Portability : non-portable (multi-param classes, functional dependencies) +-- +-- MonadState class. +-- +-- This module is inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. + +----------------------------------------------------------------------------- + +module Control.Monad.State.Class ( + -- * MonadState class + MonadState(..), + modify, + gets, + ) where + +-- --------------------------------------------------------------------------- +-- | /get/ returns the state from the internals of the monad. +-- +-- /put/ replaces the state inside the monad. + +class (Monad m) => MonadState s m | m -> s where + get :: m s + put :: s -> m () + +-- | Monadic state transformer. +-- +-- Maps an old state to a new state inside a state monad. +-- The old state is thrown away. +-- +-- > Main> :t modify ((+1) :: Int -> Int) +-- > modify (...) :: (MonadState Int a) => a () +-- +-- This says that @modify (+1)@ acts over any +-- Monad that is a member of the @MonadState@ class, +-- with an @Int@ state. + +modify :: (MonadState s m) => (s -> s) -> m () +modify f = do + s <- get + put (f s) + +-- | Gets specific component of the state, using a projection function +-- supplied. + +gets :: (MonadState s m) => (s -> a) -> m a +gets f = do + s <- get + return (f s) + diff --git a/Control/Monad/State/Lazy.hs b/Control/Monad/State/Lazy.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/State/Lazy.hs @@ -0,0 +1,283 @@ +{-# OPTIONS #-} + +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.State.Lazy +-- Copyright : (c) Mauro Jaskelioff 2008, +-- (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : mjj@cs.nott.ac.uk +-- Stability : experimental +-- Portability : non-portable (multi-param classes, functional dependencies) +-- +-- Lazy state monads. +-- +-- This module is inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. +-- +-- See below for examples. + +----------------------------------------------------------------------------- + +module Control.Monad.State.Lazy ( + module Control.Monad.State.Class, + -- * The State Monad + State(..), + evalState, + execState, + mapState, + withState, + -- * The StateT Monad + StateT(..), + evalStateT, + execStateT, + mapStateT, + withStateT, + module Control.Monad, + module Control.Monad.Fix, + module Control.Monad.Trans, + -- * Examples + -- $examples + ) where + +import Control.Monad +import Control.Monad.Fix +import Control.Monad.State.Class +import Control.Monad.Trans + +-- --------------------------------------------------------------------------- +-- | A parameterizable state monad where /s/ is the type of the state +-- to carry and /a/ is the type of the /return value/. + +newtype State s a = State { runState :: s -> (a, s) } + +-- |Evaluate this state monad with the given initial state,throwing +-- away the final state. Very much like @fst@ composed with +-- @runstate@. + +evalState :: State s a -- ^The state to evaluate + -> s -- ^An initial value + -> a -- ^The return value of the state application +evalState m s = fst (runState m s) + +-- |Execute this state and return the new state, throwing away the +-- return value. Very much like @snd@ composed with +-- @runstate@. + +execState :: State s a -- ^The state to evaluate + -> s -- ^An initial value + -> s -- ^The new state +execState m s = snd (runState m s) + +-- |Map a stateful computation from one (return value, state) pair to +-- another. For instance, to convert numberTree from a function that +-- returns a tree to a function that returns the sum of the numbered +-- tree (see the Examples section for numberTree and sumTree) you may +-- write: +-- +-- > sumNumberedTree :: (Eq a) => Tree a -> State (Table a) Int +-- > sumNumberedTree = mapState (\ (t, tab) -> (sumTree t, tab)) . numberTree + +mapState :: ((a, s) -> (b, s)) -> State s a -> State s b +mapState f m = State $ f . runState m + +-- |Apply this function to this state and return the resulting state. +withState :: (s -> s) -> State s a -> State s a +withState f m = State $ runState m . f + +instance Functor (State s) where + fmap f m = State $ \s -> let + (a, s') = runState m s + in (f a, s') + +instance Monad (State s) where + return a = State $ \s -> (a, s) + m >>= k = State $ \s -> let + (a, s') = runState m s + in runState (k a) s' + +instance MonadFix (State s) where + mfix f = State $ \s -> let (a, s') = runState (f a) s in (a, s') + +instance MonadState s (State s) where + get = State $ \s -> (s, s) + put s = State $ \_ -> ((), s) + +instance (MonadTrans t, Monad (t (State s))) + => MonadState s (t (State s)) where + get = lift $ State $ \s -> (s, s) + put s = lift $ State $ \_ -> ((), s) + +-- --------------------------------------------------------------------------- +-- | A parameterizable state monad for encapsulating an inner +-- monad. +-- +-- The StateT Monad structure is parameterized over two things: +-- +-- * s - The state. +-- +-- * m - The inner monad. +-- +-- Here are some examples of use: +-- +-- (Parser from ParseLib with Hugs) +-- +-- > type Parser a = StateT String [] a +-- > ==> StateT (String -> [(a,String)]) +-- +-- For example, item can be written as: +-- +-- > item = do (x:xs) <- get +-- > put xs +-- > return x +-- > +-- > type BoringState s a = StateT s Indentity a +-- > ==> StateT (s -> Identity (a,s)) +-- > +-- > type StateWithIO s a = StateT s IO a +-- > ==> StateT (s -> IO (a,s)) +-- > +-- > type StateWithErr s a = StateT s Maybe a +-- > ==> StateT (s -> Maybe (a,s)) + +newtype StateT s m a = StateT { runStateT :: s -> m (a,s) } + +-- |Similar to 'evalState' +evalStateT :: (Monad m) => StateT s m a -> s -> m a +evalStateT m s = do + ~(a, _) <- runStateT m s + return a + +-- |Similar to 'execState' +execStateT :: (Monad m) => StateT s m a -> s -> m s +execStateT m s = do + ~(_, s') <- runStateT m s + return s' + +-- |Similar to 'mapState' +mapStateT :: (m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b +mapStateT f m = StateT $ f . runStateT m + +-- |Similar to 'withState' +withStateT :: (s -> s) -> StateT s m a -> StateT s m a +withStateT f m = StateT $ runStateT m . f + +instance (Monad m) => Functor (StateT s m) where + fmap f m = StateT $ \s -> do + ~(x, s') <- runStateT m s + return (f x, s') + +instance (Monad m) => Monad (StateT s m) where + return a = StateT $ \s -> return (a, s) + m >>= k = StateT $ \s -> do + ~(a, s') <- runStateT m s + runStateT (k a) s' + fail str = StateT $ \_ -> fail str + +instance (MonadPlus m) => MonadPlus (StateT s m) where + mzero = StateT $ \_ -> mzero + m `mplus` n = StateT $ \s -> runStateT m s `mplus` runStateT n s + +instance (MonadFix m) => MonadFix (StateT s m) where + mfix f = StateT $ \s -> mfix $ \ ~(a, _) -> runStateT (f a) s + +instance (Monad m) => MonadState s (StateT s m) where + get = StateT $ \s -> return (s, s) + put s = StateT $ \_ -> return ((), s) + + +-- --------------------------------------------------------------------------- +-- Instances for other mtl transformers + +instance (Monad m, MonadTrans t, Monad (t (StateT s m))) + => MonadState s (t (StateT s m)) where + get = lift $ StateT $ \s -> return (s, s) + put s = lift $ StateT $ \_ -> return ((), s) + +instance MonadTrans (StateT s) where + lift m = StateT $ \s -> do + a <- m + return (a, s) + tmap f _ m = StateT $ \s -> f $ runStateT m s + +instance (MonadIO m) => MonadIO (StateT s m) where + liftIO = lift . liftIO + +-- --------------------------------------------------------------------------- +-- $examples +-- A function to increment a counter. Taken from the paper +-- /Generalising Monads to Arrows/, John +-- Hughes (), November 1998: +-- +-- > tick :: State Int Int +-- > tick = do n <- get +-- > put (n+1) +-- > return n +-- +-- Add one to the given number using the state monad: +-- +-- > plusOne :: Int -> Int +-- > plusOne n = execState tick n +-- +-- A contrived addition example. Works only with positive numbers: +-- +-- > plus :: Int -> Int -> Int +-- > plus n x = execState (sequence $ replicate n tick) x +-- +-- An example from /The Craft of Functional Programming/, Simon +-- Thompson (), +-- Addison-Wesley 1999: \"Given an arbitrary tree, transform it to a +-- tree of integers in which the original elements are replaced by +-- natural numbers, starting from 0. The same element has to be +-- replaced by the same number at every occurrence, and when we meet +-- an as-yet-unvisited element we have to find a \'new\' number to match +-- it with:\" +-- +-- > data Tree a = Nil | Node a (Tree a) (Tree a) deriving (Show, Eq) +-- > type Table a = [a] +-- +-- > numberTree :: Eq a => Tree a -> State (Table a) (Tree Int) +-- > numberTree Nil = return Nil +-- > numberTree (Node x t1 t2) +-- > = do num <- numberNode x +-- > nt1 <- numberTree t1 +-- > nt2 <- numberTree t2 +-- > return (Node num nt1 nt2) +-- > where +-- > numberNode :: Eq a => a -> State (Table a) Int +-- > numberNode x +-- > = do table <- get +-- > (newTable, newPos) <- return (nNode x table) +-- > put newTable +-- > return newPos +-- > nNode:: (Eq a) => a -> Table a -> (Table a, Int) +-- > nNode x table +-- > = case (findIndexInList (== x) table) of +-- > Nothing -> (table ++ [x], length table) +-- > Just i -> (table, i) +-- > findIndexInList :: (a -> Bool) -> [a] -> Maybe Int +-- > findIndexInList = findIndexInListHelp 0 +-- > findIndexInListHelp _ _ [] = Nothing +-- > findIndexInListHelp count f (h:t) +-- > = if (f h) +-- > then Just count +-- > else findIndexInListHelp (count+1) f t +-- +-- numTree applies numberTree with an initial state: +-- +-- > numTree :: (Eq a) => Tree a -> Tree Int +-- > numTree t = evalState (numberTree t) [] +-- +-- > testTree = Node "Zero" (Node "One" (Node "Two" Nil Nil) (Node "One" (Node "Zero" Nil Nil) Nil)) Nil +-- > numTree testTree => Node 0 (Node 1 (Node 2 Nil Nil) (Node 1 (Node 0 Nil Nil) Nil)) Nil +-- +-- sumTree is a little helper function that does not use the State monad: +-- +-- > sumTree :: (Num a) => Tree a -> a +-- > sumTree Nil = 0 +-- > sumTree (Node e t1 t2) = e + (sumTree t1) + (sumTree t2) diff --git a/Control/Monad/State/Strict.hs b/Control/Monad/State/Strict.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/State/Strict.hs @@ -0,0 +1,282 @@ +{-# OPTIONS #-} + +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.State.Strict +-- Copyright : (c) Mauro Jaskelioff 2008, +-- (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : mjj@cs.nott.ac.uk +-- Stability : experimental +-- Portability : non-portable (multi-param classes, functional dependencies) +-- +-- Strict state monads. +-- +-- This module is inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. +-- +-- See below for examples. + +----------------------------------------------------------------------------- + +module Control.Monad.State.Strict ( + module Control.Monad.State.Class, + -- * The State Monad + State(..), + evalState, + execState, + mapState, + withState, + -- * The StateT Monad + StateT(..), + evalStateT, + execStateT, + mapStateT, + withStateT, + module Control.Monad, + module Control.Monad.Fix, + module Control.Monad.Trans, + -- * Examples + -- $examples + ) where + +import Control.Monad +import Control.Monad.Fix +import Control.Monad.State.Class +import Control.Monad.Trans + +-- --------------------------------------------------------------------------- +-- | A parameterizable state monad where /s/ is the type of the state +-- to carry and /a/ is the type of the /return value/. + +newtype State s a = State { runState :: s -> (a, s) } + +-- |Evaluate this state monad with the given initial state,throwing +-- away the final state. Very much like @fst@ composed with +-- @runstate@. + +evalState :: State s a -- ^The state to evaluate + -> s -- ^An initial value + -> a -- ^The return value of the state application +evalState m s = fst (runState m s) + +-- |Execute this state and return the new state, throwing away the +-- return value. Very much like @snd@ composed with +-- @runstate@. + +execState :: State s a -- ^The state to evaluate + -> s -- ^An initial value + -> s -- ^The new state +execState m s = snd (runState m s) + +-- |Map a stateful computation from one (return value, state) pair to +-- another. For instance, to convert numberTree from a function that +-- returns a tree to a function that returns the sum of the numbered +-- tree (see the Examples section for numberTree and sumTree) you may +-- write: +-- +-- > sumNumberedTree :: (Eq a) => Tree a -> State (Table a) Int +-- > sumNumberedTree = mapState (\ (t, tab) -> (sumTree t, tab)) . numberTree + +mapState :: ((a, s) -> (b, s)) -> State s a -> State s b +mapState f m = State $ f . runState m + +-- |Apply this function to this state and return the resulting state. +withState :: (s -> s) -> State s a -> State s a +withState f m = State $ runState m . f + + +instance Functor (State s) where + fmap f m = State $ \s -> case runState m s of + (a, s') -> (f a, s') + +instance Monad (State s) where + return a = State $ \s -> (a, s) + m >>= k = State $ \s -> case runState m s of + (a, s') -> runState (k a) s' + +instance MonadFix (State s) where + mfix f = State $ \s -> let (a, s') = runState (f a) s in (a, s') + +instance MonadState s (State s) where + get = State $ \s -> (s, s) + put s = State $ \_ -> ((), s) + +instance (MonadTrans t, Monad (t (State s))) + => MonadState s (t (State s)) where + get = lift $ State $ \s -> (s, s) + put s = lift $ State $ \_ -> ((), s) + +-- --------------------------------------------------------------------------- +-- | A parameterizable state monad for encapsulating an inner +-- monad. +-- +-- The StateT Monad structure is parameterized over two things: +-- +-- * s - The state. +-- +-- * m - The inner monad. +-- +-- Here are some examples of use: +-- +-- (Parser from ParseLib with Hugs) +-- +-- > type Parser a = StateT String [] a +-- > ==> StateT (String -> [(a,String)]) +-- +-- For example, item can be written as: +-- +-- > item = do (x:xs) <- get +-- > put xs +-- > return x +-- > +-- > type BoringState s a = StateT s Indentity a +-- > ==> StateT (s -> Identity (a,s)) +-- > +-- > type StateWithIO s a = StateT s IO a +-- > ==> StateT (s -> IO (a,s)) +-- > +-- > type StateWithErr s a = StateT s Maybe a +-- > ==> StateT (s -> Maybe (a,s)) + +newtype StateT s m a = StateT { runStateT :: s -> m (a,s) } + +-- |Similar to 'evalState' +evalStateT :: (Monad m) => StateT s m a -> s -> m a +evalStateT m s = do + (a, _) <- runStateT m s + return a + +-- |Similar to 'execState' +execStateT :: (Monad m) => StateT s m a -> s -> m s +execStateT m s = do + (_, s') <- runStateT m s + return s' + +-- |Similar to 'mapState' +mapStateT :: (m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b +mapStateT f m = StateT $ f . runStateT m + +-- |Similar to 'withState' +withStateT :: (s -> s) -> StateT s m a -> StateT s m a +withStateT f m = StateT $ runStateT m . f + +instance (Monad m) => Functor (StateT s m) where + fmap f m = StateT $ \s -> do + (x, s') <- runStateT m s + return (f x, s') + +instance (Monad m) => Monad (StateT s m) where + return a = StateT $ \s -> return (a, s) + m >>= k = StateT $ \s -> do + (a, s') <- runStateT m s + runStateT (k a) s' + fail str = StateT $ \_ -> fail str + +instance (MonadPlus m) => MonadPlus (StateT s m) where + mzero = StateT $ \_ -> mzero + m `mplus` n = StateT $ \s -> runStateT m s `mplus` runStateT n s + +instance (MonadFix m) => MonadFix (StateT s m) where + mfix f = StateT $ \s -> mfix $ \ ~(a, _) -> runStateT (f a) s + +instance (Monad m) => MonadState s (StateT s m) where + get = StateT $ \s -> return (s, s) + put s = StateT $ \_ -> return ((), s) + +-- --------------------------------------------------------------------------- +-- Instances for other mtl transformers + +instance (Monad m, MonadTrans t, Monad (t (StateT s m))) + => MonadState s (t (StateT s m)) where + get = lift $ StateT $ \s -> return (s, s) + put s = lift $ StateT $ \_ -> return ((), s) + +instance MonadTrans (StateT s) where + lift m = StateT $ \s -> do + a <- m + return (a, s) + tmap f _ m = StateT $ \s -> f $ runStateT m s + + +instance (MonadIO m) => MonadIO (StateT s m) where + liftIO = lift . liftIO + +-- --------------------------------------------------------------------------- +-- $examples +-- A function to increment a counter. Taken from the paper +-- /Generalising Monads to Arrows/, John +-- Hughes (), November 1998: +-- +-- > tick :: State Int Int +-- > tick = do n <- get +-- > put (n+1) +-- > return n +-- +-- Add one to the given number using the state monad: +-- +-- > plusOne :: Int -> Int +-- > plusOne n = execState tick n +-- +-- A contrived addition example. Works only with positive numbers: +-- +-- > plus :: Int -> Int -> Int +-- > plus n x = execState (sequence $ replicate n tick) x +-- +-- An example from /The Craft of Functional Programming/, Simon +-- Thompson (), +-- Addison-Wesley 1999: \"Given an arbitrary tree, transform it to a +-- tree of integers in which the original elements are replaced by +-- natural numbers, starting from 0. The same element has to be +-- replaced by the same number at every occurrence, and when we meet +-- an as-yet-unvisited element we have to find a \'new\' number to match +-- it with:\" +-- +-- > data Tree a = Nil | Node a (Tree a) (Tree a) deriving (Show, Eq) +-- > type Table a = [a] +-- +-- > numberTree :: Eq a => Tree a -> State (Table a) (Tree Int) +-- > numberTree Nil = return Nil +-- > numberTree (Node x t1 t2) +-- > = do num <- numberNode x +-- > nt1 <- numberTree t1 +-- > nt2 <- numberTree t2 +-- > return (Node num nt1 nt2) +-- > where +-- > numberNode :: Eq a => a -> State (Table a) Int +-- > numberNode x +-- > = do table <- get +-- > (newTable, newPos) <- return (nNode x table) +-- > put newTable +-- > return newPos +-- > nNode:: (Eq a) => a -> Table a -> (Table a, Int) +-- > nNode x table +-- > = case (findIndexInList (== x) table) of +-- > Nothing -> (table ++ [x], length table) +-- > Just i -> (table, i) +-- > findIndexInList :: (a -> Bool) -> [a] -> Maybe Int +-- > findIndexInList = findIndexInListHelp 0 +-- > findIndexInListHelp _ _ [] = Nothing +-- > findIndexInListHelp count f (h:t) +-- > = if (f h) +-- > then Just count +-- > else findIndexInListHelp (count+1) f t +-- +-- numTree applies numberTree with an initial state: +-- +-- > numTree :: (Eq a) => Tree a -> Tree Int +-- > numTree t = evalState (numberTree t) [] +-- +-- > testTree = Node "Zero" (Node "One" (Node "Two" Nil Nil) (Node "One" (Node "Zero" Nil Nil) Nil)) Nil +-- > numTree testTree => Node 0 (Node 1 (Node 2 Nil Nil) (Node 1 (Node 0 Nil Nil) Nil)) Nil +-- +-- sumTree is a little helper function that does not use the State monad: +-- +-- > sumTree :: (Num a) => Tree a -> a +-- > sumTree Nil = 0 +-- > sumTree (Node e t1 t2) = e + (sumTree t1) + (sumTree t2) diff --git a/Control/Monad/Trans.hs b/Control/Monad/Trans.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Trans.hs @@ -0,0 +1,47 @@ +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.Trans +-- Copyright : (c) Mauro Jaskelioff 2008, +-- (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : mjj@cs.nott.ac.uk +-- Stability : experimental +-- Portability : portable +-- +-- The MonadTrans class. +-- +-- Inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. +----------------------------------------------------------------------------- + +module Control.Monad.Trans ( + MonadTrans(..), + MonadIO(..), + ) where + +import System.IO + +-- --------------------------------------------------------------------------- +-- MonadTrans class +-- +-- Monad to facilitate stackable Monads. +-- Provides a way of digging into an outer +-- monad, giving access to (lifting) the inner monad. + +class MonadTrans t where + lift :: Monad m => m a -> t m a + tmap :: (Monad m, Monad n) => + (forall a. m a -> n a) -> + (forall b. n b -> m b) -> + t m c -> t n c + +class (Monad m) => MonadIO m where + liftIO :: IO a -> m a + +instance MonadIO IO where + liftIO = id diff --git a/Control/Monad/Writer.hs b/Control/Monad/Writer.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Writer.hs @@ -0,0 +1,26 @@ +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.Writer +-- Copyright : (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : mjj@cs.nott.ac.uk +-- Stability : experimental +-- Portability : non-portable (multi-param classes, functional dependencies) +-- +-- The MonadWriter class. +-- +-- Inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. +----------------------------------------------------------------------------- + +module Control.Monad.Writer ( + module Control.Monad.Writer.Lazy + ) where + +import Control.Monad.Writer.Lazy + diff --git a/Control/Monad/Writer/Class.hs b/Control/Monad/Writer/Class.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Writer/Class.hs @@ -0,0 +1,58 @@ +{-# OPTIONS -fallow-undecidable-instances #-} +-- Search for -fallow-undecidable-instances to see why this is needed + +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.Writer.Class +-- Copyright : (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : libraries@haskell.org +-- Stability : experimental +-- Portability : non-portable (multi-param classes, functional dependencies) +-- +-- The MonadWriter class. +-- +-- Inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. +----------------------------------------------------------------------------- + +module Control.Monad.Writer.Class ( + MonadWriter(..), + listens, + censor, + ) where + +import Data.Monoid + +-- --------------------------------------------------------------------------- +-- MonadWriter class +-- +-- tell is like tell on the MUD's it shouts to monad +-- what you want to be heard. The monad carries this 'packet' +-- upwards, merging it if needed (hence the Monoid requirement)} +-- +-- listen listens to a monad acting, and returns what the monad "said". +-- +-- pass lets you provide a writer transformer which changes internals of +-- the written object. + +class (Monoid w, Monad m) => MonadWriter w m | m -> w where + tell :: w -> m () + listen :: m a -> m (a, w) + pass :: m (a, w -> w) -> m a + +listens :: (MonadWriter w m) => (w -> b) -> m a -> m (a, b) +listens f m = do + ~(a, w) <- listen m + return (a, f w) + +censor :: (MonadWriter w m) => (w -> w) -> m a -> m a +censor f m = pass $ do + a <- m + return (a, f) + diff --git a/Control/Monad/Writer/Lazy.hs b/Control/Monad/Writer/Lazy.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Writer/Lazy.hs @@ -0,0 +1,162 @@ +{-# OPTIONS #-} + +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.Writer.Lazy +-- Copyright : (c) Mauro Jaskleioff 2008, +-- (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : mjj@cs.nott.ac.uk +-- Stability : experimental +-- Portability : non-portable (multi-param classes, functional dependencies) +-- +-- Lazy writer monads. +-- +-- Inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. +----------------------------------------------------------------------------- + +module Control.Monad.Writer.Lazy ( + module Control.Monad.Writer.Class, + Writer(..), + execWriter, + mapWriter, + WriterT(..), + execWriterT, + mapWriterT, + module Control.Monad, + module Control.Monad.Fix, + module Control.Monad.Trans, + module Data.Monoid, + ) where +import Control.Monad +import Control.Monad.Fix +import Control.Monad.State +import Control.Monad.Trans +import Control.Monad.Writer.Class +import Data.Monoid + +-- --------------------------------------------------------------------------- +-- Our parameterizable writer monad + +newtype Writer w a = Writer { runWriter :: (a, w) } + +execWriter :: Writer w a -> w +execWriter m = snd (runWriter m) + +mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b +mapWriter f m = Writer $ f (runWriter m) + +instance Functor (Writer w) where + fmap f m = Writer $ let (a, w) = runWriter m in (f a, w) + +instance (Monoid w) => Monad (Writer w) where + return a = Writer (a, mempty) + m >>= k = Writer $ let + (a, w) = runWriter m + (b, w') = runWriter (k a) + in (b, w `mappend` w') + +instance (Monoid w) => MonadFix (Writer w) where + mfix m = Writer $ let (a, w) = runWriter (m a) in (a, w) + +instance (Monoid w) => MonadWriter w (Writer w) where + tell w = Writer ((), w) + listen m = Writer $ let (a, w) = runWriter m in ((a, w), w) + pass m = Writer $ let ((a, f), w) = runWriter m in (a, f w) + +writer2State :: (Monoid w) => Writer w a -> State w a +writer2State (Writer (a,w')) = State $ \w -> (a, w `mappend` w') + +state2Writer :: (Monoid w) => State w a -> Writer w a +state2Writer (State m) = Writer $ m mempty + +instance (Monoid w, MonadTrans t, + Monad (t (Writer w)), Monad (t (State w))) => + MonadWriter w (t (Writer w)) where + tell w = lift $ Writer ((), w) + listen m = tmap state2Writer writer2State + $ (tmap writer2State state2Writer m) + >>= \a -> lift $ State $ \w -> ((a,w), w) + pass m = tmap state2Writer writer2State + $ (tmap writer2State state2Writer m) + >>= \(a, f) -> lift $ State $ \w -> (a, f w) + +-- --------------------------------------------------------------------------- +-- Our parameterizable writer monad, with an inner monad + +newtype WriterT w m a = WriterT { runWriterT :: m (a, w) } + +execWriterT :: Monad m => WriterT w m a -> m w +execWriterT m = do + ~(_, w) <- runWriterT m + return w + +mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b +mapWriterT f m = WriterT $ f (runWriterT m) + +instance (Monad m) => Functor (WriterT w m) where + fmap f m = WriterT $ do + ~(a, w) <- runWriterT m + return (f a, w) + +instance (Monoid w, Monad m) => Monad (WriterT w m) where + return a = WriterT $ return (a, mempty) + m >>= k = WriterT $ do + ~(a, w) <- runWriterT m + ~(b, w') <- runWriterT (k a) + return (b, w `mappend` w') + fail msg = WriterT $ fail msg + +instance (Monoid w, MonadPlus m) => MonadPlus (WriterT w m) where + mzero = WriterT mzero + m `mplus` n = WriterT $ runWriterT m `mplus` runWriterT n + +instance (Monoid w, MonadFix m) => MonadFix (WriterT w m) where + mfix m = WriterT $ mfix $ \ ~(a, _) -> runWriterT (m a) + +instance (Monoid w, Monad m) => MonadWriter w (WriterT w m) where + tell w = WriterT $ return ((), w) + listen m = WriterT $ do + ~(a, w) <- runWriterT m + return ((a, w), w) + pass m = WriterT $ do + ~((a, f), w) <- runWriterT m + return (a, f w) + +-- --------------------------------------------------------------------------- +writerT2StateT :: (Monad m, Monoid w) => WriterT w m a -> StateT w m a +writerT2StateT (WriterT m) = StateT $ \w -> + liftM (\(a,w') -> (a, w `mappend` w')) m + +stateT2WriterT :: (Monoid w) => StateT w m a -> WriterT w m a +stateT2WriterT (StateT m) = WriterT $ m mempty + + +instance (Monoid w, Monad m, MonadTrans t, + Monad (t (WriterT w m)), Monad (t (StateT w m))) => + MonadWriter w (t (WriterT w m)) where + tell w = lift $ WriterT $ return ((), w) + listen m = tmap stateT2WriterT writerT2StateT + $ (tmap writerT2StateT stateT2WriterT m) + >>= \a -> lift $ StateT $ \w -> return ((a,w), w) + pass m = tmap stateT2WriterT writerT2StateT + $ (tmap writerT2StateT stateT2WriterT m) + >>= \(a, f) -> lift $ StateT $ \w -> return (a, f w) + +-- --------------------------------------------------------------------------- + +instance (Monoid w) => MonadTrans (WriterT w) where + lift m = WriterT $ do + a <- m + return (a, mempty) + tmap f _ m = WriterT $ f (runWriterT m) + +instance (Monoid w, MonadIO m) => MonadIO (WriterT w m) where + liftIO = lift . liftIO + diff --git a/Control/Monad/Writer/Strict.hs b/Control/Monad/Writer/Strict.hs new file mode 100644 --- /dev/null +++ b/Control/Monad/Writer/Strict.hs @@ -0,0 +1,166 @@ +{-# OPTIONS #-} + +----------------------------------------------------------------------------- +-- | +-- Module : Control.Monad.Writer.Strict +-- Copyright : (c) Mauro Jaskelioff 2008, +-- (c) Andy Gill 2001, +-- (c) Oregon Graduate Institute of Science and Technology, 2001 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : mjj@cs.nott.ac.uk +-- Stability : experimental +-- Portability : non-portable (multi-param classes, functional dependencies) +-- +-- Strict writer monads. +-- +-- Inspired by the paper +-- /Functional Programming with Overloading and +-- Higher-Order Polymorphism/, +-- Mark P Jones () +-- Advanced School of Functional Programming, 1995. +----------------------------------------------------------------------------- + +module Control.Monad.Writer.Strict ( + module Control.Monad.Writer.Class, + Writer(..), + execWriter, + mapWriter, + WriterT(..), + execWriterT, + mapWriterT, + module Control.Monad, + module Control.Monad.Fix, + module Control.Monad.Trans, + module Data.Monoid, + ) where + +import Control.Monad +import Control.Monad.Fix +import Control.Monad.State.Strict +import Control.Monad.Trans +import Control.Monad.Writer.Class +import Data.Monoid + +-- --------------------------------------------------------------------------- +-- Our parameterizable writer monad + +newtype Writer w a = Writer { runWriter :: (a, w) } + +execWriter :: Writer w a -> w +execWriter m = snd (runWriter m) + +mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b +mapWriter f m = Writer $ f (runWriter m) + +instance Functor (Writer w) where + fmap f m = Writer $ case runWriter m of + (a, w) -> (f a, w) + +instance (Monoid w) => Monad (Writer w) where + return a = Writer (a, mempty) + m >>= k = Writer $ case runWriter m of + (a, w) -> case runWriter (k a) of + (b, w') -> (b, w `mappend` w') + +instance (Monoid w) => MonadFix (Writer w) where + mfix m = Writer $ let (a, w) = runWriter (m a) in (a, w) + +instance (Monoid w) => MonadWriter w (Writer w) where + tell w = Writer ((), w) + listen m = Writer $ case runWriter m of + (a, w) -> ((a, w), w) + pass m = Writer $ case runWriter m of + ((a, f), w) -> (a, f w) + +writer2State :: (Monoid w) => Writer w a -> State w a +writer2State (Writer (a,w')) = State $ \w -> (a, w `mappend` w') + +state2Writer :: (Monoid w) => State w a -> Writer w a +state2Writer (State m) = Writer $ m mempty + +instance (Monoid w, MonadTrans t, + Monad (t (Writer w)), Monad (t (State w))) => + MonadWriter w (t (Writer w)) where + tell w = lift $ Writer ((), w) + listen m = tmap state2Writer writer2State + $ (tmap writer2State state2Writer m) + >>= \a -> lift $ State $ \w -> ((a,w), w) + pass m = tmap state2Writer writer2State + $ (tmap writer2State state2Writer m) + >>= \(a, f) -> lift $ State $ \w -> (a, f w) + +-- --------------------------------------------------------------------------- +-- Our parameterizable writer monad, with an inner monad + +newtype WriterT w m a = WriterT { runWriterT :: m (a, w) } + +execWriterT :: Monad m => WriterT w m a -> m w +execWriterT m = do + (_, w) <- runWriterT m + return w + +mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b +mapWriterT f m = WriterT $ f (runWriterT m) + +instance (Monad m) => Functor (WriterT w m) where + fmap f m = WriterT $ do + (a, w) <- runWriterT m + return (f a, w) + +instance (Monoid w, Monad m) => Monad (WriterT w m) where + return a = WriterT $ return (a, mempty) + m >>= k = WriterT $ do + (a, w) <- runWriterT m + (b, w') <- runWriterT (k a) + return (b, w `mappend` w') + fail msg = WriterT $ fail msg + +instance (Monoid w, MonadPlus m) => MonadPlus (WriterT w m) where + mzero = WriterT mzero + m `mplus` n = WriterT $ runWriterT m `mplus` runWriterT n + +instance (Monoid w, MonadFix m) => MonadFix (WriterT w m) where + mfix m = WriterT $ mfix $ \ ~(a, _) -> runWriterT (m a) + +instance (Monoid w, Monad m) => MonadWriter w (WriterT w m) where + tell w = WriterT $ return ((), w) + listen m = WriterT $ do + (a, w) <- runWriterT m + return ((a, w), w) + pass m = WriterT $ do + ((a, f), w) <- runWriterT m + return (a, f w) + +-- --------------------------------------------------------------------------- +-- Instances for other mtl transformers + +writerT2StateT :: (Monad m, Monoid w) => WriterT w m a -> StateT w m a +writerT2StateT (WriterT m) = StateT $ \w -> + liftM (\(a,w') -> (a, w `mappend` w')) m + +stateT2WriterT :: (Monoid w) => StateT w m a -> WriterT w m a +stateT2WriterT (StateT m) = WriterT $ m mempty + + +instance (Monoid w, Monad m, MonadTrans t, + Monad (t (WriterT w m)), Monad (t (StateT w m))) => + MonadWriter w (t (WriterT w m)) where + tell w = lift $ WriterT $ return ((), w) + listen m = tmap stateT2WriterT writerT2StateT + $ (tmap writerT2StateT stateT2WriterT m) + >>= \a -> lift $ StateT $ \w -> return ((a,w), w) + pass m = tmap stateT2WriterT writerT2StateT + $ (tmap writerT2StateT stateT2WriterT m) + >>= \(a, f) -> lift $ StateT $ \w -> return (a, f w) + +instance (Monoid w) => MonadTrans (WriterT w) where + lift m = WriterT $ do + a <- m + return (a, mempty) + tmap f _ m = WriterT $ f (runWriterT m) + + +instance (Monoid w, MonadIO m) => MonadIO (WriterT w m) where + liftIO = lift . liftIO + diff --git a/LICENSE b/LICENSE new file mode 100644 --- /dev/null +++ b/LICENSE @@ -0,0 +1,31 @@ +The Glasgow Haskell Compiler License + +Copyright 2004, The University Court of the University of Glasgow. +All rights reserved. + +Redistribution and use in source and binary forms, with or without +modification, are permitted provided that the following conditions are met: + +- Redistributions of source code must retain the above copyright notice, +this list of conditions and the following disclaimer. + +- Redistributions in binary form must reproduce the above copyright notice, +this list of conditions and the following disclaimer in the documentation +and/or other materials provided with the distribution. + +- Neither name of the University nor the names of its contributors may be +used to endorse or promote products derived from this software without +specific prior written permission. + +THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY COURT OF THE UNIVERSITY OF +GLASGOW AND THE CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, +INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND +FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE +UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW OR THE CONTRIBUTORS BE LIABLE +FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR +SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER +CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH +DAMAGE. diff --git a/Setup.hs b/Setup.hs new file mode 100644 --- /dev/null +++ b/Setup.hs @@ -0,0 +1,6 @@ +module Main (main) where + +import Distribution.Simple + +main :: IO () +main = defaultMain diff --git a/mmtl.cabal b/mmtl.cabal new file mode 100644 --- /dev/null +++ b/mmtl.cabal @@ -0,0 +1,48 @@ +name: mmtl +version: 0.1 +license: BSD3 +license-file: LICENSE +author: Mauro Jaskelioff +maintainer: mjj@cs.nott.ac.uk +category: Control +synopsis: Modular Monad transformer library +description: + A modular monad transformer library, (almost) a drop-in replacement for + the monad transformer library (mtl). It provides a uniform lifting of + operations through any monad transformer. + + Known differences with mtl: + - It provides a uniform lifting of operations for + any monad transformer. + - It does not provide a RWS monad (but you can build it yourself ;) + - The class MonadTrans requires a new member function tmap. + - The lifting of callCC through StateT coincides with + the lifting in MonadLib, but not with the lifting in mtl. +build-type: Simple +ghc-options: -Wall +exposed-modules: + Control.Monad.Codensity + Control.Monad.Cont + Control.Monad.Cont.Class + Control.Monad.Error + Control.Monad.Error.Class + Control.Monad.Identity + Control.Monad.List + Control.Monad.Reader + Control.Monad.Reader.Class + Control.Monad.State + Control.Monad.State.Class + Control.Monad.State.Lazy + Control.Monad.State.Strict + Control.Monad.Trans + Control.Monad.Writer + Control.Monad.Writer.Class + Control.Monad.Writer.Lazy + Control.Monad.Writer.Strict +build-depends: base +extensions: MultiParamTypeClasses, + FunctionalDependencies, + FlexibleInstances, + FlexibleContexts, + TypeSynonymInstances, + Rank2Types