----------------------------------------------------------------------------- -- | -- Module : Control.Monad.Trans.RWS.Lazy -- Copyright : (c) Andy Gill 2001, -- (c) Oregon Graduate Institute of Science and Technology, 2001 -- License : BSD-style (see the file LICENSE) -- -- Maintainer : ross@soi.city.ac.uk -- Stability : experimental -- Portability : portable -- -- A monad transformer that combines 'ReaderT', 'WriterT' and 'StateT'. -- This version is lazy; for a strict version, see -- "Control.Monad.Trans.RWS.Strict", which has the same interface. ----------------------------------------------------------------------------- module Control.Monad.Trans.RWS.Lazy ( -- * The RWS monad RWS, rws, runRWS, evalRWS, execRWS, mapRWS, withRWS, -- * The RWST monad transformer RWST(..), evalRWST, execRWST, mapRWST, withRWST, -- * Reader operations reader, ask, local, asks, -- * Writer operations writer, tell, listen, listens, pass, censor, -- * State operations state, get, put, modify, gets, -- * Lifting other operations liftCallCC, liftCallCC', liftCatch, ) where import Control.Monad.IO.Class import Control.Monad.Trans.Class import Data.Functor.Identity import Control.Applicative import Control.Monad import Control.Monad.Fix import Data.Monoid -- | A monad containing an environment of type @r@, output of type @w@ -- and an updatable state of type @s@. type RWS r w s = RWST r w s Identity -- | Construct an RWS computation from a function. -- (The inverse of 'runRWS'.) rws :: (r -> s -> (a, s, w)) -> RWS r w s a rws f = RWST (\ r s -> Identity (f r s)) -- | Unwrap an RWS computation as a function. -- (The inverse of 'rws'.) runRWS :: RWS r w s a -> r -> s -> (a, s, w) runRWS m r s = runIdentity (runRWST m r s) -- | Evaluate a computation with the given initial state and environment, -- returning the final value and output, discarding the final state. evalRWS :: RWS r w s a -- ^RWS computation to execute -> r -- ^initial environment -> s -- ^initial value -> (a, w) -- ^final value and output evalRWS m r s = let (a, _, w) = runRWS m r s in (a, w) -- | Evaluate a computation with the given initial state and environment, -- returning the final state and output, discarding the final value. execRWS :: RWS r w s a -- ^RWS computation to execute -> r -- ^initial environment -> s -- ^initial value -> (s, w) -- ^final state and output execRWS m r s = let (_, s', w) = runRWS m r s in (s', w) -- | Map the return value, final state and output of a computation using -- the given function. -- -- * @'runRWS' ('mapRWS' f m) r s = f ('runRWS' m r s)@ mapRWS :: ((a, s, w) -> (b, s, w')) -> RWS r w s a -> RWS r w' s b mapRWS f = mapRWST (Identity . f . runIdentity) -- | @'withRWS' f m@ executes action @m@ with an initial environment -- and state modified by applying @f@. -- -- * @'runRWS' ('withRWS' f m) r s = 'uncurry' ('runRWS' m) (f r s)@ withRWS :: (r' -> s -> (r, s)) -> RWS r w s a -> RWS r' w s a withRWS = withRWST -- --------------------------------------------------------------------------- -- | A monad transformer adding reading an environment of type @r@, -- collecting an output of type @w@ and updating a state of type @s@ -- to an inner monad @m@. newtype RWST r w s m a = RWST { runRWST :: r -> s -> m (a, s, w) } -- | Evaluate a computation with the given initial state and environment, -- returning the final value and output, discarding the final state. evalRWST :: (Monad m) => RWST r w s m a -- ^computation to execute -> r -- ^initial environment -> s -- ^initial value -> m (a, w) -- ^computation yielding final value and output evalRWST m r s = do ~(a, _, w) <- runRWST m r s return (a, w) -- | Evaluate a computation with the given initial state and environment, -- returning the final state and output, discarding the final value. execRWST :: (Monad m) => RWST r w s m a -- ^computation to execute -> r -- ^initial environment -> s -- ^initial value -> m (s, w) -- ^computation yielding final state and output execRWST m r s = do ~(_, s', w) <- runRWST m r s return (s', w) -- | Map the inner computation using the given function. -- -- * @'runRWST' ('mapRWST' f m) r s = f ('runRWST' m r s)@ mapRWST :: (m (a, s, w) -> n (b, s, w')) -> RWST r w s m a -> RWST r w' s n b mapRWST f m = RWST $ \r s -> f (runRWST m r s) -- | @'withRWST' f m@ executes action @m@ with an initial environment -- and state modified by applying @f@. -- -- * @'runRWST' ('withRWST' f m) r s = 'uncurry' ('runRWST' m) (f r s)@ withRWST :: (r' -> s -> (r, s)) -> RWST r w s m a -> RWST r' w s m a withRWST f m = RWST $ \r s -> uncurry (runRWST m) (f r s) instance (Functor m) => Functor (RWST r w s m) where fmap f m = RWST $ \r s -> fmap (\ ~(a, s', w) -> (f a, s', w)) $ runRWST m r s instance (Monoid w, Functor m, Monad m) => Applicative (RWST r w s m) where pure = return (<*>) = ap instance (Monoid w, Functor m, MonadPlus m) => Alternative (RWST r w s m) where empty = mzero (<|>) = mplus instance (Monoid w, Monad m) => Monad (RWST r w s m) where return a = RWST $ \_ s -> return (a, s, mempty) m >>= k = RWST $ \r s -> do ~(a, s', w) <- runRWST m r s ~(b, s'',w') <- runRWST (k a) r s' return (b, s'', w `mappend` w') fail msg = RWST $ \_ _ -> fail msg instance (Monoid w, MonadPlus m) => MonadPlus (RWST r w s m) where mzero = RWST $ \_ _ -> mzero m `mplus` n = RWST $ \r s -> runRWST m r s `mplus` runRWST n r s instance (Monoid w, MonadFix m) => MonadFix (RWST r w s m) where mfix f = RWST $ \r s -> mfix $ \ ~(a, _, _) -> runRWST (f a) r s instance (Monoid w) => MonadTrans (RWST r w s) where lift m = RWST $ \_ s -> do a <- m return (a, s, mempty) instance (Monoid w, MonadIO m) => MonadIO (RWST r w s m) where liftIO = lift . liftIO -- --------------------------------------------------------------------------- -- Reader operations -- | Constructor for computations in the reader monad (equivalent to 'asks'). reader :: (Monoid w, Monad m) => (r -> a) -> RWST r w s m a reader = asks -- | Fetch the value of the environment. ask :: (Monoid w, Monad m) => RWST r w s m r ask = RWST $ \r s -> return (r, s, mempty) -- | Execute a computation in a modified environment -- -- * @'runRWST' ('local' f m) r s = 'runRWST' m (f r) s@ local :: (Monoid w, Monad m) => (r -> r) -> RWST r w s m a -> RWST r w s m a local f m = RWST $ \r s -> runRWST m (f r) s -- | Retrieve a function of the current environment. -- -- * @'asks' f = 'liftM' f 'ask'@ asks :: (Monoid w, Monad m) => (r -> a) -> RWST r w s m a asks f = RWST $ \r s -> return (f r, s, mempty) -- --------------------------------------------------------------------------- -- Writer operations -- | Construct a writer computation from a (result, output) pair. writer :: Monad m => (a, w) -> RWST r w s m a writer (a, w) = RWST $ \_ s -> return (a, s, w) -- | @'tell' w@ is an action that produces the output @w@. tell :: (Monoid w, Monad m) => w -> RWST r w s m () tell w = RWST $ \_ s -> return ((),s,w) -- | @'listen' m@ is an action that executes the action @m@ and adds its -- output to the value of the computation. -- -- * @'runRWST' ('listen' m) r s = 'liftM' (\\(a, w) -> ((a, w), w)) ('runRWST' m r s)@ listen :: (Monoid w, Monad m) => RWST r w s m a -> RWST r w s m (a, w) listen m = RWST $ \r s -> do ~(a, s', w) <- runRWST m r s return ((a, w), s', w) -- | @'listens' f m@ is an action that executes the action @m@ and adds -- the result of applying @f@ to the output to the value of the computation. -- -- * @'listens' f m = 'liftM' (id *** f) ('listen' m)@ -- -- * @'runRWST' ('listens' f m) r s = 'liftM' (\\(a, w) -> ((a, f w), w)) ('runRWST' m r s)@ listens :: (Monoid w, Monad m) => (w -> b) -> RWST r w s m a -> RWST r w s m (a, b) listens f m = RWST $ \r s -> do ~(a, s', w) <- runRWST m r s return ((a, f w), s', w) -- | @'pass' m@ is an action that executes the action @m@, which returns -- a value and a function, and returns the value, applying the function -- to the output. -- -- * @'runRWST' ('pass' m) r s = 'liftM' (\\((a, f), w) -> (a, f w)) ('runRWST' m r s)@ pass :: (Monoid w, Monad m) => RWST r w s m (a, w -> w) -> RWST r w s m a pass m = RWST $ \r s -> do ~((a, f), s', w) <- runRWST m r s return (a, s', f w) -- | @'censor' f m@ is an action that executes the action @m@ and -- applies the function @f@ to its output, leaving the return value -- unchanged. -- -- * @'censor' f m = 'pass' ('liftM' (\\x -> (x,f)) m)@ -- -- * @'runRWST' ('censor' f m) r s = 'liftM' (\\(a, w) -> (a, f w)) ('runRWST' m r s)@ censor :: (Monoid w, Monad m) => (w -> w) -> RWST r w s m a -> RWST r w s m a censor f m = RWST $ \r s -> do ~(a, s', w) <- runRWST m r s return (a, s', f w) -- --------------------------------------------------------------------------- -- State operations -- | Construct a state monad computation from a state transformer function. state :: (Monoid w, Monad m) => (s -> (a,s)) -> RWST r w s m a state f = RWST $ \_ s -> let (a,s') = f s in return (a, s', mempty) -- | Fetch the current value of the state within the monad. get :: (Monoid w, Monad m) => RWST r w s m s get = RWST $ \_ s -> return (s, s, mempty) -- | @'put' s@ sets the state within the monad to @s@. put :: (Monoid w, Monad m) => s -> RWST r w s m () put s = RWST $ \_ _ -> return ((), s, mempty) -- | @'modify' f@ is an action that updates the state to the result of -- applying @f@ to the current state. -- -- * @'modify' f = 'get' >>= ('put' . f)@ modify :: (Monoid w, Monad m) => (s -> s) -> RWST r w s m () modify f = RWST $ \_ s -> return ((), f s, mempty) -- | Get a specific component of the state, using a projection function -- supplied. -- -- * @'gets' f = 'liftM' f 'get'@ gets :: (Monoid w, Monad m) => (s -> a) -> RWST r w s m a gets f = RWST $ \_ s -> return (f s, s, mempty) -- | Uniform lifting of a @callCC@ operation to the new monad. -- This version rolls back to the original state on entering the -- continuation. liftCallCC :: (Monoid w) => ((((a,s,w) -> m (b,s,w)) -> m (a,s,w)) -> m (a,s,w)) -> ((a -> RWST r w s m b) -> RWST r w s m a) -> RWST r w s m a liftCallCC callCC f = RWST $ \r s -> callCC $ \c -> runRWST (f (\a -> RWST $ \_ _ -> c (a, s, mempty))) r s -- | In-situ lifting of a @callCC@ operation to the new monad. -- This version uses the current state on entering the continuation. liftCallCC' :: (Monoid w) => ((((a,s,w) -> m (b,s,w)) -> m (a,s,w)) -> m (a,s,w)) -> ((a -> RWST r w s m b) -> RWST r w s m a) -> RWST r w s m a liftCallCC' callCC f = RWST $ \r s -> callCC $ \c -> runRWST (f (\a -> RWST $ \_ s' -> c (a, s', mempty))) r s -- | Lift a @catchError@ operation to the new monad. liftCatch :: (m (a,s,w) -> (e -> m (a,s,w)) -> m (a,s,w)) -> RWST l w s m a -> (e -> RWST l w s m a) -> RWST l w s m a liftCatch catchError m h = RWST $ \r s -> runRWST m r s `catchError` \e -> runRWST (h e) r s