Control/Monad/Trans/RWS/Lazy.hs

    -----------------------------------------------------------------------------
    -- |
    -- 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