Control.Monad.Trans.RWS.CPS

A monad transformer that combines ReaderT, WriterT and StateT. This version uses continuation-passing-style for the writer part to achieve constant space usage. This transformer can be used as a drop-in replacement for Control.Monad.Trans.RWS.Strict.

The RWS monad

type RWS r w s = RWST r w s Identity

A monad containing an environment of type r, output of type w and an updatable state of type s.

rws :: Monoid w => (r -> s -> (a, s, w)) -> RWS r w s a

Construct an RWS computation from a function. (The inverse of runRWS.)

runRWS :: Monoid w => RWS r w s a -> r -> s -> (a, s, w)

Unwrap an RWS computation as a function. (The inverse of rws.)

evalRWS

Arguments

:: Monoid w

=> RWS r w s a

RWS computation to execute

-> r

initial environment

-> s

initial value

-> (a, w)

final value and output

Evaluate a computation with the given initial state and environment, returning the final value and output, discarding the final state.

execRWS

Arguments

:: Monoid w

=> RWS r w s a

RWS computation to execute

-> r

initial environment

-> s

initial value

-> (s, w)

final state and output

Evaluate a computation with the given initial state and environment, returning the final state and output, discarding the final value.

mapRWS :: (Monoid w, Monoid w') => ((a, s, w) -> (b, s, w')) -> RWS r w s a -> RWS r w' s b

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)

withRWS :: (r' -> s -> (r, s)) -> RWS r w s a -> RWS r' w s a

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)

The RWST monad transformer

data RWST r w s m a

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.

Instances

MonadTrans (RWST r w s)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

lift :: Monad m => m a -> RWST r w s m a

Monad m => Monad (RWST r w s m)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

(>>=) :: RWST r w s m a -> (a -> RWST r w s m b) -> RWST r w s m b

(>>) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m b

return :: a -> RWST r w s m a

fail :: String -> RWST r w s m a

Functor m => Functor (RWST r w s m)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

fmap :: (a -> b) -> RWST r w s m a -> RWST r w s m b

(<$) :: a -> RWST r w s m b -> RWST r w s m a

MonadFix m => MonadFix (RWST r w s m)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

mfix :: (a -> RWST r w s m a) -> RWST r w s m a

MonadFail m => MonadFail (RWST r w s m)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

fail :: String -> RWST r w s m a

(Functor m, Monad m) => Applicative (RWST r w s m)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

pure :: a -> RWST r w s m a

(<*>) :: RWST r w s m (a -> b) -> RWST r w s m a -> RWST r w s m b

liftA2 :: (a -> b -> c) -> RWST r w s m a -> RWST r w s m b -> RWST r w s m c

(*>) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m b

(<*) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m a

MonadIO m => MonadIO (RWST r w s m)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

liftIO :: IO a -> RWST r w s m a

(Functor m, MonadPlus m) => Alternative (RWST r w s m)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

empty :: RWST r w s m a

(<|>) :: RWST r w s m a -> RWST r w s m a -> RWST r w s m a

some :: RWST r w s m a -> RWST r w s m [a]

many :: RWST r w s m a -> RWST r w s m [a]

(Functor m, MonadPlus m) => MonadPlus (RWST r w s m)

Defined in Control.Monad.Trans.RWS.CPS.Internal

Methods

mzero :: RWST r w s m a

mplus :: RWST r w s m a -> RWST r w s m a -> RWST r w s m a

rwsT :: (Functor m, Monoid w) => (r -> s -> m (a, s, w)) -> RWST r w s m a

The RWST constructor is deliberately not exported in the CPS module to avoid exposing the hidden state w. rwsT provides a safe way to construct a RWST with the same api as the original RWST.

runRWST :: Monoid w => RWST r w s m a -> r -> s -> m (a, s, w)

Unwrap an RWST computation as a function.

evalRWST

Arguments

:: (Monad m, Monoid w)

=> RWST r w s m a

computation to execute

-> r

initial environment

-> s

initial value

-> m (a, w)

computation yielding final value and output

Evaluate a computation with the given initial state and environment, returning the final value and output, discarding the final state.

execRWST

Arguments

:: (Monad m, Monoid w)

=> RWST r w s m a

computation to execute

-> r

initial environment

-> s

initial value

-> m (s, w)

computation yielding final state and output

Evaluate a computation with the given initial state and environment, returning the final state and output, discarding the final value.

mapRWST :: (Monad n, Monoid w, Monoid w') => (m (a, s, w) -> n (b, s, w')) -> RWST r w s m a -> RWST r w' s n b

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

withRWST :: (r' -> s -> (r, s)) -> RWST r w s m a -> RWST r' w s m a

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)

Reader operations

reader :: Monad m => (r -> a) -> RWST r w s m a

Constructor for computations in the reader monad (equivalent to asks).

ask :: Monad m => RWST r w s m r

Fetch the value of the environment.

local :: (r -> r) -> RWST r w s m a -> RWST r w s m a

Execute a computation in a modified environment

• runRWST (local f m) r s = runRWST m (f r) s

asks :: Monad m => (r -> a) -> RWST r w s m a

Retrieve a function of the current environment.

• asks f = liftM f ask

Writer operations

writer :: (Monoid w, Monad m) => (a, w) -> RWST r w s m a

Construct a writer computation from a (result, output) pair.

tell :: (Monoid w, Monad m) => w -> RWST r w s m ()

tell w is an action that produces the output w.

listen :: (Monoid w, Monad m) => RWST r w s m a -> RWST r w s m (a, 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)

listens :: (Monoid w, Monad m) => (w -> b) -> RWST r w s m a -> RWST r w s m (a, b)

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)

pass :: (Monoid w, Monoid w', Monad m) => RWST r w s m (a, w -> w') -> RWST r w' s m a

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)

censor :: (Monoid w, Monad m) => (w -> w) -> RWST r w s m a -> RWST r w s m a

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)

State operations

state :: Monad m => (s -> (a, s)) -> RWST r w s m a

Construct a state monad computation from a state transformer function.

get :: Monad m => RWST r w s m s

Fetch the current value of the state within the monad.

put :: Monad m => s -> RWST r w s m ()

put s sets the state within the monad to s.

modify :: Monad m => (s -> s) -> RWST r w s m ()

modify f is an action that updates the state to the result of applying f to the current state.

• modify f = get >>= (put . f)

gets :: Monad m => (s -> a) -> RWST r w s m a

Get a specific component of the state, using a projection function supplied.

• gets f = liftM f get

Lifting other operations

liftCallCC :: CallCC m (a, s, w) (b, s, w) -> CallCC (RWST r w s m) a b

Uniform lifting of a callCC operation to the new monad. This version rolls back to the original state on entering the continuation.

liftCallCC' :: CallCC m (a, s, w) (b, s, w) -> CallCC (RWST r w s m) a b

In-situ lifting of a callCC operation to the new monad. This version uses the current state on entering the continuation.

liftCatch :: Catch e m (a, s, w) -> Catch e (RWST r w s m) a

Lift a catchE operation to the new monad.