Control.Monad.Trans.MSF.RWS

This module combines the wrapping and running functions for the Reader, Writer and State monad layers in a single layer.

It is based on the _strict_ RWS monad Strict, so when combining it with other modules such as mtl's, the strict version has to be included, i.e. Strict instead of RWS or Lazy.

Documentation

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

Wrap an MSF with explicit state variables in RWST monad.

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

Run the RWST layer by making the state variables explicit.

newtype RWST r w s (m :: Type -> Type) 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.

Constructors

RWST

Fields

• runRWST :: r -> s -> m (a, s, w)

Instances

(Monoid w, MonadSplit g m) => MonadSplit g (RWST r w s m)

Defined in Control.Monad.Random.Class

Methods

getSplit :: RWST r w s m g

(Monoid w, MonadBase b m) => MonadBase b (RWST r w s m)

Defined in Control.Monad.Base

Methods

liftBase :: b α -> RWST r w s m α

Monoid w => MonadTrans (RWST r w s)

Defined in Control.Monad.Trans.RWS.Strict

Methods

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

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

Defined in Control.Monad.Trans.RWS.Strict

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.Strict

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

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

Defined in Control.Monad.Trans.RWS.Strict

Methods

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

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

Defined in Control.Monad.Trans.RWS.Strict

Methods

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

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

Defined in Control.Monad.Trans.RWS.Strict

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

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

Defined in Control.Monad.Trans.RWS.Strict

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

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

Defined in Control.Monad.Trans.RWS.Strict

Methods

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

(Monoid w, MonadRandom m) => MonadRandom (RWST r w s m)

Defined in Control.Monad.Random.Class

Methods

getRandomR :: Random a => (a, a) -> RWST r w s m a

getRandom :: Random a => RWST r w s m a

getRandomRs :: Random a => (a, a) -> RWST r w s m [a]

getRandoms :: Random a => RWST r w s m [a]

(Monoid w, MonadInterleave m) => MonadInterleave (RWST r w s m)

Defined in Control.Monad.Random.Class

Methods

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

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

Defined in Control.Monad.Trans.RWS.Strict

Methods

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

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

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

Defined in Control.Monad.Trans.RWS.Strict

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]

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

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

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

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

evalRWS

Arguments

:: 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

:: 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 :: ((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)

evalRWST

Arguments

:: 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

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

execRWST

Arguments

:: 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

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

mapRWST :: (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)

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)

liftCallCC' :: Monoid w => 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.

gets :: (Monoid w, 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

modify :: (Monoid w, 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)

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

put s sets the state within the monad to s.

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

Fetch the current value of the state within the monad.

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

Construct a state monad computation from a state transformer function.

censor :: 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)

pass :: 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)

listens :: 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)

listen :: 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)

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

tell w is an action that produces the output w.

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

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

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

Retrieve a function of the current environment.

• asks f = liftM f ask

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

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

Fetch the value of the environment.

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

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