transformers-0.3.0.0: Concrete functor and monad transformers

Portabilityportable
Stabilityexperimental
Maintainer[email protected]
Safe HaskellSafe-Inferred

Control.Monad.Trans.RWS.Strict

Contents

Description

A monad transformer that combines ReaderT, WriterT and StateT. This version is strict; for a lazy version, see Control.Monad.Trans.RWS.Lazy, which has the same interface.

Synopsis

The RWS monad

type RWS r w s = RWST r w s IdentitySource

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 aSource

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

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

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

evalRWSSource

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.

execRWSSource

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 bSource

Map the return value, final state and output of a computation using the given function.

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

withRWS f m executes action m with an initial environment and state modified by applying f.

The RWST monad transformer

newtype RWST r w s m a Source

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 => MonadTrans (RWST r w s) 
(Monoid w, Monad m) => Monad (RWST r w s m) 
Functor m => Functor (RWST r w s m) 
(Monoid w, MonadFix m) => MonadFix (RWST r w s m) 
(Monoid w, MonadPlus m) => MonadPlus (RWST r w s m) 
(Monoid w, Functor m, Monad m) => Applicative (RWST r w s m) 
(Monoid w, Functor m, MonadPlus m) => Alternative (RWST r w s m) 
(Monoid w, MonadIO m) => MonadIO (RWST r w s m) 

evalRWSTSource

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.

execRWSTSource

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 bSource

Map the inner computation using the given function.

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

withRWST f m executes action m with an initial environment and state modified by applying f.

Reader operations

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

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

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

Fetch the value of the environment.

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

Execute a computation in a modified environment

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

Retrieve a function of the current environment.

Writer operations

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

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

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

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)Source

listen m is an action that executes the action m and adds its output to the value of the computation.

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

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.

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

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.

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

censor f m is an action that executes the action m and applies the function f to its output, leaving the return value unchanged.

State operations

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

Construct a state monad computation from a state transformer function.

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

Fetch the current value of the state within the monad.

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

put s sets the state within the monad to s.

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

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

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

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

Lifting other operations

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 aSource

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 aSource

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

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 aSource

Lift a catchError operation to the new monad.