Portability	non-portable (multi-param classes, functional dependencies)
Stability	experimental
Maintainer	libraries@haskell.org
Safe Haskell	Safe-Inferred

Control.Monad.RWS.Lazy

Contents

The RWS monad
The RWST monad transformer
Lazy Reader-writer-state monads

Description

Lazy RWS monad.

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

Synopsis

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 :: (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)

The RWST monad transformer

newtype 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.

Constructors

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

Instances

(Monoid w, Monad m) => MonadRWS r w s (RWST r w s m)
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)
(Monad m, Monoid w) => MonadState s (RWST r w s m)
(Monad m, Monoid w) => MonadReader r (RWST r w s m)
(Monoid w, MonadError e m) => MonadError e (RWST r w s m)
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)
(Monoid w, MonadCont m) => MonadCont (RWST r w s m)

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)

Lazy Reader-writer-state monads

module Control.Monad.RWS.Class

module Control.Monad

module Control.Monad.Fix

module Control.Monad.Trans

module Data.Monoid