Portability	portable
Stability	experimental
Maintainer	libraries@haskell.org

Control.Monad.Trans.RWS.Lazy

Contents

The RWS monad
The RWST monad transformer
Reader operations
Writer operations
State operations
Lifting other operations

Description

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.

Synopsis

The RWS monad

type RWS r w s = RWST r w s Identity Source

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.)

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

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

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

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

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)

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

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

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

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

Reader operations

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

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.

listens f m = liftM (id *** f) (listen m)

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.

censor f m = pass (liftM (\x -> (x,f)) m)

State operations

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.

```
gets f = liftM f get
```

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.