Portability	portable
Stability	experimental
Maintainer	ross@soi.city.ac.uk
Safe Haskell	Safe-Infered

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

type RWS r w s = RWST r w s Identity
rws :: (r -> s -> (a, s, w)) -> RWS r w s a
runRWS :: RWS r w s a -> r -> s -> (a, s, w)
evalRWS :: RWS r w s a -> r -> s -> (a, w)
execRWS :: RWS r w s a -> r -> s -> (s, w)
mapRWS :: ((a, s, w) -> (b, s, w')) -> RWS r w s a -> RWS r w' s b
withRWS :: (r' -> s -> (r, s)) -> RWS r w s a -> RWS r' w s a
newtype RWST r w s m a = RWST {
- runRWST :: r -> s -> m (a, s, w)
}
evalRWST :: Monad m => RWST r w s m a -> r -> s -> m (a, w)
execRWST :: Monad m => RWST r w s m a -> r -> s -> m (s, w)
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
reader :: (Monoid w, Monad m) => (r -> a) -> RWST r w s m a
ask :: (Monoid w, Monad m) => RWST r w s m r
local :: (Monoid w, Monad m) => (r -> r) -> RWST r w s m a -> RWST r w s m a
asks :: (Monoid w, Monad m) => (r -> a) -> RWST r w s m a
writer :: Monad m => (a, w) -> RWST r w s m a
tell :: (Monoid w, Monad m) => w -> RWST r w s m ()
listen :: (Monoid w, Monad m) => RWST r w s m a -> RWST r w s m (a, w)
listens :: (Monoid w, Monad m) => (w -> b) -> RWST r w s m a -> RWST r w s m (a, b)
pass :: (Monoid w, Monad m) => RWST r w s m (a, w -> w) -> RWST r w s m a
censor :: (Monoid w, Monad m) => (w -> w) -> RWST r w s m a -> RWST r w s m a
state :: (Monoid w, Monad m) => (s -> (a, s)) -> RWST r w s m a
get :: (Monoid w, Monad m) => RWST r w s m s
put :: (Monoid w, Monad m) => s -> RWST r w s m ()
modify :: (Monoid w, Monad m) => (s -> s) -> RWST r w s m ()
gets :: (Monoid w, Monad m) => (s -> a) -> RWST r w s m a
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 a
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 a
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 a

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 Source

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 Source

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.

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 aSource

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 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 Source

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 Source

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.

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 aSource

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 :: (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

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

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

Retrieve a function of the current environment.

```
asks  f = liftM  f ask 
```

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.

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)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)

runRWST  (listens  f m) r s = liftM  (\(a, w) -> ((a, f w), w)) (runRWST  m r s)

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.

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 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)

runRWST  (censor  f m) r s = liftM  (\(a, w) -> (a, f w)) (runRWST  m r s)

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.

```
modify  f = get  >>= (put  . f)
```

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.