Copyright	(c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001
License	BSD-style (see the file LICENSE)
Maintainer	[email protected]
Stability	experimental
Portability	portable
Safe Haskell	Safe
Language	Haskell98

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 a Source

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

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

execRWS Source

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 Source

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 Source

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.

evalRWST Source

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

execRWST Source

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 Source

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 Source

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 a Source

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

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

Fetch the value of the environment.

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

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 a Source

Retrieve a function of the current environment.

• asks f = liftM f ask

Writer operations

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

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 a Source

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 a Source

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 a Source

Construct a state monad computation from a state transformer function.

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

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 a Source

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

• gets f = liftM f get

Lifting other operations

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

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 => CallCC m (a, s, w) (b, s, w) -> CallCC (RWST r w s m) a b Source

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

liftCatch :: Catch e m (a, s, w) -> Catch e (RWST r w s m) a Source

Lift a catchE operation to the new monad.