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)

newtype RWST r w s (m :: Type -> Type) 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 :: forall r' s r w (m :: Type -> Type) a. (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 :: forall w (m :: Type -> Type) r a s. (Monoid w, Monad m) => (r -> a) -> RWST r w s m a Source

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

ask :: forall w (m :: Type -> Type) r s. (Monoid w, Monad m) => RWST r w s m r Source

Fetch the value of the environment.

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

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

tell :: forall (m :: Type -> Type) w r s. Monad m => w -> RWST r w s m () Source

tell w is an action that produces the output w.

listen :: forall (m :: Type -> Type) r w s a. 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 :: forall (m :: Type -> Type) w b r s a. 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 :: forall (m :: Type -> Type) r w s a. 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 :: forall (m :: Type -> Type) w r s a. 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 :: forall w (m :: Type -> Type) s a r. (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 :: forall w (m :: Type -> Type) r s. (Monoid w, Monad m) => RWST r w s m s Source

Fetch the current value of the state within the monad.

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

put s sets the state within the monad to s.

modify :: forall w (m :: Type -> Type) s r. (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 :: forall w (m :: Type -> Type) s a r. (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

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. The uniformity property (see Control.Monad.Signatures) implies that the lifted catchE discards any output or changes to the state from the body on entering the handler.