A monad containing an environment of type r, output of type w and an updatable state of type s.

Construct an RWS computation from a function. (The inverse of runRWS .)

Unwrap an RWS computation as a function. (The inverse of rws .)

Arguments

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

Arguments

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

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 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)

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.

Instances

Construct an RWST computation from a function. (The inverse of runRWST .)

Unwrap an RWST computation as a function. (The inverse of rwsT .)

Arguments

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

Arguments

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

Map the inner computation using the given function.

• runRWST (mapRWST f m) r s = f (runRWST m r s) mapRWST :: (m (a, s, w) -> n (b, s, w')) -> RWST r w s m a -> RWST r w' s n b

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)

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

Fetch the value of the environment.

Execute a computation in a modified environment

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

Retrieve a function of the current environment.

• asks  f = liftM  f ask 

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

tell w is an action that produces the output w.

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 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 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 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)

Construct a state monad computation from a state transformer function.

Fetch the current value of the state within the monad.

put s sets the state within the monad to s.

modify f is an action that updates the state to the result of applying f to the current state.

• modify  f = get  >>= (put  . f)

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

• gets  f = liftM  f get 

Uniform lifting of a callCC operation to the new monad. This version rolls back to the original state on entering the continuation.

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

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.