Control.Monad.RWS.CPS

Stricter RWS monad using continuation-passing-style for the writer output.

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

The RWS monad

type RWS r w s = RWST r w s Identity

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

rws :: Monoid w => (r -> s -> (a, s, w)) -> RWS r w s a

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

runRWS :: Monoid w => RWS r w s a -> r -> s -> (a, s, w)

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

evalRWS

Arguments

:: Monoid w

=> 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

Arguments

:: Monoid w

=> 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 :: (Monoid w, Monoid w') => ((a, s, w) -> (b, s, w')) -> RWS r w s a -> RWS r w' s b

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

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

data RWST r w s m a :: * -> * -> * -> (* -> *) -> * -> *

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

MonadTrans (RWST r w s)

Methods

lift :: Monad m => m a -> RWST r w s m a

Monad m => Monad (RWST r w s m)

Methods

(>>=) :: RWST r w s m a -> (a -> RWST r w s m b) -> RWST r w s m b

(>>) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m b

return :: a -> RWST r w s m a

fail :: String -> RWST r w s m a

Functor m => Functor (RWST r w s m)

Methods

fmap :: (a -> b) -> RWST r w s m a -> RWST r w s m b

(<$) :: a -> RWST r w s m b -> RWST r w s m a

MonadFix m => MonadFix (RWST r w s m)

Methods

mfix :: (a -> RWST r w s m a) -> RWST r w s m a

MonadFail m => MonadFail (RWST r w s m)

Methods

fail :: String -> RWST r w s m a

(Functor m, Monad m) => Applicative (RWST r w s m)

Methods

pure :: a -> RWST r w s m a

(<*>) :: RWST r w s m (a -> b) -> RWST r w s m a -> RWST r w s m b

(*>) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m b

(<*) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m a

MonadIO m => MonadIO (RWST r w s m)

Methods

liftIO :: IO a -> RWST r w s m a

(Functor m, MonadPlus m) => Alternative (RWST r w s m)

Methods

empty :: RWST r w s m a

(<|>) :: RWST r w s m a -> RWST r w s m a -> RWST r w s m a

some :: RWST r w s m a -> RWST r w s m [a]

many :: RWST r w s m a -> RWST r w s m [a]

(Functor m, MonadPlus m) => MonadPlus (RWST r w s m)

Methods

mzero :: RWST r w s m a

mplus :: RWST r w s m a -> RWST r w s m a -> RWST r w s m a

rwsT :: (Functor m, Monoid w) => (r -> s -> m (a, s, w)) -> RWST r w s m a

The RWST constructor is deliberately not exported in the CPS module to avoid exposing the hidden state w. rwsT provides a safe way to construct a RWST with the same api as the original RWST.

runRWST :: Monoid w => RWST r w s m a -> r -> s -> m (a, s, w)

Unwrap an RWST computation as a function.

evalRWST

Arguments

:: (Monad m, Monoid w)

=> 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

Arguments

:: (Monad m, Monoid w)

=> 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 :: (Monad n, Monoid w, Monoid w') => (m (a, s, w) -> n (b, s, w')) -> RWST r w s m a -> RWST r w' s n b

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

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)

Orphan instances

(Monad m, Monoid w) => MonadRWS r w s (RWST r w s m)

MonadError e m => MonadError e (RWST r w s m)

Methods

throwError :: e -> RWST r w s m a

catchError :: RWST r w s m a -> (e -> RWST r w s m a) -> RWST r w s m a

Monad m => MonadReader r (RWST r w s m)

Methods

ask :: RWST r w s m r

local :: (r -> r) -> RWST r w s m a -> RWST r w s m a

reader :: (r -> a) -> RWST r w s m a

Monad m => MonadState s (RWST r w s m)

Methods

get :: RWST r w s m s

put :: s -> RWST r w s m ()

state :: (s -> (a, s)) -> RWST r w s m a

(Monoid w, Monad m) => MonadWriter w (RWST r w s m)

Methods

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

tell :: w -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, w)

pass :: RWST r w s m (a, w -> w) -> RWST r w s m a

MonadCont m => MonadCont (RWST r w s m)

Methods

callCC :: ((a -> RWST r w s m b) -> RWST r w s m a) -> RWST r w s m a