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

type EnvType (RWST r w s m)

type EnvType (RWST r w s m) = r

type StateType (RWST r w s m)

type StateType (RWST r w s m) = s

type WriterType (RWST r w s m)

type WriterType (RWST r w s m) = w

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

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)

Strict Reader-writer-state monads

Orphan instances

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

Associated Types

type EnvType (RWST r w s m :: * -> *) :: *

Methods

ask :: RWST r w s m (EnvType (RWST r w s m))

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

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

Associated Types

type StateType (RWST r w s m :: * -> *) :: *

Methods

get :: RWST r w s m (StateType (RWST r w s m))

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

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

Associated Types

type WriterType (RWST r w s m :: * -> *) :: *

Methods

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

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

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