Moduletransformers-0.6.1.1Haskell2010

Control.Monad.Trans.RWS.Lazy

A monad transformer that combines ReaderT, WriterT and StateT. This version is lazy; for a constant-space version with almost the same interface, see Control.Monad.Trans.RWS.CPS.

2 types
28 values

Packagetransformers-0.6.1.1
Exports30
LanguageHaskell2010
LicenceBSD-3-Clause
SourceLazy.hs

The RWS monad

7 declarations

typetype 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.

valuerws :: (r -> s -> (a, s, w)) -> RWS r w s a

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

valuerunRWS :: RWS r w s a -> r -> s -> (a, s, w)

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

valueevalRWS

:: 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.

valueexecRWS

:: 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.

valuemapRWS :: ((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)

valuewithRWS :: (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

5 declarations

newtypenewtype RWST r w s (m :: Type -> Type) 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.

Constructors

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

Instances12MonadTrans, Monad, Functor, MonadFix, MonadFail, Applicative, …

Monoid w => MonadTrans (RWST r w s)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
(Monoid w, Monad m) => Monad (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
Functor m => Functor (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
(Monoid w, MonadFix m) => MonadFix (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
(Monoid w, MonadFail m) => MonadFail (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
(Monoid w, Functor m, Monad m) => Applicative (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
(Monoid w, Functor m, MonadPlus m) => Alternative (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
(Monoid w, MonadPlus m) => MonadPlus (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
(Monoid w, MonadIO m) => MonadIO (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
Contravariant m => Contravariant (RWST r w s m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
Generic (RWST r w s m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy
type Rep (RWST r w s m a) = D1 ('MetaData "RWST" "Control.Monad.Trans.RWS.Lazy" "transformers-0.6.1.1-a11a" 'True) (C1 ('MetaCons "RWST" 'PrefixI 'True) (S1 ('MetaSel ('Just "runRWST") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (r -> s -> m (a, s, w)))))Defined in transformers-0.6.1.1 · Control.Monad.Trans.RWS.Lazy

valueevalRWST

:: Monad m
=> 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.

valueexecRWST

:: Monad m
=> 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.

valuemapRWST

:: 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)

valuewithRWST :: (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)

Reader operations

4 declarations

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

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

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

Fetch the value of the environment.

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

Execute a computation in a modified environment

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

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

Retrieve a function of the current environment.

```
asks f = liftM f ask
```

Writer operations

6 declarations

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

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

valuetell :: Monad m => w -> RWST r w s m ()

tell w is an action that produces the output w.

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

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

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)

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

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)

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

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

5 declarations

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

Construct a state monad computation from a state transformer function.

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

Fetch the current value of the state within the monad.

valueput :: (Monoid w, Monad m) => s -> RWST r w s m ()

put s sets the state within the monad to s.

valuemodify :: (Monoid w, Monad m) => (s -> s) -> RWST r w s m ()

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

```
modify f = get >>= (put . f)
```

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

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

```
gets f = liftM f get
```

Lifting other operations

3 declarations

valueliftCallCC

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

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

valueliftCallCC'

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

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

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

Lift a catchE operation to the new monad.