Copyright	(c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001
License	BSD-style (see the file LICENSE)
Maintainer	libraries@haskell.org
Stability	experimental
Portability	non-portable (multi-param classes, functional dependencies)
Safe Haskell	Safe
Language	Haskell2010

Control.Monad.Writer.CPS

Contents

MonadWriter class
The Writer monad
The WriterT monad transformer

Description

Strict writer monads that use continuation-passing-style to achieve constant space usage.

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

Since: mtl-2.3, transformers-0.5.6

Synopsis

class (Monoid w, Monad m) => MonadWriter w m | m -> w where
- writer :: (a, w) -> m a
- tell :: w -> m ()
- listen :: m a -> m (a, w)
- pass :: m (a, w -> w) -> m a
listens :: MonadWriter w m => (w -> b) -> m a -> m (a, b)
censor :: MonadWriter w m => (w -> w) -> m a -> m a
type Writer w = WriterT w Identity
runWriter :: Monoid w => Writer w a -> (a, w)
execWriter :: Monoid w => Writer w a -> w
mapWriter :: (Monoid w, Monoid w') => ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
data WriterT w (m :: Type -> Type) a
execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w
mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
module Control.Monad.Trans

MonadWriter class

class (Monoid w, Monad m) => MonadWriter w m | m -> w where Source #

Minimal complete definition

(writer | tell), listen, pass

Methods

writer :: (a, w) -> m a Source #

writer (a,w) embeds a simple writer action.

tell :: w -> m () Source #

tell w is an action that produces the output w.

listen :: m a -> m (a, w) Source #

listen m is an action that executes the action m and adds its output to the value of the computation.

pass :: m (a, w -> w) -> 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.

Instances

Instances details

MonadWriter w m => MonadWriter w (MaybeT m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> MaybeT m a Source # tell :: w -> MaybeT m () Source # listen :: MaybeT m a -> MaybeT m (a, w) Source # pass :: MaybeT m (a, w -> w) -> MaybeT m a Source #
Monoid w => MonadWriter w ((,) w) Source #	Since: 2.2.2
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> (w, a) Source # tell :: w -> (w, ()) Source # listen :: (w, a) -> (w, (a, w)) Source # pass :: (w, (a, w -> w)) -> (w, a) Source #
(Monoid w, MonadWriter w m) => MonadWriter w (AccumT w m) Source #	There are two valid instances for `AccumT`. It could either: Lift the operations to the inner `MonadWriter` Handle the operations itself, à la a `WriterT`. This instance chooses (1), reflecting that the intent of `AccumT` as a type is different than that of `WriterT`. Since: 2.3
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> AccumT w m a Source # tell :: w -> AccumT w m () Source # listen :: AccumT w m a -> AccumT w m (a, w) Source # pass :: AccumT w m (a, w -> w) -> AccumT w m a Source #
MonadWriter w m => MonadWriter w (StateT s m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> StateT s m a Source # tell :: w -> StateT s m () Source # listen :: StateT s m a -> StateT s m (a, w) Source # pass :: StateT s m (a, w -> w) -> StateT s m a Source #
MonadWriter w m => MonadWriter w (StateT s m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> StateT s m a Source # tell :: w -> StateT s m () Source # listen :: StateT s m a -> StateT s m (a, w) Source # pass :: StateT s m (a, w -> w) -> StateT s m a Source #
MonadWriter w m => MonadWriter w (ReaderT r m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> ReaderT r m a Source # tell :: w -> ReaderT r m () Source # listen :: ReaderT r m a -> ReaderT r m (a, w) Source # pass :: ReaderT r m (a, w -> w) -> ReaderT r m a Source #
MonadWriter w m => MonadWriter w (IdentityT m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> IdentityT m a Source # tell :: w -> IdentityT m () Source # listen :: IdentityT m a -> IdentityT m (a, w) Source # pass :: IdentityT m (a, w -> w) -> IdentityT m a Source #
MonadWriter w m => MonadWriter w (ExceptT e m) Source #	Since: 2.2
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> ExceptT e m a Source # tell :: w -> ExceptT e m () Source # listen :: ExceptT e m a -> ExceptT e m (a, w) Source # pass :: ExceptT e m (a, w -> w) -> ExceptT e m a Source #
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> WriterT w m a Source # tell :: w -> WriterT w m () Source # listen :: WriterT w m a -> WriterT w m (a, w) Source # pass :: WriterT w m (a, w -> w) -> WriterT w m a Source #
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> WriterT w m a Source # tell :: w -> WriterT w m () Source # listen :: WriterT w m a -> WriterT w m (a, w) Source # pass :: WriterT w m (a, w -> w) -> WriterT w m a Source #
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source #	Since: 2.3
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> WriterT w m a Source # tell :: w -> WriterT w m () Source # listen :: WriterT w m a -> WriterT w m (a, w) Source # pass :: WriterT w m (a, w -> w) -> WriterT w m a Source #
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> RWST r w s m a Source # tell :: w -> RWST r w s m () Source # listen :: RWST r w s m a -> RWST r w s m (a, w) Source # pass :: RWST r w s m (a, w -> w) -> RWST r w s m a Source #
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) Source #
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> RWST r w s m a Source # tell :: w -> RWST r w s m () Source # listen :: RWST r w s m a -> RWST r w s m (a, w) Source # pass :: RWST r w s m (a, w -> w) -> RWST r w s m a Source #
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) Source #	Since: 2.3
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> RWST r w s m a Source # tell :: w -> RWST r w s m () Source # listen :: RWST r w s m a -> RWST r w s m (a, w) Source # pass :: RWST r w s m (a, w -> w) -> RWST r w s m a Source #

listens :: MonadWriter w m => (w -> b) -> m a -> 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)

censor :: MonadWriter w m => (w -> w) -> m a -> 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)

The Writer monad

type Writer w = WriterT w Identity #

A writer monad parameterized by the type w of output to accumulate.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

runWriter :: Monoid w => Writer w a -> (a, w) #

Unwrap a writer computation as a (result, output) pair. (The inverse of writer.)

execWriter :: Monoid w => Writer w a -> w #

Extract the output from a writer computation.

```
execWriter m = snd (runWriter m)
```

mapWriter :: (Monoid w, Monoid w') => ((a, w) -> (b, w')) -> Writer w a -> Writer w' b #

Map both the return value and output of a computation using the given function.

runWriter (mapWriter f m) = f (runWriter m)

The WriterT monad transformer

data WriterT w (m :: Type -> Type) a #

A writer monad parameterized by:

w - the output to accumulate.
m - The inner monad.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

Instances

Instances details

MonadAccum w' m => MonadAccum w' (WriterT w m) Source #	Since: 2.3
Instance details Defined in Control.Monad.Accum Methods look :: WriterT w m w' Source # add :: w' -> WriterT w m () Source # accum :: (w' -> (a, w')) -> WriterT w m a Source #
(Monoid w, MonadError e m) => MonadError e (WriterT w m) Source #	Since: 2.3
Instance details Defined in Control.Monad.Error.Class Methods throwError :: e -> WriterT w m a Source # catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a Source #
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) Source #	Since: 2.3
Instance details Defined in Control.Monad.Reader.Class Methods ask :: WriterT w m r Source # local :: (r -> r) -> WriterT w m a -> WriterT w m a Source # reader :: (r -> a) -> WriterT w m a Source #
MonadSelect w' m => MonadSelect w' (WriterT w m) Source #	'Readerizes' the writer: the 'ranking' function can see the value that's been accumulated (of type `w`), but can't add anything to the log. Effectively, can be thought of as 'extending' the 'ranking' by all values of `w`, but which `w` gets given to any rank calls is predetermined by the 'outer writer' (and cannot change). Since: 2.3
Instance details Defined in Control.Monad.Select Methods select :: ((a -> w') -> a) -> WriterT w m a Source #
(Monoid w, MonadState s m) => MonadState s (WriterT w m) Source #	Since: 2.3
Instance details Defined in Control.Monad.State.Class Methods get :: WriterT w m s Source # put :: s -> WriterT w m () Source # state :: (s -> (a, s)) -> WriterT w m a Source #
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source #	Since: 2.3
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> WriterT w m a Source # tell :: w -> WriterT w m () Source # listen :: WriterT w m a -> WriterT w m (a, w) Source # pass :: WriterT w m (a, w -> w) -> WriterT w m a Source #
MonadTrans (WriterT w)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods lift :: Monad m => m a -> WriterT w m a #
Monad m => Monad (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods (>>=) :: WriterT w m a -> (a -> WriterT w m b) -> WriterT w m b # (>>) :: WriterT w m a -> WriterT w m b -> WriterT w m b # return :: a -> WriterT w m a #
Functor m => Functor (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods fmap :: (a -> b) -> WriterT w m a -> WriterT w m b # (<$) :: a -> WriterT w m b -> WriterT w m a #
MonadFix m => MonadFix (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods mfix :: (a -> WriterT w m a) -> WriterT w m a #
MonadFail m => MonadFail (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods fail :: String -> WriterT w m a #
(Functor m, Monad m) => Applicative (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods pure :: a -> WriterT w m a # (<>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b # liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c # (>) :: WriterT w m a -> WriterT w m b -> WriterT w m b # (<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a #
MonadIO m => MonadIO (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods liftIO :: IO a -> WriterT w m a #
(Functor m, MonadPlus m) => Alternative (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods empty :: WriterT w m a # (<\|>) :: WriterT w m a -> WriterT w m a -> WriterT w m a # some :: WriterT w m a -> WriterT w m [a] # many :: WriterT w m a -> WriterT w m [a] #
(Functor m, MonadPlus m) => MonadPlus (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods mzero :: WriterT w m a # mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a #
(Monoid w, MonadCont m) => MonadCont (WriterT w m) Source #	Since: 2.3
Instance details Defined in Control.Monad.Cont.Class Methods callCC :: ((a -> WriterT w m b) -> WriterT w m a) -> WriterT w m a Source #

execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w #

Extract the output from a writer computation.

execWriterT m = liftM snd (runWriterT m)

mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b #

Map both the return value and output of a computation using the given function.

runWriterT (mapWriterT f m) = f (runWriterT m)

module Control.Monad.Trans