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	Haskell98

Control.Monad.Writer.Lazy

Contents

MonadWriter class
The Writer monad
The WriterT monad transformer

Description

Lazy writer monads.

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.

Synopsis

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

MonadWriter w m => MonadWriter w (MaybeT m) Source
MonadWriter w m => MonadWriter w (IdentityT m) Source
MonadWriter w m => MonadWriter w (StateT s m) Source
MonadWriter w m => MonadWriter w (StateT s m) Source
MonadWriter w m => MonadWriter w (ReaderT r m) Source
MonadWriter w m => MonadWriter w (ExceptT e m) Source
(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) Source
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) Source
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) 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 :: Writer w a -> (a, w)

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

execWriter :: Writer w a -> w

Extract the output from a writer computation.

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

mapWriter :: ((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

newtype WriterT w m 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.

Constructors

WriterT
Fields runWriterT :: m (a, w)

Instances

(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source
(Monoid w, MonadState s m) => MonadState s (WriterT w m) Source
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) Source
(Monoid w, MonadError e m) => MonadError e (WriterT w m) Source
Monoid w => MonadTrans (WriterT w)
(Monoid w, Monad m) => Monad (WriterT w m)
Functor m => Functor (WriterT w m)
(Monoid w, MonadFix m) => MonadFix (WriterT w m)
(Monoid w, Applicative m) => Applicative (WriterT w m)
Foldable f => Foldable (WriterT w f)
Traversable f => Traversable (WriterT w f)
(Monoid w, Alternative m) => Alternative (WriterT w m)
(Monoid w, MonadPlus m) => MonadPlus (WriterT w m)
(Eq w, Eq1 m) => Eq1 (WriterT w m)
(Ord w, Ord1 m) => Ord1 (WriterT w m)
(Read w, Read1 m) => Read1 (WriterT w m)
(Show w, Show1 m) => Show1 (WriterT w m)
(Monoid w, MonadIO m) => MonadIO (WriterT w m)
(Monoid w, MonadCont m) => MonadCont (WriterT w m) Source
(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a)
(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a)
(Read w, Read1 m, Read a) => Read (WriterT w m a)
(Show w, Show1 m, Show a) => Show (WriterT w m a)

runWriterT :: WriterT w m a -> m (a, w)

execWriterT :: Monad m => WriterT w m a -> m w

Extract the output from a writer computation.

execWriterT m = liftM snd (runWriterT m)

mapWriterT :: (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

module Control.Monad.Fix

module Control.Monad.Trans

module Data.Monoid