Control.Monad.Writer.Lazy

mtl-2.1.3.1: Monad classes, using functional dependencies

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-Inferred
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

- 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 :: Writer w a -> (a, w)
- execWriter :: Writer w a -> w
- mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
- newtype WriterT w m a :: * -> (* -> *) -> * -> * = WriterT {
  - runWriterT :: m (a, w)
  }
- execWriterT :: Monad m => WriterT w m a -> m w
- mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
- module Control.Monad
- module Control.Monad.Fix
- module Control.Monad.Trans
- module Data.Monoid

# MonadWriter class

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

Minimal complete definition

listen, pass

Methods

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

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

tell :: w -> m ()

``tell` w` is an action that produces the output `w`.

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

``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

``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) |
MonadWriter w m => MonadWriter w (IdentityT m) |
MonadWriter w m => MonadWriter w (StateT s m) |
MonadWriter w m => MonadWriter w (StateT s m) |
MonadWriter w m => MonadWriter w (ReaderT r m) |
(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) |
(Monoid w, Monad m) => MonadWriter w (WriterT w m) |
(Monoid w, Monad m) => MonadWriter w (WriterT w m) |
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) |
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) |

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

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

# 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) |
(Monoid w, MonadState s m) => MonadState s (WriterT w m) |
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) |
(Monoid w, MonadError e m) => MonadError e (WriterT w m) |
Monoid w => MonadTrans (WriterT w) |
(Monoid w, Alternative m) => Alternative (WriterT w m) |
(Monoid w, Monad m) => Monad (WriterT w m) |
Functor m => Functor (WriterT w m) |
(Monoid w, MonadFix m) => MonadFix (WriterT w m) |
(Monoid w, MonadPlus m) => MonadPlus (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, MonadIO m) => MonadIO (WriterT w m) |
(Monoid w, MonadCont m) => MonadCont (WriterT w m) |

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