Control.Monad.Writer.Lazy

monads-tf-0.1.0.2: Monad classes, using type families

Copyright | (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001
License | BSD-style (see the file LICENSE)
Maintainer | ross@soi.city.ac.uk
Stability | experimental
Portability | non-portable (type families)
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

- class (Monoid (WriterType m), Monad m) => MonadWriter m where
  - type WriterType m
  - tell :: WriterType m -> m ()
  - listen :: m a -> m (a, WriterType m)
  - pass :: m (a, WriterType m -> WriterType m) -> m a
- listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)
- censor :: MonadWriter m => (WriterType m -> WriterType m) -> 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 (WriterType m), Monad m) => MonadWriter m where

Associated Types

type WriterType m

Methods

tell :: WriterType m -> m ()

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

pass :: m (a, WriterType m -> WriterType m) -> m a

Instances

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

listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)

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

# 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 => 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) |
(Monoid w, MonadError m) => MonadError (WriterT w m) |
(Monoid w, MonadReader m) => MonadReader (WriterT w m) |
(Monoid w, MonadState m) => MonadState (WriterT w m) |
(Monoid w, Monad m) => MonadWriter (WriterT w m) |
(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) |
type ErrorType (WriterT w m) = ErrorType m |
type EnvType (WriterT w m) = EnvType m |
type StateType (WriterT w m) = StateType m |
type WriterType (WriterT w m) = 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