Portability	non-portable (multi-param classes, functional dependencies)
Stability	experimental
Maintainer	libraries@haskell.org

Control.Monad.Writer.Strict

Contents

MonadWriter class
The Writer monad
The WriterT monad transformer

Description

Strict 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 whereSource

Methods

tell :: w -> m ()Source

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

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

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)Source

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

The Writer monad

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

The WriterT monad transformer

newtype WriterT w m a

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, 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)
(Monoid w, Alternative m) => Alternative (WriterT w m)
(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

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