Control.Monad.Writer

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

Description

The MonadWriter class.

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://www.cse.ogi.edu/~mpj/) Advanced School of Functional Programming, 1995.

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

Methods

tell :: w -> m ()

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

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

Instances

(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m)

(Monoid w) => MonadWriter w (RWS r w s)

(Monoid w, Monad m) => MonadWriter w (RWST r w s m)

(MonadWriter w m) => MonadWriter w (StateT s m)

(Monoid w) => MonadWriter w (Writer w)

(Monoid w, Monad m) => MonadWriter w (WriterT w m)

(MonadWriter w m) => MonadWriter w (ReaderT r m)

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

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

newtype Writer w a

Constructors

Writer

runWriter :: (a, w)

Instances

Functor (Writer w)

(Monoid w) => Monad (Writer w)

(Monoid w) => MonadFix (Writer w)

(Monoid w) => MonadWriter w (Writer w)

execWriter :: Writer w a -> w

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

newtype WriterT w m a

Constructors

WriterT

runWriterT :: (m (a, w))

Instances

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

(Monoid w, MonadError e m) => MonadError e (WriterT w m)

(Monoid w, MonadState s m) => MonadState s (WriterT w m)

(Monad m) => Functor (WriterT w m)

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

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

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

(Monoid w, Monad m) => MonadWriter w (WriterT w m)

(Monoid w) => MonadTrans (WriterT w)

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

(Monoid w, MonadReader r m) => MonadReader r (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

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