Control.Monad.Writer.Lazy

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.

MonadWriter class

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

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)

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

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