Portability  portable 

Stability  experimental 
Maintainer  libraries@haskell.org 
Lazy writer monads.
Inspired by the paper /Functional Programming with Overloading and HigherOrder Polymorphism/, Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995.
 type Writer w = WriterT w Identity
 writer :: (a, w) > Writer w a
 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
 tell :: (Monoid w, Monad m) => w > WriterT w m ()
 listen :: (Monoid w, Monad m) => WriterT w m a > WriterT w m (a, w)
 pass :: (Monoid w, Monad m) => WriterT w m (a, w > w) > WriterT w m a
 listens :: (Monoid w, Monad m) => (w > b) > WriterT w m a > WriterT w m (a, b)
 censor :: (Monoid w, Monad m) => (w > w) > WriterT w m a > WriterT w m a
 liftCallCC :: Monoid w => ((((a, w) > m (b, w)) > m (a, w)) > m (a, w)) > ((a > WriterT w m b) > WriterT w m a) > WriterT w m a
 liftCatch :: (m (a, w) > (e > m (a, w)) > m (a, w)) > WriterT w m a > (e > WriterT w m a) > WriterT w m a
The Writer monad
execWriter :: Writer w a > wSource
The WriterT monad transformer
WriterT  

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) 
execWriterT :: Monad m => WriterT w m a > m wSource
mapWriterT :: (m (a, w) > n (b, w')) > WriterT w m a > WriterT w' n bSource