Portability  nonportable (multiparam classes, functional dependencies) 

Stability  experimental 
Maintainer  libraries@haskell.org 
Safe Haskell  SafeInferred 
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.
 class (Monoid w, Monad m) => MonadWriter w m  m > w where
 listens :: MonadWriter w m => (w > b) > m a > m (a, b)
 censor :: MonadWriter w m => (w > w) > 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.Trans
MonadWriter class
class (Monoid w, Monad m) => MonadWriter w m  m > w whereSource
embeds a simple writer action.
writer
(a,w)
is an action that produces the output tell
ww
.
listen :: m a > m (a, w)Source
is an action that executes the action listen
mm
and adds
its output to the value of the computation.
pass :: m (a, w > w) > m aSource
is an action that executes the action pass
mm
, which
returns a value and a function, and returns the value, applying
the function to the output.
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
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)
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
.
WriterT  

(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)  
Foldable f => Foldable (WriterT w f)  
Traversable f => Traversable (WriterT w f)  
(Monoid w, MonadIO m) => MonadIO (WriterT w m)  
(Monoid w, Alternative m) => Alternative (WriterT w m)  
(Monoid w, MonadCont m) => MonadCont (WriterT w m) 
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.Trans