Portability  portable 

Stability  experimental 
Maintainer  libraries@haskell.org 
The lazy WriterT
monad transformer, which adds collection of
outputs (such as a count or string output) to a given monad.
This version builds its output lazily; for a strict version, see Control.Monad.Trans.Writer.Strict, which has the same interface.
This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead.
 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)
 listens :: (Monoid w, Monad m) => (w > b) > WriterT w m a > WriterT w m (a, b)
 pass :: (Monoid w, Monad m) => WriterT w m (a, w > w) > WriterT w m a
 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
writer :: (a, w) > Writer w aSource
Construct a writer computation from a (result, output) pair.
(The inverse of runWriter
.)
runWriter :: Writer w a > (a, w)Source
Unwrap a writer computation as a (result, output) pair.
(The inverse of writer
.)
execWriter :: Writer w a > wSource
Extract the output from a writer computation.
execWriter
m =snd
(runWriter
m)
The WriterT monad transformer
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 => 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
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 bSource
Map both the return value and output of a computation using the given function.

)runWriterT
(mapWriterT
f m) = f (runWriterT
m
Writer operations
tell :: (Monoid w, Monad m) => w > WriterT w m ()Source
is an action that produces the output tell
ww
.
listen :: (Monoid w, Monad m) => WriterT w m a > WriterT w m (a, w)Source
is an action that executes the action listen
mm
and adds its
output to the value of the computation.
runWriterT
(listen
m) =liftM
(\(a, w) > ((a, w), w)) (runWriterT
m)
listens :: (Monoid w, Monad m) => (w > b) > WriterT w m a > WriterT w m (a, b)Source
is an action that executes the action listens
f mm
and adds
the result of applying f
to the output to the value of the computation.
listens
f m =liftM
(id *** f) (listen
m)runWriterT
(listens
f m) =liftM
(\(a, w) > ((a, f w), w)) (runWriterT
m)
pass :: (Monoid w, Monad m) => WriterT w m (a, w > w) > WriterT 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.
runWriterT
(pass
m) =liftM
(\((a, f), w) > (a, f w)) (runWriterT
m)
censor :: (Monoid w, Monad m) => (w > w) > WriterT w m a > WriterT w m aSource
is an action that executes the action censor
f mm
and
applies the function f
to its output, leaving the return value
unchanged.
censor
f m =pass
(liftM
(\x > (x,f)) m)runWriterT
(censor
f m) =liftM
(\(a, w) > (a, f w)) (runWriterT
m)