Portability  portable 

Stability  experimental 
Maintainer  ross@soi.city.ac.uk 
Safe Haskell  SafeInferred 
The lazy WriterT
monad transformer, which adds collection of
outputs (such as a count or string output) to a given monad.
This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead.
This version builds its output lazily; for a strict version with the same interface, see Control.Monad.Trans.Writer.Strict.
 type Writer w = WriterT w Identity
 writer :: Monad m => (a, w) > WriterT w m 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 => CallCC m (a, w) (b, w) > CallCC (WriterT w m) a b
 liftCatch :: Catch e m (a, w) > Catch e (WriterT w m) a
The Writer monad
writer :: Monad m => (a, w) > WriterT w m 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)  
Foldable f => Foldable (WriterT w f)  
Traversable f => Traversable (WriterT w f)  
(Monoid w, Alternative m) => Alternative (WriterT w m)  
(Monoid w, MonadIO m) => MonadIO (WriterT w m)  
(Show w, Show1 m) => Show1 (WriterT w m)  
(Read w, Read1 m) => Read1 (WriterT w m)  
(Ord w, Ord1 m) => Ord1 (WriterT w m)  
(Eq w, Eq1 m) => Eq1 (WriterT w m)  
(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a)  
(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a)  
(Read w, Read1 m, Read a) => Read (WriterT w m a)  
(Show w, Show1 m, Show a) => Show (WriterT w m a) 
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)