| Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 | 
|---|---|
| License | BSD-style (see the file LICENSE) | 
| Maintainer | R.Paterson@city.ac.uk | 
| Stability | experimental | 
| Portability | portable | 
| Safe Haskell | Safe | 
| Language | Haskell98 | 
Control.Monad.Trans.Writer.Lazy
Description
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 constant-space version with almost the same interface, see Control.Monad.Trans.Writer.CPS.
Synopsis
- 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 :: Monad m => w -> WriterT w m ()
- listen :: Monad m => WriterT w m a -> WriterT w m (a, w)
- listens :: Monad m => (w -> b) -> WriterT w m a -> WriterT w m (a, b)
- pass :: Monad m => WriterT w m (a, w -> w) -> WriterT w m a
- censor :: 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 a #
Construct a writer computation from a (result, output) pair.
 (The inverse of runWriter.)
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.
- execWriterm =- snd(- runWriterm)
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.
Constructors
| WriterT | |
| Fields 
 | |
Instances
execWriterT :: Monad m => WriterT w m a -> m w #
Extract the output from a writer computation.
- execWriterTm =- liftM- snd(- runWriterTm)
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(- mapWriterTf m) = f (- runWriterTm)
Writer operations
listen :: Monad m => WriterT w m a -> WriterT w m (a, w) #
listen mm and adds its
 output to the value of the computation.
- runWriterT(- listenm) =- liftM(\ (a, w) -> ((a, w), w)) (- runWriterTm)
listens :: Monad m => (w -> b) -> WriterT w m a -> WriterT w m (a, b) #
listens f mm and adds
 the result of applying f to the output to the value of the computation.
- listensf m =- liftM(id *** f) (- listenm)
- runWriterT(- listensf m) =- liftM(\ (a, w) -> ((a, f w), w)) (- runWriterTm)
pass :: Monad m => WriterT w m (a, w -> w) -> WriterT w m a #
pass mm, which returns
 a value and a function, and returns the value, applying the function
 to the output.
- runWriterT(- passm) =- liftM(\ ((a, f), w) -> (a, f w)) (- runWriterTm)
censor :: Monad m => (w -> w) -> WriterT w m a -> WriterT w m a #
censor f mm and
 applies the function f to its output, leaving the return value
 unchanged.
- censorf m =- pass(- liftM(\ x -> (x,f)) m)
- runWriterT(- censorf m) =- liftM(\ (a, w) -> (a, f w)) (- runWriterTm)