| 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 Source #
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 -> w Source #
Extract the output from a writer computation.
-
execWriterm =snd(runWriterm)
The WriterT monad transformer
newtype WriterT w m a Source #
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 Source #
Extract the output from a writer computation.
-
execWriterTm =liftMsnd(runWriterTm)
mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b Source #
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) Source #
is an action that executes the action
listen
m
m
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) Source #
is an action that executes the action
listens
f m
m
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 Source #
is an action that executes the action
pass
m
m
, 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 Source #
is an action that executes the action
censor
f m
m
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)