Copyright | (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001 |
---|---|
License | BSD-style (see the file LICENSE) |
Maintainer | ross@soi.city.ac.uk |
Stability | experimental |
Portability | portable |
Safe Haskell | Safe |
Language | Haskell98 |
The strict 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 strictly; for a lazy version with the same interface, see Control.Monad.Trans.Writer.Lazy. Although the output is built strictly, it is not possible to achieve constant space behaviour with this transformer: for that, use Control.Monad.Trans.State.Strict instead.
- 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 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.
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, 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, MonadPlus m) => MonadPlus (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 w Source
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 Source
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 a Source
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 a Source
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)