Portability | non-portable (multi-param classes, functional dependencies) |
---|---|
Stability | experimental |
Maintainer | libraries@haskell.org |
Control.Monad.Writer.Lazy
Description
Lazy writer monads.
Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995.
- class (Monoid w, Monad m) => MonadWriter w m | m -> w where
- listens :: MonadWriter w m => (w -> b) -> m a -> m (a, b)
- censor :: MonadWriter w m => (w -> w) -> m a -> m a
- 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
- module Control.Monad
- module Control.Monad.Fix
- module Control.Monad.Trans
- module Data.Monoid
MonadWriter class
class (Monoid w, Monad m) => MonadWriter w m | m -> w whereSource
Methods
is an action that produces the output tell
ww
.
listen :: m a -> m (a, w)Source
is an action that executes the action listen
mm
and adds
its output to the value of the computation.
pass :: m (a, w -> 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.
listens :: MonadWriter w m => (w -> b) -> m a -> m (a, b)Source
censor :: MonadWriter w m => (w -> w) -> m a -> m aSource
The Writer monad
writer :: (a, w) -> Writer w 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.
execWriter
m =snd
(runWriter
m)
The WriterT monad transformer
newtype WriterT w m a
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
|
execWriterT :: Monad m => WriterT w m a -> m w
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
Map both the return value and output of a computation using the given function.
-
)runWriterT
(mapWriterT
f m) = f (runWriterT
m
module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans
module Data.Monoid