| Portability | non-portable (multi-param classes, functional dependencies) |
|---|---|
| Stability | experimental |
| Maintainer | libraries@haskell.org |
| Safe Haskell | Safe-Infered |
Control.Monad.Writer.Strict
Description
Strict 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.
Instances
| MonadWriter w m => MonadWriter w (MaybeT m) | |
| MonadWriter w m => MonadWriter w (IdentityT m) | |
| MonadWriter w m => MonadWriter w (StateT s m) | |
| MonadWriter w m => MonadWriter w (StateT s m) | |
| MonadWriter w m => MonadWriter w (ReaderT r m) | |
| (Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) | |
| (Monoid w, Monad m) => MonadWriter w (WriterT w m) | |
| (Monoid w, Monad m) => MonadWriter w (WriterT w m) | |
| (Monoid w, Monad m) => MonadWriter w (RWST r w s m) | |
| (Monoid w, Monad m) => MonadWriter w (RWST r w s m) |
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.
execWriterm =snd(runWriterm)
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
| |
Instances
| (Monoid w, Monad m) => MonadWriter w (WriterT w m) | |
| (Monoid w, MonadState s m) => MonadState s (WriterT w m) | |
| (Monoid w, MonadReader r m) => MonadReader r (WriterT w m) | |
| (Monoid w, MonadError e m) => MonadError e (WriterT w m) | |
| 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) | |
| (Monoid w, Alternative m) => Alternative (WriterT w m) | |
| (Monoid w, MonadIO m) => MonadIO (WriterT w m) | |
| (Monoid w, MonadCont m) => MonadCont (WriterT w m) |
execWriterT :: Monad m => WriterT w m a -> m w
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
Map both the return value and output of a computation using the given function.
-
)runWriterT(mapWriterTf m) = f (runWriterTm
module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans
module Data.Monoid