monads-fd-0.1.0.1: Monad classes, using functional dependenciesContentsIndex
Control.Monad.Writer.Lazy
Portabilitynon-portable (multi-param classes, functional dependencies)
Stabilityexperimental
Maintainerlibraries@haskell.org
Contents
MonadWriter class
The Writer monad
The WriterT monad transformer
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.

Synopsis
class (Monoid w, Monad m) => MonadWriter w m | m -> w where
tell :: w -> m ()
listen :: m a -> m (a, w)
pass :: m (a, w -> w) -> m a
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
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 where
Methods
tell :: w -> m ()
tell w is an action that produces the output w.
listen :: m a -> m (a, w)
listen m is an action that executes the action m and adds its output to the value of the computation.
pass :: m (a, w -> w) -> m a
pass m is an action that executes the action m, which returns a value and a function, and returns the value, applying the function to the output.
show/hide 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)

listens f m is an action that executes the action m and adds the result of applying f to the output to the value of the computation.

censor :: MonadWriter w m => (w -> w) -> m a -> m a

censor f m is an action that executes the action m and applies the function f to its output, leaving the return value unchanged.

The Writer monad
type Writer w = WriterT w Identity

A writer monad parameterized by the type w of output to accumulate.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

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.

mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b

Map both the return value and output of a computation using the given function.

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
runWriterT :: m (a, w)
show/hide Instances
execWriterT :: Monad m => WriterT w m a -> m w

Extract the output from a writer computation.

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.

module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans
module Data.Monoid
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