monads-tf-0.1.0.0: Monad classes, using type familiesContentsIndex
Control.Monad.Writer.Lazy
Portabilitynon-portable (type families)
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 (WriterType m), Monad m) => MonadWriter m where
type WriterType m
tell :: WriterType m -> m ()
listen :: m a -> m (a, WriterType m)
pass :: m (a, WriterType m -> WriterType m) -> m a
listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)
censor :: MonadWriter m => (WriterType m -> WriterType m) -> 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 (WriterType m), Monad m) => MonadWriter m where
Associated Types
type WriterType m
Methods
tell :: WriterType m -> m ()
listen :: m a -> m (a, WriterType m)
pass :: m (a, WriterType m -> WriterType m) -> m a
show/hide Instances
listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)
censor :: MonadWriter m => (WriterType m -> WriterType m) -> m a -> m a
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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