monads-tf-0.1.0.1: Monad classes, using type families

Portabilitynon-portable (type families)
Stabilityexperimental
Maintainerross@soi.city.ac.uk
Safe HaskellSafe-Inferred

Control.Monad.Writer.Lazy

Contents

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

MonadWriter class

class (Monoid (WriterType m), Monad m) => MonadWriter m whereSource

Associated Types

type WriterType m Source

Methods

tell :: WriterType m -> m ()Source

listen :: m a -> m (a, WriterType m)Source

pass :: m (a, WriterType m -> WriterType m) -> m aSource

Instances

listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)Source

censor :: MonadWriter m => (WriterType m -> WriterType m) -> m a -> m aSource

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 

Fields

runWriterT :: m (a, w)
 

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.