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Control.Monad.Trans.Writer.CPS

Copyright (c) Daniel Mendler 2016
(c) Andy Gill 2001
(c) Oregon Graduate Institute of Science and Technology 2001
License BSD-style (see the file LICENSE)
Maintainer [email protected]
Stability experimental
Portability portable
Safe Haskell Safe
Language Haskell98

Description

The strict WriterT monad transformer, which adds collection of outputs (such as a count or string output) to a given monad.

This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead.

This version builds its output strictly and uses continuation-passing-style to achieve constant space usage. This transformer can be used as a drop-in replacement for Control.Monad.Trans.Writer.Strict.

The Writer monad

type Writer w = WriterT w Identity Source

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.

writer :: (Monoid w, Monad m) => (a, w) -> WriterT w m a Source

Construct a writer computation from a (result, output) pair. (The inverse of runWriter.)

runWriter :: Monoid w => Writer w a -> (a, w) Source

Unwrap a writer computation as a (result, output) pair. (The inverse of writer.)

execWriter :: Monoid w => Writer w a -> w Source

Extract the output from a writer computation.

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

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

The WriterT monad transformer

data WriterT w m a Source

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.

Instances
Instances details
MonadTrans (WriterT w)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

lift :: Monad m => m a -> WriterT w m a Source

Monad m => Monad (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

(>>=) :: WriterT w m a -> (a -> WriterT w m b) -> WriterT w m b Source

(>>) :: WriterT w m a -> WriterT w m b -> WriterT w m b Source

return :: a -> WriterT w m a Source

Functor m => Functor (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

fmap :: (a -> b) -> WriterT w m a -> WriterT w m b Source

(<$) :: a -> WriterT w m b -> WriterT w m a Source

MonadFix m => MonadFix (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

mfix :: (a -> WriterT w m a) -> WriterT w m a Source

MonadFail m => MonadFail (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

fail :: String -> WriterT w m a Source

(Functor m, Monad m) => Applicative (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

pure :: a -> WriterT w m a Source

(<*>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b Source

liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c Source

(*>) :: WriterT w m a -> WriterT w m b -> WriterT w m b Source

(<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a Source

MonadIO m => MonadIO (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

liftIO :: IO a -> WriterT w m a Source

(Functor m, MonadPlus m) => Alternative (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

empty :: WriterT w m a Source

(<|>) :: WriterT w m a -> WriterT w m a -> WriterT w m a Source

some :: WriterT w m a -> WriterT w m [a] Source

many :: WriterT w m a -> WriterT w m [a] Source

(Functor m, MonadPlus m) => MonadPlus (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.CPS

Methods

mzero :: WriterT w m a Source

mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a Source

writerT :: (Functor m, Monoid w) => m (a, w) -> WriterT w m a Source

Construct a writer computation from a (result, output) computation. (The inverse of runWriterT.)

runWriterT :: Monoid w => WriterT w m a -> m (a, w) Source

Unwrap a writer computation. (The inverse of writerT.)

execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w Source

Extract the output from a writer computation.

mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b Source

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

Writer operations

tell :: (Monoid w, Monad m) => w -> WriterT w m () Source

tell w is an action that produces the output w.

listen :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w) Source

listen m is an action that executes the action m and adds its output to the value of the computation.

listens :: (Monoid w, Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b) Source

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.

pass :: (Monoid w, Monoid w', Monad m) => WriterT w m (a, w -> w') -> WriterT w' m a Source

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.

censor :: (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a Source

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

Lifting other operations

liftCallCC :: CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b Source

Uniform lifting of a callCC operation to the new monad. This version rolls back to the original state on entering the continuation.

liftCatch :: Catch e m (a, w) -> Catch e (WriterT w m) a Source

Lift a catchE operation to the new monad.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.8.3/docs/html/libraries/transformers-0.5.6.2/Control-Monad-Trans-Writer-CPS.html