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	mail@daniel-mendler.de
Stability	experimental
Portability	non-portable (multi-param classes, functional dependencies)
Safe Haskell	Safe
Language	Haskell2010

Control.Monad.Writer.CPS

Contents

The Writer monad
The WriterT monad transformer
Orphan instances

Description

Stricter writer monad using continuation-passing-style for the writer output.

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

type Writer w = WriterT w Identity
runWriter :: Monoid w => Writer w a -> (a, w)
execWriter :: Monoid w => Writer w a -> w
mapWriter :: (Monoid w, Monoid w') => ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
data WriterT w (m :: Type -> Type) a
writerT :: (Functor m, Monoid w) => m (a, w) -> WriterT w m a
runWriterT :: Monoid w => WriterT w m a -> m (a, w)
execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w
mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
module Data.Monoid
module Control.Monad.Writer.Class
module Control.Monad.Trans
module Control.Monad.Fix
module Control.Monad

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 :: Monoid w => Writer w a -> (a, w) #

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

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

Extract the output from a writer computation.

```
execWriter m = snd (runWriter m)
```

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

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

runWriter (mapWriter f m) = f (runWriter m)

The WriterT monad transformer

data WriterT w (m :: Type -> Type) 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.

Instances

(Monad m, Monoid w) => MonadWriter w (WriterT w m) Source #
Instance details Defined in Control.Monad.Writer.CPS Methods writer :: (a, w) -> WriterT w m a # tell :: w -> WriterT w m () # listen :: WriterT w m a -> WriterT w m (a, w) # pass :: WriterT w m (a, w -> w) -> WriterT w m a #
MonadState s m => MonadState s (WriterT w m) Source #
Instance details Defined in Control.Monad.Writer.CPS Methods get :: WriterT w m s # put :: s -> WriterT w m () # state :: (s -> (a, s)) -> WriterT w m a #
(MonadReader r m, Monoid w) => MonadReader r (WriterT w m) Source #
Instance details Defined in Control.Monad.Writer.CPS Methods ask :: WriterT w m r # local :: (r -> r) -> WriterT w m a -> WriterT w m a # reader :: (r -> a) -> WriterT w m a #
MonadError e m => MonadError e (WriterT w m) Source #
Instance details Defined in Control.Monad.Writer.CPS Methods throwError :: e -> WriterT w m a # catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a #
MonadTrans (WriterT w)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods lift :: Monad m => m a -> WriterT w m a #
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 # (>>) :: WriterT w m a -> WriterT w m b -> WriterT w m b # return :: a -> WriterT w m a # fail :: String -> WriterT w m a #
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 # (<$) :: a -> WriterT w m b -> WriterT w m a #
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 #
MonadFail m => MonadFail (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods fail :: String -> WriterT w m a #
(Functor m, Monad m) => Applicative (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods pure :: a -> WriterT w m a # (<>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b # liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c # (>) :: WriterT w m a -> WriterT w m b -> WriterT w m b # (<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a #
MonadIO m => MonadIO (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods liftIO :: IO a -> WriterT w m a #
(Functor m, MonadPlus m) => Alternative (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods empty :: WriterT w m a # (<\|>) :: WriterT w m a -> WriterT w m a -> WriterT w m a # some :: WriterT w m a -> WriterT w m [a] # many :: WriterT w m a -> WriterT w m [a] #
(Functor m, MonadPlus m) => MonadPlus (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods mzero :: WriterT w m a # mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a #
MonadCont m => MonadCont (WriterT w m) Source #
Instance details Defined in Control.Monad.Writer.CPS Methods callCC :: ((a -> WriterT w m b) -> WriterT w m a) -> WriterT w m a #

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

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

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

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

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

Extract the output from a writer computation.

execWriterT m = liftM snd (runWriterT m)

mapWriterT :: (Monad n, Monoid w, Monoid w') => (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 (mapWriterT f m) = f (runWriterT m)

module Data.Monoid

module Control.Monad.Writer.Class

module Control.Monad.Trans

module Control.Monad.Fix

module Control.Monad

Orphan instances

(Monad m, Monoid w) => MonadWriter w (WriterT w m) Source #
Instance details Methods writer :: (a, w) -> WriterT w m a # tell :: w -> WriterT w m () # listen :: WriterT w m a -> WriterT w m (a, w) # pass :: WriterT w m (a, w -> w) -> WriterT w m a #
MonadState s m => MonadState s (WriterT w m) Source #
Instance details Methods get :: WriterT w m s # put :: s -> WriterT w m () # state :: (s -> (a, s)) -> WriterT w m a #
(MonadReader r m, Monoid w) => MonadReader r (WriterT w m) Source #
Instance details Methods ask :: WriterT w m r # local :: (r -> r) -> WriterT w m a -> WriterT w m a # reader :: (r -> a) -> WriterT w m a #
MonadError e m => MonadError e (WriterT w m) Source #
Instance details Methods throwError :: e -> WriterT w m a # catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a #
MonadCont m => MonadCont (WriterT w m) Source #
Instance details Methods callCC :: ((a -> WriterT w m b) -> WriterT w m a) -> WriterT w m a #