mtl-2.2.2: Monad classes, using functional dependencies
Copyright (c) Andy Gill 2001
(c) Oregon Graduate Institute of Science and Technology 2001
License BSD-style (see the file LICENSE)
Maintainer libraries@haskell.org
Stability experimental
Portability non-portable (multi-param classes, functional dependencies)
Safe Haskell Safe
Language Haskell2010

Control.Monad.Writer.Strict

Description

Strict 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 w, Monad m) => MonadWriter w m | m -> w where Source #

Minimal complete definition

( writer | tell ), listen , pass

Methods

writer :: (a, w) -> m a Source #

writer (a,w) embeds a simple writer action.

tell :: w -> m () Source #

tell w is an action that produces the output w .

listen :: m a -> m (a, w) Source #

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 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.

Instances

Instances details
MonadWriter w m => MonadWriter w ( MaybeT m) Source #
Instance details

Defined in Control.Monad.Writer.Class

Monoid w => MonadWriter w ( (,) w) Source #

NOTE : This instance is only defined for base >= 4.9.0 .

Since: 2.2.2

Instance details

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> (w, a) Source #

tell :: w -> (w, ()) Source #

listen :: (w, a) -> (w, (a, w)) Source #

pass :: (w, (a, w -> w)) -> (w, a) Source #

MonadWriter w m => MonadWriter w ( StateT s m) Source #
Instance details

Defined in Control.Monad.Writer.Class

MonadWriter w m => MonadWriter w ( StateT s m) Source #
Instance details

Defined in Control.Monad.Writer.Class

MonadWriter w m => MonadWriter w ( ReaderT r m) Source #
Instance details

Defined in Control.Monad.Writer.Class

MonadWriter w m => MonadWriter w ( IdentityT m) Source #
Instance details

Defined in Control.Monad.Writer.Class

MonadWriter w m => MonadWriter w ( ExceptT e m) Source #

Since: 2.2

Instance details

Defined in Control.Monad.Writer.Class

( Error e, MonadWriter w m) => MonadWriter w ( ErrorT e m) Source #
Instance details

Defined in Control.Monad.Writer.Class

( Monoid w, Monad m) => MonadWriter w ( WriterT w m) Source #
Instance details

Defined in Control.Monad.Writer.Class

( Monoid w, Monad m) => MonadWriter w ( WriterT w m) Source #
Instance details

Defined in Control.Monad.Writer.Class

( Monoid w, Monad m) => MonadWriter w ( RWST r w s m) Source #
Instance details

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> RWST r w s m a Source #

tell :: w -> RWST r w s m () Source #

listen :: RWST r w s m a -> RWST r w s m (a, w) Source #

pass :: RWST r w s m (a, w -> w) -> RWST r w s m a Source #

( Monoid w, Monad m) => MonadWriter w ( RWST r w s m) Source #
Instance details

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> RWST r w s m a Source #

tell :: w -> RWST r w s m () Source #

listen :: RWST r w s m a -> RWST r w s m (a, w) Source #

pass :: RWST r w s m (a, w -> w) -> RWST r w s m a Source #

listens :: MonadWriter w m => (w -> b) -> m a -> 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.

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

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 .

runWriter :: Writer w a -> (a, w) Source #

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

execWriter :: Writer w a -> w Source #

Extract the output from a writer computation.

mapWriter :: ((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

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

Constructors

WriterT

Fields

Instances

Instances details
( Monoid w, MonadError e m) => MonadError e ( WriterT w m) Source #
Instance details

Defined in Control.Monad.Error.Class

( Monoid w, MonadReader r m) => MonadReader r ( WriterT w m) Source #
Instance details

Defined in Control.Monad.Reader.Class

( Monoid w, MonadState s m) => MonadState s ( WriterT w m) Source #
Instance details

Defined in Control.Monad.State.Class

( Monoid w, Monad m) => MonadWriter w ( WriterT w m) Source #
Instance details

Defined in Control.Monad.Writer.Class

Monoid w => MonadTrans ( WriterT w)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

Methods

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

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

Defined in Control.Monad.Trans.Writer.Strict

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

Defined in Control.Monad.Trans.Writer.Strict

Methods

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

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

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

Defined in Control.Monad.Trans.Writer.Strict

Methods

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

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

Defined in Control.Monad.Trans.Writer.Strict

( Monoid w, Applicative m) => Applicative ( WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

Foldable f => Foldable ( WriterT w f)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

Traversable f => Traversable ( WriterT w f)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

Methods

traverse :: Applicative f0 => (a -> f0 b) -> WriterT w f a -> f0 ( WriterT w f b) Source #

sequenceA :: Applicative f0 => WriterT w f (f0 a) -> f0 ( WriterT w f a) Source #

mapM :: Monad m => (a -> m b) -> WriterT w f a -> m ( WriterT w f b) Source #

sequence :: Monad m => WriterT w f (m a) -> m ( WriterT w f a) Source #

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

Defined in Control.Monad.Trans.Writer.Strict

Methods

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

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

( Eq w, Eq1 m) => Eq1 ( WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

Methods

liftEq :: (a -> b -> Bool ) -> WriterT w m a -> WriterT w m b -> Bool Source #

( Ord w, Ord1 m) => Ord1 ( WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

( Read w, Read1 m) => Read1 ( WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

( Show w, Show1 m) => Show1 ( WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

( Monoid w, MonadZip m) => MonadZip ( WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

Methods

mzip :: WriterT w m a -> WriterT w m b -> WriterT w m (a, b) Source #

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

munzip :: WriterT w m (a, b) -> ( WriterT w m a, WriterT w m b) Source #

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

Defined in Control.Monad.Trans.Writer.Strict

( Monoid w, Alternative m) => Alternative ( WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

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

Defined in Control.Monad.Trans.Writer.Strict

( Monoid w, MonadCont m) => MonadCont ( WriterT w m) Source #
Instance details

Defined in Control.Monad.Cont.Class

Methods

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

( Eq w, Eq1 m, Eq a) => Eq ( WriterT w m a)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

( Ord w, Ord1 m, Ord a) => Ord ( WriterT w m a)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

( Read w, Read1 m, Read a) => Read ( WriterT w m a)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

( Show w, Show1 m, Show a) => Show ( WriterT w m a)
Instance details

Defined in Control.Monad.Trans.Writer.Strict

execWriterT :: Monad m => WriterT w m a -> m w Source #

Extract the output from a writer computation.

mapWriterT :: (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.