Control.Monad.Writer.Strict

Copyright	(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	non-portable (multi-param classes, functional dependencies)
Safe Haskell	Safe
Language	Haskell2010

MonadWriter class
The Writer monad
The WriterT monad transformer

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.

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)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> MaybeT m a Source tell :: w -> MaybeT m () Source listen :: MaybeT m a -> MaybeT m (a, w) Source pass :: MaybeT m (a, w -> w) -> MaybeT m a Source
Monoid w => MonadWriter w ((,) w)	NOTE: This instance is only defined for `base >= 4.9.0`. Since: mtl-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)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> StateT s m a Source tell :: w -> StateT s m () Source listen :: StateT s m a -> StateT s m (a, w) Source pass :: StateT s m (a, w -> w) -> StateT s m a Source
MonadWriter w m => MonadWriter w (StateT s m)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> StateT s m a Source tell :: w -> StateT s m () Source listen :: StateT s m a -> StateT s m (a, w) Source pass :: StateT s m (a, w -> w) -> StateT s m a Source
MonadWriter w m => MonadWriter w (ReaderT r m)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> ReaderT r m a Source tell :: w -> ReaderT r m () Source listen :: ReaderT r m a -> ReaderT r m (a, w) Source pass :: ReaderT r m (a, w -> w) -> ReaderT r m a Source
MonadWriter w m => MonadWriter w (IdentityT m)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> IdentityT m a Source tell :: w -> IdentityT m () Source listen :: IdentityT m a -> IdentityT m (a, w) Source pass :: IdentityT m (a, w -> w) -> IdentityT m a Source
MonadWriter w m => MonadWriter w (ExceptT e m)	Since: mtl-2.2
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> ExceptT e m a Source tell :: w -> ExceptT e m () Source listen :: ExceptT e m a -> ExceptT e m (a, w) Source pass :: ExceptT e m (a, w -> w) -> ExceptT e m a Source
(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> ErrorT e m a Source tell :: w -> ErrorT e m () Source listen :: ErrorT e m a -> ErrorT e m (a, w) Source pass :: ErrorT e m (a, w -> w) -> ErrorT e m a Source
(Monoid w, Monad m) => MonadWriter w (WriterT w m)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> WriterT w m a Source tell :: w -> WriterT w m () Source listen :: WriterT w m a -> WriterT w m (a, w) Source pass :: WriterT w m (a, w -> w) -> WriterT w m a Source
(Monoid w, Monad m) => MonadWriter w (WriterT w m)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> WriterT w m a Source tell :: w -> WriterT w m () Source listen :: WriterT w m a -> WriterT w m (a, w) Source pass :: WriterT w m (a, w -> w) -> WriterT w m a Source
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)
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)
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.

listens f m = liftM (id *** f) (listen m)

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.

censor f m = pass (liftM (\x -> (x,f)) m)

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.

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

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.

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

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 runWriterT :: m (a, w)

Instances

Instances details

(Monoid w, MonadError e m) => MonadError e (WriterT w m)
Instance details Defined in Control.Monad.Error.Class Methods throwError :: e -> WriterT w m a Source catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a Source
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)
Instance details Defined in Control.Monad.Reader.Class Methods ask :: WriterT w m r Source local :: (r -> r) -> WriterT w m a -> WriterT w m a Source reader :: (r -> a) -> WriterT w m a Source
(Monoid w, MonadState s m) => MonadState s (WriterT w m)
Instance details Defined in Control.Monad.State.Class Methods get :: WriterT w m s Source put :: s -> WriterT w m () Source state :: (s -> (a, s)) -> WriterT w m a Source
(Monoid w, Monad m) => MonadWriter w (WriterT w m)
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> WriterT w m a Source tell :: w -> WriterT w m () Source listen :: WriterT w m a -> WriterT w m (a, w) Source pass :: WriterT w m (a, w -> w) -> WriterT w m a Source
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 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.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 Methods fail :: String -> WriterT w m a Source
(Monoid w, Applicative m) => Applicative (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict 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
Foldable f => Foldable (WriterT w f)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods fold :: Monoid m => WriterT w f m -> m Source foldMap :: Monoid m => (a -> m) -> WriterT w f a -> m Source foldMap' :: Monoid m => (a -> m) -> WriterT w f a -> m Source foldr :: (a -> b -> b) -> b -> WriterT w f a -> b Source foldr' :: (a -> b -> b) -> b -> WriterT w f a -> b Source foldl :: (b -> a -> b) -> b -> WriterT w f a -> b Source foldl' :: (b -> a -> b) -> b -> WriterT w f a -> b Source foldr1 :: (a -> a -> a) -> WriterT w f a -> a Source foldl1 :: (a -> a -> a) -> WriterT w f a -> a Source toList :: WriterT w f a -> [a] Source null :: WriterT w f a -> Bool Source length :: WriterT w f a -> Int Source elem :: Eq a => a -> WriterT w f a -> Bool Source maximum :: Ord a => WriterT w f a -> a Source minimum :: Ord a => WriterT w f a -> a Source sum :: Num a => WriterT w f a -> a Source product :: Num a => WriterT w f a -> a Source
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 Methods liftCompare :: (a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering Source
(Read w, Read1 m) => Read1 (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (WriterT w m a) Source liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [WriterT w m a] Source liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (WriterT w m a) Source liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [WriterT w m a] Source
(Show w, Show1 m) => Show1 (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> WriterT w m a -> ShowS Source liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [WriterT w m a] -> ShowS Source
(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 Methods liftIO :: IO a -> WriterT w m a Source
(Monoid w, Alternative m) => Alternative (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict 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
(Monoid w, MonadPlus m) => MonadPlus (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods mzero :: WriterT w m a Source mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a Source
(Monoid w, MonadCont m) => MonadCont (WriterT w m)
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 Methods (==) :: WriterT w m a -> WriterT w m a -> Bool (/=) :: WriterT w m a -> WriterT w m a -> Bool
(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods compare :: WriterT w m a -> WriterT w m a -> Ordering (<) :: WriterT w m a -> WriterT w m a -> Bool (<=) :: WriterT w m a -> WriterT w m a -> Bool (>) :: WriterT w m a -> WriterT w m a -> Bool (>=) :: WriterT w m a -> WriterT w m a -> Bool max :: WriterT w m a -> WriterT w m a -> WriterT w m a min :: WriterT w m a -> WriterT w m a -> WriterT w m a
(Read w, Read1 m, Read a) => Read (WriterT w m a)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods readsPrec :: Int -> ReadS (WriterT w m a) Source readList :: ReadS [WriterT w m a] Source readPrec :: ReadPrec (WriterT w m a) Source readListPrec :: ReadPrec [WriterT w m a] Source
(Show w, Show1 m, Show a) => Show (WriterT w m a)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods showsPrec :: Int -> WriterT w m a -> ShowS Source show :: WriterT w m a -> String Source showList :: [WriterT w m a] -> ShowS Source

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

Extract the output from a writer computation.

execWriterT m = liftM snd (runWriterT m)

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.

runWriterT (mapWriterT f m) = f (runWriterT m)

module Control.Monad

module Control.Monad.Fix

module Control.Monad.Trans

module Data.Monoid

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.10.2/docs/html/libraries/mtl-2.2.2/Control-Monad-Writer-Strict.html

Control.Monad.Writer.Strict

Contents

Description

MonadWriter class

Minimal complete definition

Methods

Instances

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

The Writer monad

The WriterT monad transformer

Constructors

Fields

Instances

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods

Methods