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/Haskell 8

Control.Monad.Writer.Lazy

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

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.

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
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 (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

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

(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.

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 (m (a, w))
Instances
(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.Lazy

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.Lazy

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

fail :: String -> WriterT w m a Source

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

Defined in Control.Monad.Trans.Writer.Lazy

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.Lazy

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.Lazy

Methods

fail :: String -> WriterT w m a Source

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

Defined in Control.Monad.Trans.Writer.Lazy

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.Lazy

Methods

fold :: Monoid m => WriterT w f m -> 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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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

Defined in Control.Monad.Trans.Writer.Lazy

Methods

showsPrec :: Int -> WriterT w m a -> ShowS Source

show :: WriterT w m a -> String Source

showList :: [WriterT w m a] -> ShowS Source

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

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.

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/8.6.1/docs/html/libraries/mtl-2.2.2/Control-Monad-Writer-Lazy.html