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.Lazy

Contents

MonadWriter class
The Writer monad
The WriterT monad transformer

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.

Synopsis

class (Monoid w, Monad m) => MonadWriter w m | m -> w where
listens :: MonadWriter w m => (w -> b) -> m a -> m (a, b)
censor :: MonadWriter w m => (w -> w) -> m a -> m a
type Writer w = WriterT w Identity
runWriter :: Writer w a -> (a, w)
execWriter :: Writer w a -> w
mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
newtype WriterT w (m :: * -> *) a = WriterT (m (a, w))
runWriterT :: WriterT w m a -> m (a, w)
execWriterT :: Monad m => WriterT w m a -> m w
mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans
module Data.Monoid

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) Source #
Instance details 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) Source #	NOTE: This instance is only defined for `base >= 4.9.0`. Since: 2.2.2
Instance details 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 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) Source #
Instance details 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) Source #
Instance details 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) Source #	Since: 2.2
Instance details 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) Source #
Instance details 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) Source #
Instance details 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) Source #
Instance details 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) Source #
Instance details 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) Source #
Instance details 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 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 #

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) #

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

execWriter :: Writer w a -> w #

Extract the output from a writer computation.

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

mapWriter :: ((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

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

Constructors

WriterT (m (a, w))

Instances

(Monoid w, MonadError e m) => MonadError e (WriterT w m) Source #
Instance details 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) Source #
Instance details 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) Source #
Instance details 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) Source #
Instance details 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 Methods lift :: Monad m => m a -> WriterT w m a #
(Monoid w, Monad m) => Monad (WriterT w m)
Instance details 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 Methods fmap :: (a -> b) -> WriterT w m a -> WriterT w m b # (<$) :: a -> WriterT w m b -> WriterT w m a #
(Monoid w, MonadFix m) => MonadFix (WriterT w m)
Instance details Methods mfix :: (a -> WriterT w m a) -> WriterT w m a #
(Monoid w, MonadFail m) => MonadFail (WriterT w m)
Instance details Methods fail :: String -> WriterT w m a #
(Monoid w, Applicative m) => Applicative (WriterT w m)
Instance details 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 #
Foldable f => Foldable (WriterT w f)
Instance details Methods fold :: Monoid m => WriterT w f m -> m # foldMap :: Monoid m => (a -> m) -> WriterT w f a -> m # foldr :: (a -> b -> b) -> b -> WriterT w f a -> b # foldr' :: (a -> b -> b) -> b -> WriterT w f a -> b # foldl :: (b -> a -> b) -> b -> WriterT w f a -> b # foldl' :: (b -> a -> b) -> b -> WriterT w f a -> b # foldr1 :: (a -> a -> a) -> WriterT w f a -> a # foldl1 :: (a -> a -> a) -> WriterT w f a -> a # toList :: WriterT w f a -> [a] # null :: WriterT w f a -> Bool # length :: WriterT w f a -> Int # elem :: Eq a => a -> WriterT w f a -> Bool # maximum :: Ord a => WriterT w f a -> a # minimum :: Ord a => WriterT w f a -> a # sum :: Num a => WriterT w f a -> a # product :: Num a => WriterT w f a -> a #
Traversable f => Traversable (WriterT w f)
Instance details Methods traverse :: Applicative f0 => (a -> f0 b) -> WriterT w f a -> f0 (WriterT w f b) # sequenceA :: Applicative f0 => WriterT w f (f0 a) -> f0 (WriterT w f a) # mapM :: Monad m => (a -> m b) -> WriterT w f a -> m (WriterT w f b) # sequence :: Monad m => WriterT w f (m a) -> m (WriterT w f a) #
(Eq w, Eq1 m) => Eq1 (WriterT w m)
Instance details Methods liftEq :: (a -> b -> Bool) -> WriterT w m a -> WriterT w m b -> Bool #
(Ord w, Ord1 m) => Ord1 (WriterT w m)
Instance details Methods liftCompare :: (a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering #
(Read w, Read1 m) => Read1 (WriterT w m)
Instance details Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (WriterT w m a) # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [WriterT w m a] # liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (WriterT w m a) # liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [WriterT w m a] #
(Show w, Show1 m) => Show1 (WriterT w m)
Instance details Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> WriterT w m a -> ShowS # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [WriterT w m a] -> ShowS #
(Monoid w, MonadZip m) => MonadZip (WriterT w m)
Instance details Methods mzip :: WriterT w m a -> WriterT w m b -> WriterT w m (a, b) # mzipWith :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c # munzip :: WriterT w m (a, b) -> (WriterT w m a, WriterT w m b) #
(Monoid w, MonadIO m) => MonadIO (WriterT w m)
Instance details Methods liftIO :: IO a -> WriterT w m a #
(Monoid w, Alternative m) => Alternative (WriterT w m)
Instance details 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] #
(Monoid w, MonadPlus m) => MonadPlus (WriterT w m)
Instance details Methods mzero :: WriterT w m a # mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a #
(Monoid w, MonadCont m) => MonadCont (WriterT w m) Source #
Instance details 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 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 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 Methods readsPrec :: Int -> ReadS (WriterT w m a) # readList :: ReadS [WriterT w m a] # readPrec :: ReadPrec (WriterT w m a) # readListPrec :: ReadPrec [WriterT w m a] #
(Show w, Show1 m, Show a) => Show (WriterT w m a)
Instance details Methods showsPrec :: Int -> WriterT w m a -> ShowS # show :: WriterT w m a -> String # showList :: [WriterT w m a] -> ShowS #

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

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

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 #

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