Control.Monad.Writer.Strict

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

Contents

- 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.

Synopsis

- class (Monoid w, Monad m) => MonadWriter w m | m -> w where
  - writer :: (a, w) -> m a
  - tell :: w -> m ()
  - listen :: m a -> m (a, w)
  - pass :: m (a, w -> w) -> m a
- 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 :: Type -> Type) a = WriterT {
  - runWriterT :: 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 #

Minimal complete definition

(writer | tell), listen, pass

Methods

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

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

tell :: w -> m () #

``tell` w` is an action that produces the output `w`.

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

``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 #

``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 # tell :: w -> MaybeT m () # listen :: MaybeT m a -> MaybeT m (a, w) # pass :: MaybeT m (a, w -> w) -> MaybeT m a #
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) # tell :: w -> (w, ()) # listen :: (w, a) -> (w, (a, w)) # pass :: (w, (a, w -> w)) -> (w, a) #
MonadWriter w m => MonadWriter w (StateT s m) # |
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> StateT s m a # tell :: w -> StateT s m () # listen :: StateT s m a -> StateT s m (a, w) # pass :: StateT s m (a, w -> w) -> StateT s m a #
MonadWriter w m => MonadWriter w (StateT s m) # |
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> StateT s m a # tell :: w -> StateT s m () # listen :: StateT s m a -> StateT s m (a, w) # pass :: StateT s m (a, w -> w) -> StateT s m a #
MonadWriter w m => MonadWriter w (IdentityT m) # |
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> IdentityT m a # tell :: w -> IdentityT m () # listen :: IdentityT m a -> IdentityT m (a, w) # pass :: IdentityT m (a, w -> w) -> IdentityT m a #
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 # tell :: w -> ExceptT e m () # listen :: ExceptT e m a -> ExceptT e m (a, w) # pass :: ExceptT e m (a, w -> w) -> ExceptT e m a #
(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 # tell :: w -> ErrorT e m () # listen :: ErrorT e m a -> ErrorT e m (a, w) # pass :: ErrorT e m (a, w -> w) -> ErrorT e m a #
(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 # 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 #
(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 # 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 #
MonadWriter w m => MonadWriter w (ReaderT r m) # |
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> ReaderT r m a # tell :: w -> ReaderT r m () # listen :: ReaderT r m a -> ReaderT r m (a, w) # pass :: ReaderT r m (a, w -> w) -> ReaderT r m a #
(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 # tell :: w -> RWST r w s m () # listen :: RWST r w s m a -> RWST r w s m (a, w) # pass :: RWST r w s m (a, w -> w) -> RWST r w s m a #
(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 # tell :: w -> RWST r w s m () # listen :: RWST r w s m a -> RWST r w s m (a, w) # pass :: RWST r w s m (a, w -> w) -> RWST r w s m a #

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

``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 #

``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

(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 # catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a #
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: WriterT w m r # local :: (r -> r) -> WriterT w m a -> WriterT w m a # reader :: (r -> a) -> WriterT w m a #
(Monoid w, MonadState s m) => MonadState s (WriterT w m) # |
Instance details Defined in Control.Monad.State.Class Methods get :: WriterT w m s # put :: s -> WriterT w m () # state :: (s -> (a, s)) -> WriterT w m a #
(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 # 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 #
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 # fail :: String -> 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 # 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 #
(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