mtl-2.2.1: 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 | Haskell98

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
- 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 {
  - 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 Source #

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 # |
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 #
MonadWriter w m => MonadWriter w (StateT s m) Source # |
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 # |
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 # |
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 # |
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 # |
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 # |
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 # |
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 # |
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 # |
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 # |
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 |

Fields

- runWriterT :: m (a, w)

Instances

(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source # |
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, MonadState s m) => MonadState s (WriterT w m) Source # |
Methods get :: WriterT w m s Source # put :: s -> WriterT w m () Source # state :: (s -> (a, s)) -> WriterT w m a Source #
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) Source # |
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, MonadError e m) => MonadError e (WriterT w m) Source # |
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 => MonadTrans (WriterT w) |
Methods lift :: Monad m => m a -> WriterT w m a #
(Monoid w, Monad m) => Monad (WriterT w m) |
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) |
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) |
Methods mfix :: (a -> WriterT w m a) -> WriterT w m a #
(Monoid w, MonadFail m) => MonadFail (WriterT w m) |
Methods fail :: String -> WriterT w m a #
(Monoid w, Applicative m) => Applicative (WriterT w m) |
Methods pure :: a -> WriterT w m a # (<*>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b # (*>) :: 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) |

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) |
Methods traverse :: Applicative f => (a -> f b) -> WriterT w f a -> f (WriterT w f b) # sequenceA :: Applicative f => WriterT w f (f a) -> f (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) |
Methods liftEq :: (a -> b -> Bool) -> WriterT w m a -> WriterT w m b -> Bool #
(Ord w, Ord1 m) => Ord1 (WriterT w m) |
Methods liftCompare :: (a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering #
(Read w, Read1 m) => Read1 (WriterT w m) |
Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (WriterT w m a) # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [WriterT w m a] #
(Show w, Show1 m) => Show1 (WriterT w m) |
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) |
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) |
Methods liftIO :: IO a -> WriterT w m a #
(Monoid w, Alternative m) => Alternative (WriterT w m) |
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) |
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 # |
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) |
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) |
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) |
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) |
Methods showsPrec :: Int -> WriterT w m a -> ShowS # show :: WriterT w m a -> String # showList :: [WriterT w m a] -> ShowS #

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