transformers-0.5.2.0: Concrete functor and monad transformers

Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001
License | BSD-style (see the file LICENSE)
Maintainer | R.Paterson@city.ac.uk
Stability | experimental
Portability | portable
Safe Haskell | Safe
Language | Haskell98

Control.Monad.Trans.Writer.Lazy

Contents

- The Writer monad
- The WriterT monad transformer
- Writer operations
- Lifting other operations

Description

The lazy `WriterT` monad transformer, which adds collection of outputs (such as a count or string output) to a given monad.

This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead.

This version builds its output lazily; for a strict version with the same interface, see Control.Monad.Trans.Writer.Strict.

Synopsis

- type Writer w = WriterT w Identity
- writer :: Monad m => (a, w) -> WriterT w m a
- 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
- tell :: Monad m => w -> WriterT w m ()
- listen :: Monad m => WriterT w m a -> WriterT w m (a, w)
- listens :: Monad m => (w -> b) -> WriterT w m a -> WriterT w m (a, b)
- pass :: Monad m => WriterT w m (a, w -> w) -> WriterT w m a
- censor :: Monad m => (w -> w) -> WriterT w m a -> WriterT w m a
- liftCallCC :: Monoid w => CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b
- liftCatch :: Catch e m (a, w) -> Catch e (WriterT w m) a

# 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`.

writer :: Monad m => (a, w) -> WriterT w m a #

Construct a writer computation from a (result, output) pair. (The inverse of `runWriter`.)

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 => 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 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) # |
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) # |
Methods mfix :: (a -> WriterT w m a) -> WriterT w m a Source #
(Monoid w, MonadFail m) => MonadFail (WriterT w m) # |
Methods fail :: String -> WriterT w m a Source #
(Monoid w, Applicative m) => Applicative (WriterT w m) # |
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) # |

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) # |
Methods traverse :: Applicative f => (a -> f b) -> WriterT w f a -> f (WriterT w f b) Source # sequenceA :: Applicative f => WriterT w f (f a) -> f (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 #
(Eq w, Eq1 m) => Eq1 (WriterT w m) # |
Methods liftEq :: (a -> b -> Bool) -> WriterT w m a -> WriterT w m b -> Bool Source #
(Ord w, Ord1 m) => Ord1 (WriterT w m) # |
Methods liftCompare :: (a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering Source #
(Read w, Read1 m) => Read1 (WriterT w m) # |
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) # |
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) # |
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) # |
Methods liftIO :: IO a -> WriterT w m a Source #
(Monoid w, Alternative m) => Alternative (WriterT w m) # |
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) # |
Methods mzero :: WriterT w m a Source # mplus :: WriterT w m a -> 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) 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) # |
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 #

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)

# Writer operations

tell :: Monad m => w -> WriterT w m () #

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

listen :: Monad m => WriterT w m a -> WriterT w m (a, w) #

``listen` m` is an action that executes the action `m` and adds its output to the value of the computation.

-     runWriterT (listen m) = liftM (\ (a, w) -> ((a, w), w)) (runWriterT m)

listens :: Monad m => (w -> b) -> WriterT w m a -> WriterT w 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)

-     runWriterT (listens f m) = liftM (\ (a, w) -> ((a, f w), w)) (runWriterT m)

pass :: Monad m => WriterT w m (a, w -> w) -> WriterT 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.

-     runWriterT (pass m) = liftM (\ ((a, f), w) -> (a, f w)) (runWriterT m)

censor :: Monad m => (w -> w) -> WriterT w m a -> WriterT w 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)

-     runWriterT (censor f m) = liftM (\ (a, w) -> (a, f w)) (runWriterT m)

# Lifting other operations

liftCallCC :: Monoid w => CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b #

Lift a `callCC` operation to the new monad.

liftCatch :: Catch e m (a, w) -> Catch e (WriterT w m) a #

Lift a `catchE` operation to the new monad.