transformers-0.5.5.0: Concrete functor and monad transformers

Copyright(c) Andy Gill 2001
(c) Oregon Graduate Institute of Science and Technology 2001
LicenseBSD-style (see the file LICENSE)
MaintainerR.Paterson@city.ac.uk
Stabilityexperimental
Portabilityportable
Safe HaskellSafe
LanguageHaskell98

Control.Monad.Trans.Writer.Lazy

Contents

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

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.

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

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

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 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

Instances

Monoid w => MonadTrans (WriterT w) Source # 

Methods

lift :: Monad m => m a -> WriterT w m a Source #

(Monoid w, Monad m) => Monad (WriterT w m) Source # 

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) Source # 

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) Source # 

Methods

mfix :: (a -> WriterT w m a) -> WriterT w m a #

(Monoid w, MonadFail m) => MonadFail (WriterT w m) Source # 

Methods

fail :: String -> WriterT w m a #

(Monoid w, Applicative m) => Applicative (WriterT w m) Source # 

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) Source # 

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) Source # 

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) Source # 

Methods

liftEq :: (a -> b -> Bool) -> WriterT w m a -> WriterT w m b -> Bool #

(Ord w, Ord1 m) => Ord1 (WriterT w m) Source # 

Methods

liftCompare :: (a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering #

(Read w, Read1 m) => Read1 (WriterT w m) Source # 

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) Source # 

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) Source # 

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) Source # 

Methods

liftIO :: IO a -> WriterT w m a #

(Monoid w, Alternative m) => Alternative (WriterT w m) Source # 

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) Source # 

Methods

mzero :: WriterT w m a #

mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a #

(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a) Source # 

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) Source # 

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) Source # 

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) Source # 

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 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.

Writer operations

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

tell w is an action that produces the output w.

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

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

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

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

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

Lifting other operations

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

Lift a callCC operation to the new monad.

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

Lift a catchE operation to the new monad.