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Control.Monad.Trans.Writer.Lazy

Copyright (c) Andy Gill 2001
(c) Oregon Graduate Institute of Science and Technology 2001
License BSD-style (see the file LICENSE)
Maintainer [email protected]
Stability experimental
Portability portable
Safe Haskell Safe
Language Haskell98

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.

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)
Instance details

Defined in Control.Monad.Trans.Writer.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

Methods

fail :: String -> WriterT w m a Source

(Monoid w, Applicative m) => Applicative (WriterT w m)
Instance details

Defined in Control.Monad.Trans.Writer.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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.Lazy

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)
Instance details

Defined in Control.Monad.Trans.Writer.Lazy

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.Lazy

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.Lazy

(Show w, Show1 m, Show a) => Show (WriterT w m a)
Instance details

Defined in Control.Monad.Trans.Writer.Lazy

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.

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.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/transformers-0.5.5.0/Control-Monad-Trans-Writer-Lazy.html