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GHC 10.0.0.20260917 · lts/ghc-10.0.x · 77a8662 · 2026-10-31

Moduletransformers-0.6.3.0Haskell2010

Control.Monad.Trans.Writer.CPS

The strict 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 strictly and uses continuation-passing-style to achieve constant space usage. This transformer can be used as a drop-in replacement for Control.Monad.Trans.Writer.Strict.

Contents

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

Synopsis

type Writer
value writer
value runWriter
value execWriter
value mapWriter
newtype WriterT
value writerT
value runWriterT
value execWriterT
value mapWriterT
value tell
value listen
value listens
value pass
value censor
value liftCallCC
value liftCatch

Packagetransformers-0.6.3.0
Exports17
LanguageHaskell2010
LicenceBSD-3-Clause
SourceCPS.hs

The Writer monad

typetype 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 m >>= k combines the outputs of the subcomputations using mappend (also known as <>):

image: images/bind-WriterT.svg

valuewriter :: (Monoid w, Monad m) => (a, w) -> WriterT w m a

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

valuerunWriter :: Monoid w => Writer w a -> (a, w)

Unwrap a writer computation as a (result, output) pair. (The inverse of writer.)

valueexecWriter :: Monoid w => Writer w a -> w

Extract the output from a writer computation.

```
execWriter m = snd (runWriter m)
```

valuemapWriter

:: (Monoid w, Monoid w')
=> (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

newtypenewtype WriterT w (m :: Type -> Type) a

A writer monad parameterized by:

w - the output to accumulate.
m - The inner monad.

The return function produces the output mempty, while m >>= k combines the outputs of the subcomputations using mappend (also known as <>):

image: images/bind-WriterT.svg

Instances11MonadTrans, Monad, Functor, MonadFix, MonadFail, Applicative, …

MonadTrans (WriterT w)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
Monad m => Monad (WriterT w m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
Functor m => Functor (WriterT w m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
MonadFix m => MonadFix (WriterT w m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
MonadFail m => MonadFail (WriterT w m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
(Functor m, Monad m) => Applicative (WriterT w m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
(Functor m, MonadPlus m) => Alternative (WriterT w m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
(Functor m, MonadPlus m) => MonadPlus (WriterT w m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
MonadIO m => MonadIO (WriterT w m)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
Generic (WriterT w m a)Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS
type Rep (WriterT w m a) = D1 ('MetaData "WriterT" "Control.Monad.Trans.Writer.CPS" "transformers-0.6.3.0-34b4" 'True) (C1 ('MetaCons "WriterT" 'PrefixI 'True) (S1 ('MetaSel ('Just "unWriterT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (w -> m (a, w)))))Defined in transformers-0.6.3.0 · Control.Monad.Trans.Writer.CPS

valuewriterT :: (Functor m, Monoid w) => m (a, w) -> WriterT w m a

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

valuerunWriterT :: Monoid w => WriterT w m a -> m (a, w)

Unwrap a writer computation. (The inverse of writerT.)

valueexecWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w

Extract the output from a writer computation.

execWriterT m = liftM snd (runWriterT m)

valuemapWriterT

:: (Monad n, Monoid w, Monoid w')
=> 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

valuetell :: (Monoid w, Monad m) => w -> WriterT w m ()

tell w is an action that produces the output w.

valuelisten :: (Monoid w, 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)

valuelistens

:: (Monoid w, 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)

valuepass

:: (Monoid w, Monoid w', 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)

valuecensor :: (Monoid w, 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

valueliftCallCC :: CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b

Uniform lifting of a callCC operation to the new monad. The uniformity property (see Control.Monad.Signatures) implies that the lifted callCC discards any output from the body on entering the saved continuation.

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

Lift a catchE operation to the new monad. The uniformity property (see Control.Monad.Signatures) implies that the lifted catchE discards any output from the body on entering the handler.