Moduletransformers-0.6.1.1Haskell2010

Control.Monad.Trans.Writer.Lazy

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 constant-space version with almost the same interface, see Control.Monad.Trans.Writer.CPS.

2 types
13 values

Packagetransformers-0.6.1.1
Exports15
LanguageHaskell2010
LicenceBSD-3-Clause
SourceLazy.hs

The Writer monad

5 declarations

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

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

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

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

valueexecWriter :: Writer w a -> w

Extract the output from a writer computation.

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

valuemapWriter :: ((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

3 declarations

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

Constructors

WriterT
- runWriterT :: m (a, w)

Instances23MonadTrans, Monad, Functor, MonadFix, MonadFail, Applicative, …

Monoid w => MonadTrans (WriterT w)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Monoid w, Monad m) => Monad (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
Functor m => Functor (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Monoid w, MonadFix m) => MonadFix (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Monoid w, MonadFail m) => MonadFail (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Monoid w, Applicative m) => Applicative (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
Foldable f => Foldable (WriterT w f)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
Traversable f => Traversable (WriterT w f)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Monoid w, Alternative m) => Alternative (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Monoid w, MonadPlus m) => MonadPlus (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Monoid w, MonadIO m) => MonadIO (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Monoid w, MonadZip m) => MonadZip (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Eq w, Eq1 m) => Eq1 (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Ord w, Ord1 m) => Ord1 (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Read w, Read1 m) => Read1 (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Show w, Show1 m) => Show1 (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
Contravariant m => Contravariant (WriterT w m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Read w, Read1 m, Read a) => Read (WriterT w m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
(Show w, Show1 m, Show a) => Show (WriterT w m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
Generic (WriterT w m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
type Rep (WriterT w m a) = D1 ('MetaData "WriterT" "Control.Monad.Trans.Writer.Lazy" "transformers-0.6.1.1-a11a" 'True) (C1 ('MetaCons "WriterT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runWriterT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (m (a, w)))))Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy

valueexecWriterT :: Monad m => WriterT w m a -> m w

Extract the output from a writer computation.

execWriterT m = liftM snd (runWriterT m)

valuemapWriterT :: (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

5 declarations

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

tell w is an action that produces the output w.

valuelisten :: 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 :: 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 :: 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 :: 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

2 declarations

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

Lift a callCC operation to the new monad.

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

Lift a catchE operation to the new monad.