Control.Monad.Trans.Writer · rerebase-1.21.2

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

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

#

Lift a callCC operation to the new monad.

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)

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)

valueexecWriter :: Writer w a -> w

#

Extract the output from a writer computation.

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

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

#

Extract the output from a writer computation.

execWriterT m = liftM snd (runWriterT 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)

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)

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

#

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

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)

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)

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

#

tell w is an action that produces the output w.

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

#

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

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)

Instances52MonadAccum, MonadError, MonadReader, MonadState, MonadWriter, MonadSelect, …

(MonadAccum w' m, Monoid w) => MonadAccum w' (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Accum
(Monoid w, MonadError e m) => MonadError e (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Error.Class
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
(Monoid w, MonadState s m) => MonadState s (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.State.Class
(Monoid w, Monad m) => MonadWriter w (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
(MonadSelect w' m, Monoid w) => MonadSelect w' (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Select
'Readerizes' the writer: the 'ranking' function can see the value that's been accumulated (of type w), but can't add anything to the log. Effectively, can be thought of as 'extending' the 'ranking' by all values of w, but which w gets given to any rank calls is predetermined by the 'outer writer' (and cannot change).
(Monoid w, MonadBase b m) => MonadBase b (WriterT w m)Defined in transformers-base-0.4.6 · Control.Monad.Base
(Functor f, MonadFree f m, Monoid w) => MonadFree f (WriterT w m)Defined in free-5.2 · Control.Monad.Free.Class
Monoid w => MonadTrans (WriterT w)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Writer.Lazy
Monoid w => BindTrans (WriterT w)Defined in semigroupoids-6.0.1 · Data.Functor.Bind.Trans
(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
(Monoid w, MonadCont m) => MonadCont (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Cont.Class
(MonadCatch m, Monoid w) => MonadCatch (WriterT w m)Defined in exceptions-0.10.9 · Control.Monad.Catch
(MonadMask m, Monoid w) => MonadMask (WriterT w m)Defined in exceptions-0.10.9 · Control.Monad.Catch
(MonadThrow m, Monoid w) => MonadThrow (WriterT w m)Defined in exceptions-0.10.9 · Control.Monad.Catch
(Monoid w, PrimMonad m) => PrimMonad (WriterT w m)Defined in primitive-0.9.1.0 · Control.Monad.Primitive
Decidable m => Decidable (WriterT w m)Defined in contravariant-1.5.5 · Data.Functor.Contravariant.Divisible
Divisible m => Divisible (WriterT w m)Defined in contravariant-1.5.5 · Data.Functor.Contravariant.Divisible
Alt f => Alt (WriterT w f)Defined in semigroupoids-6.0.1 · Data.Functor.Alt
(Apply m, Semigroup w) => Apply (WriterT w m)Defined in semigroupoids-6.0.1 · Data.Functor.Bind.Class
A WriterT w m is not Applicative unless its w is a Monoid, but it is an instance of Apply
(Bind m, Semigroup w) => Bind (WriterT w m)Defined in semigroupoids-6.0.1 · Data.Functor.Bind.Class
A WriterT w m is not a Monad unless its w is a Monoid, but it is an instance of Bind
Conclude m => Conclude (WriterT w m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Conclude
Decide m => Decide (WriterT w m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Decide
Divise m => Divise (WriterT w m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Divise
Plus f => Plus (WriterT w f)Defined in semigroupoids-6.0.1 · Data.Functor.Plus
Invariant m => Invariant (WriterT w m)Defined in invariant-0.6.4 · Data.Functor.Invariant
from the transformers package
(Monoid w, Selective f) => Selective (WriterT w f)Defined in selective-0.7.0.1 · Control.Selective
Copointed m => Copointed (WriterT w m)Defined in pointed-5.0.4 · Data.Copointed
(Default w, Pointed m) => Pointed (WriterT w m)Defined in pointed-5.0.4 · Data.Pointed
Adjunction m w => Adjunction (WriterT s m) (TracedT s w)Defined in adjunctions-4.4.3 · Data.Functor.Adjunction
(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
type PrimState (WriterT w m) = PrimState mDefined in primitive-0.9.1.0 · Control.Monad.Primitive

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

#

Lift a catchE operation to the new monad.