Arrow Libraries (arrows package)ParentContentsIndex
Control.Arrow.Transformer.Writer
Portability non-portable (multi-parameter type classes)
Stability experimental
Maintainer ross@soi.city.ac.uk
Description
Arrow transformer that adds accumulation of output.
Synopsis
data WriterArrow w a b c
runWriter :: (Arrow a, Monoid w) => WriterArrow w a e b -> a e (b, w)
class (ArrowWriter w a, Arrow a') => ArrowAddWriter w a a' | a -> a' where
liftWriter :: a' e b -> a e b
elimWriter :: a e b -> a' e (b, w)
module Control.Arrow.Operations
module Control.Arrow.Transformer
Documentation
data WriterArrow w a b c
An arrow type that augments an existing arrow with accumulating output. The ArrowWriter class contains the relevant operations.
Instances
(Arrow a, Monoid w) => Arrow (WriterArrow w a)
(ArrowAddError ex a a', Monoid w) => ArrowAddError ex (WriterArrow w a) (WriterArrow w a')
(ArrowAddError ex a a', Monoid w) => ArrowAddError ex (WriterArrow w a) (WriterArrow w a')
(ArrowAddReader r a a', Monoid w) => ArrowAddReader r (WriterArrow w a) (WriterArrow w a')
(ArrowAddReader r a a', Monoid w) => ArrowAddReader r (WriterArrow w a) (WriterArrow w a')
(ArrowAddState s a a', Monoid w) => ArrowAddState s (WriterArrow w a) (WriterArrow w a')
(ArrowAddState s a a', Monoid w) => ArrowAddState s (WriterArrow w a) (WriterArrow w a')
(Arrow a, Monoid w) => ArrowAddWriter w (WriterArrow w a) a
(ArrowApply a, Monoid w) => ArrowApply (WriterArrow w a)
(ArrowChoice a, Monoid w) => ArrowChoice (WriterArrow w a)
(ArrowCircuit a, Monoid w) => ArrowCircuit (WriterArrow w a)
(ArrowError ex a, Monoid w) => ArrowError ex (WriterArrow w a)
(ArrowLoop a, Monoid w) => ArrowLoop (WriterArrow w a)
(ArrowPlus a, Monoid w) => ArrowPlus (WriterArrow w a)
(ArrowReader r a, Monoid w) => ArrowReader r (WriterArrow w a)
(ArrowState s a, Monoid w) => ArrowState s (WriterArrow w a)
(Arrow a, Monoid w) => ArrowTransformer (WriterArrow w) a
(Arrow a, Monoid w) => ArrowWriter w (WriterArrow w a)
(ArrowZero a, Monoid w) => ArrowZero (WriterArrow w a)
runWriter :: (Arrow a, Monoid w) => WriterArrow w a e b -> a e (b, w)

Encapsulation of a writer computation, providing the accumulated output.

Typical usage in arrow notation: 

	proc p -> do
		...
		(result, output) <- (|runWriter cmd|)
class (ArrowWriter w a, Arrow a') => ArrowAddWriter w a a' | a -> a' where

Adding a WriterArrow to an arrow type, but not necessarily as the outer arrow transformer.

Typically a composite arrow type is built by applying a series of arrow transformer to a base arrow (usually either a function arrow or a Kleisli arrow. One can add a transformer to the top of this stack using the lift method of the ArrowTransformer class, or remove a state transformer from the top of the stack using the runWriter encapsulation operator. The methods of this class add and remove state transformers anywhere in the stack. In the instance 

	instance Arrow a => ArrowAddWriter w (ArrowWriter w a) a

they are equivalent to lift and runWriter respectively. Instances are lifted through other transformers with 

	instance ArrowAddWriter w a a' =>
		ArrowAddWriter w (FooArrow a) (FooArrow a')
Methods
liftWriter :: a' e b -> a e b

Lift a computation from an arrow to one with added output.

Typical usage in arrow notation: 

	proc p -> ...
		(|liftWriter cmd|)
elimWriter :: a e b -> a' e (b, w)

Elimination of an output writer from a computation, providing the accumulated output.

Typical usage in arrow notation: 

	proc p -> do
		...
		(result, output) <- (|elimWriter cmd|)
Instances
ArrowAddWriter s a a' => ArrowAddWriter s (ReaderArrow r a) (ReaderArrow r a')
ArrowAddWriter w a a' => ArrowAddWriter w (Automaton a) (Automaton a')
(ArrowAddWriter w a a', ArrowChoice a, ArrowChoice a') => ArrowAddWriter w (ErrorArrow ex a) (ErrorArrow ex a')
ArrowAddWriter w a a' => ArrowAddWriter w (StateArrow s a) (StateArrow s a')
(ArrowAddWriter w a a', Sequence f) => ArrowAddWriter w (StaticArrow f a) (StaticArrow f a')
(Arrow a, Monoid w) => ArrowAddWriter w (WriterArrow w a) a
module Control.Arrow.Operations
module Control.Arrow.Transformer
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