arrows-0.4.4.2: Arrow classes and transformers

Copyright (c) Ross Paterson 2003 BSD-style (see the LICENSE file in the distribution) R.Paterson@city.ac.uk experimental non-portable (multi-parameter type classes) Safe Haskell98

Control.Arrow.Transformer.Writer

Description

Arrow transformer that adds accumulation of output.

Synopsis

# Documentation

newtype WriterArrow w a b c Source #

An arrow type that augments an existing arrow with accumulating output. The ArrowWriter class contains the relevant operations.

Constructors

 WriterArrow (a b (c, w))

Instances

runWriter :: (Arrow a, Monoid w) => WriterArrow w a e b -> a e (b, w) Source #

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 Source #

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')

Minimal complete definition

Methods

liftWriter :: a' e b -> a e b Source #

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) Source #

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