arrows-0.4.4.1: Arrow classes and transformers

Portabilitynon-portable (multi-parameter type classes)
Stabilityexperimental
Maintainerross@soi.city.ac.uk
Safe HaskellSafe-Inferred

Control.Arrow.Operations

Contents

Description

Subclasses of Arrow providing additional operations.

The signatures are designed to be compatible with the proposed notation for arrows, cf. http://www.haskell.org/arrows/.

Synopsis

Conventions

The arrow classes defined in this module have names like ArrowFoo, and contain operations specific to such arrows. Some of these include a method newFoo, which maps computations to computations of the same arrow type, but exposing some of the internals of the arrow.

Arrow transformers have names like BarArrow, and are instances of appropriate arrow classes. For each arrow transformer, there is typically an encapsulation operator runBar that removes that transformer from the outside of an arrow type. The lift method of the ArrowTransformer class adds an arrow transformer to the outside of an arrow type.

Typically a composite arrow type is built by applying a series of arrow transformers to a base arrow (usually either a function arrow or a Kleisli arrow. The lift method and the runBar function operate only on the arrow transformer at the top of this stack. For more sophisticated manipulation of this stack of arrow transformers, many arrow transformers provide an ArrowAddBar class, with methods methods liftBar and elimBar to add and remove the transformer anywhere in the stack.

State transformers

class Arrow a => ArrowState s a | a -> s whereSource

An arrow type that provides a modifiable state, based of section 9 of Generalising Monads to Arrows, by John Hughes, Science of Computer Programming 37:67-111, May 2000.

Methods

fetch :: a e sSource

Obtain the current value of the state.

store :: a s ()Source

Assign a new value to the state.

Instances

State readers

class Arrow a => ArrowReader r a | a -> r whereSource

An arrow type that provides a read-only state (an environment). If you also need to modify the state, use ArrowState.

Methods

readState :: a b rSource

Obtain the current value of the state.

newReader :: a e b -> a (e, r) bSource

Run a subcomputation in the same arrow, but with a different environment. The environment of the outer computation is unaffected.

Typical usage in arrow notation: 

	proc p -> ...
		(|newReader cmd|) env

Instances

Monoid writers

class (Monoid w, Arrow a) => ArrowWriter w a | a -> w whereSource

An arrow type that collects additional output (of some Monoid type).

Methods

write :: a w ()Source

Add a piece of additional output.

newWriter :: a e b -> a e (b, w)Source

Run a subcomputation in the same arrow, making its additional output accessible.

Typical usage in arrow notation: 

	proc p -> do
		...
		(value, output) <- (|newWriter cmd|)

Instances

Errors

class Arrow a => ArrowError ex a | a -> ex whereSource

An arrow type that includes errors (or exceptions).

Minimal definition: raise and tryInUnless.

TODO: the operations here are inconsistent with other arrow transformers.

Methods

raise :: a ex bSource

Raise an error.

handleSource

Arguments

:: a e b

computation that may raise errors

-> a (e, ex) b

computation to handle errors

-> a e b 

Traditional exception construct.

Typical usage in arrow notation: 

	proc p -> ...
		body `handle` \ex -> handler

tryInUnlessSource

Arguments

:: a e b

computation that may raise errors

-> a (e, b) c

computation to receive successful results

-> a (e, ex) c

computation to handle errors

-> a e c 

Exception construct in the style of Exceptional Syntax, by Nick Benton and Andrew Kennedy, JFP 11(4):395-410, July 2001.

Typical usage in arrow notation: 

	proc p -> ...
		(|tryInUnless
			body
			(\res -> success)
			(\ex -> handler)
		|)

newError :: a e b -> a e (Either ex b)Source

Handler that returns the error as a value.

Instances

tryInUnlessDefaultSource

Arguments

:: (ArrowError ex a, ArrowChoice a) 
=> a e b

computation that may raise errors

-> a (e, b) c

computation to receive successful results

-> a (e, ex) c

computation to handle errors

-> a e c 

A suitable value for tryInUnless when the arrow type belongs to ArrowChoice. To use it, you must define either handle or newError.

Synchronous circuits

class ArrowLoop a => ArrowCircuit a whereSource

An arrow type that can be used to interpret synchronous circuits.

Methods

delaySource

Arguments

:: b

the value to return initially.

-> a b b

an arrow that propagates its input with a one-tick delay.

A delay component.

Instances