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.State

Description

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

Synopsis

# Documentation

newtype StateArrow s a b c Source #

An arrow type that augments an existing arrow with a modifiable state. The ArrowState class contains the operations on this state.

Constructors

 StateArrow (a (b, s) (c, s))

Instances

runState :: Arrow a => StateArrow s a e b -> a (e, s) (b, s) Source #

Encapsulation of a state-using computation, exposing the initial and final states.

Typical usage in arrow notation:

   proc p -> do
...
(result, final_state) <- (|runState cmd|) init_state

class (ArrowState s a, Arrow a') => ArrowAddState s a a' | a -> a' where Source #

Adding a StateArrow 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 runState encapsulation operator. The methods of this class add and remove state transformers anywhere in the stack. In the instance

   instance Arrow a => ArrowAddState s (ArrowState s a) a

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

   instance ArrowAddState s a a' =>
ArrowAddState s (FooArrow a) (FooArrow a')

Minimal complete definition

Methods

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

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

Typical usage in arrow notation:

   proc p -> ...
(|liftState cmd|)

elimState :: a e b -> a' (e, s) (b, s) Source #

Elimination of a state transformer from a computation, exposing the initial and final states.

Typical usage in arrow notation:

   proc p -> do
...
(result, final_state) <- (|elimState cmd|) init_state

Instances