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Control.Arrow.ArrowState

Portability | multy parameter classes and functional depenedencies required
Stability | experimental
Maintainer | Uwe Schmidt (uwe\@fh-wedel.de)

Description
Arrows for managing an explicit state State arrows work similar to state monads. A state value is threaded through the application of arrows.
Synopsis

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

changeState :: (s -> b -> s) -> a b b
accessState :: (s -> b -> c) -> a b c
getState :: a b s
setState :: a s s
nextState :: (s -> s) -> a b s

Documentation

class Arrow a => ArrowState s a | a -> s where | Source

The interface for accessing and changing the state component. Multi parameter classes and functional dependencies are required.
Methods

changeState :: (s -> b -> s) -> a b b | Source

change the state of a state arrow by applying a function for computing a new state from the old and the arrow input. Result is the arrow input

accessState :: (s -> b -> c) -> a b c | Source

access the state with a function using the arrow input as data for selecting state components.

getState :: a b s | Source

read the complete state, ignore arrow input definition: `getState = accessState (\ s x -> s)`

setState :: a s s | Source

overwrite the old state definition: `setState = changeState (\ s x -> x)`

nextState :: (s -> s) -> a b s | Source

change state (and ignore input) and return new state

convenience function, usefull for generating e.g. unique identifiers:

example with SLA state list arrows

     newId :: SLA Int b String
     newId = nextState (+1)
             >>>
             arr (('#':) . show)

     runSLA 0 (newId <+> newId <+> newId) undefined
       = ["#1", "#2", "#3"]

Instances

ArrowState s (SLA s)
ArrowState s (IOSLA s)

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