hxt-9.2.2: A collection of tools for processing XML with Haskell.

Portabilitymulti parameter classes and functional depenedencies required
MaintainerUwe Schmidt (uwe\@fh-wedel.de)
Safe HaskellSafe-Infered



Arrows for managing an explicit state

State arrows work similar to state monads. A state value is threaded through the application of arrows.



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

The interface for accessing and changing the state component.

Multi parameter classes and functional dependencies are required.


changeState :: (s -> b -> s) -> a b bSource

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 cSource

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

getState :: a b sSource

read the complete state, ignore arrow input

definition: getState = accessState (\ s x -> s)

setState :: a s sSource

overwrite the old state

definition: setState = changeState (\ s x -> x)

nextState :: (s -> s) -> a b sSource

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"]