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

CopyrightCopyright (C) 2005 Uwe Schmidt
MaintainerUwe Schmidt (uwe\@fh-wedel.de)
Portabilitymulti parameter classes and functional depenedencies required
Safe HaskellSafe-Inferred



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 where Source

The interface for accessing and changing the state component.

Multi parameter classes and functional dependencies are required.

Minimal complete definition

changeState, accessState


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"]