| Portability | multi parameter classes and functional depenedencies required |
|---|---|
| Stability | experimental |
| Maintainer | Uwe Schmidt (uwe\@fh-wedel.de) |
Control.Arrow.ArrowState
Description
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
- 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 whereSource
The interface for accessing and changing the state component.
Multi parameter classes and functional dependencies are required.
Methods
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.
read the complete state, ignore arrow input
definition: getState = accessState (\ s x -> s)
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"]
Instances
| ArrowState s (SLA s) | |
| ArrowState s (IOSLA s) |