Control.Arrow.ArrowState · hxt-9.3.1.22

classclass Arrow a => ArrowState s (a :: Type -> Type -> Type) | a -> s where

The interface for accessing and changing the state component.

Multi parameter classes and functional dependencies are required.

changeState :: (s -> b -> s) -> a b b
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
access the state with a function using the arrow input as data for selecting state components.
getState :: a b s
read the complete state, ignore arrow input
definition: getState = accessState (\ s x -> s)
setState :: a s s
overwrite the old state
definition: setState = changeState (\ s x -> x)
nextState :: (s -> s) -> a b s
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"]
```

Instances2ArrowState

ArrowState s (IOSLA s)Defined in hxt-9.3.1.22 · Control.Arrow.IOStateListArrow
ArrowState s (SLA s)Defined in hxt-9.3.1.22 · Control.Arrow.StateListArrow