Kairos

A functional reactive programming (FRP) library for Kotlin.

This library is experimental and should not be used for general production code. The APIs within are subject to change, and there may be bugs.

About FRP

Functional reactive programming is a type of reactive programming system that follows a set of clear and composable rules, without sacrificing consistency. FRP exposes an API that should be familiar to those versed in Kotlin Flow.

Details for nerds

Kairos implements an applicative / monadic flavor of FRP, using a push-pull methodology to allow for efficient updates.

"Real" functional reactive programming should be specified with denotational semantics (wikipedia): you can view the semantics for Kairos here.

Usage

First, stand up a new FrpNetwork. All reactive events and state is kept consistent within a single network.

val coroutineScope: CoroutineScope = ...
val frpNetwork = coroutineScope.newFrpNetwork()

You can use the FrpNetwork to stand-up a network of reactive events and state. Events are modeled with TFlow (short for "transactional flow"), and state TState (short for "transactional state").

suspend fun activate(network: FrpNetwork) {
    network.activateSpec {
        val input = network.mutableTFlow<Unit>()
        // Launch a long-running side-effect that emits to the network
        // every second.
        launchEffect {
            while (true) {
                input.emit(Unit)
                delay(1.seconds)
            }
        }
        // Accumulate state
        val count: TState<Int> = input.fold { _, i -> i + 1 }
        // Observe events to perform side-effects in reaction to them
        input.observe {
            println("Got event ${count.sample()} at time: ${System.currentTimeMillis()}")
        }
    }
}

FrpNetwork.activateSpec will suspend indefinitely; cancelling the invocation will tear-down all effects and obervers running within the lambda.

Resources

• Cheatsheet for those coming from Kotlin Flow