Michael Gikaru thealmikey

A composable pattern for pure state machines with effects

State machines are everywhere in interactive systems, but they're rarely defined clearly and explicitly. Given some big blob of code including implicit state machines, which transitions are possible and under what conditions? What effects take place on what transitions?

There are existing design patterns for state machines, but all the patterns I've seen complect side effects with the structure of the state machine itself. Instances of these patterns are difficult to test without mocking, and they end up with more dependencies. Worse, the classic patterns compose poorly: hierarchical state machines are typically not straightforward extensions. The functional programming world has solutions, but they don't transpose neatly enough to be broadly usable in mainstream languages.

Here I present a composable pattern for pure state machiness with effects,

Applied Functional Programming with Scala - Notes

1. Mastering Functions

A function is a mapping from one set, called a domain, to another set, called the codomain. A function associates every element in the domain with exactly one element in the codomain. In Scala, both domain and codomain are types.

val square : Int => Int = x => x * x

	package chat

	import org.zeromq.ZMQ
	import kotlin.concurrent.thread

	fun main(args: Array<String>): Unit =
	ZMQ.context(1).use { ctx ->
	thread {
	ctx.socket(ZMQ.SUB).use { stream ->
	stream.connect(MESSAGE_STREAM)

	import scalanative.native._
	import SDL._
	import SDLExtra._

	@extern
	@link("SDL2")
	object SDL {
	type Window = CStruct0
	type Renderer = CStruct0

	package so

	import org.neo4j.ogm.annotation.GeneratedValue
	import org.neo4j.ogm.annotation.Id
	import org.neo4j.ogm.annotation.NodeEntity
	import org.neo4j.ogm.annotation.Relationship
	import org.neo4j.ogm.config.Configuration
	import org.neo4j.ogm.session.SessionFactory

	@NodeEntity