Mark Sands marksands

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,

require-from-twitter

Since Twitter doesn't have an edit button, it's a suitable host for JavaScript modules.

How to use

Source tweet: https://twitter.com/rauchg/status/712799807073419264

const leftPad = await requireFromTwitter('712799807073419264');

	NSDate *date = [NSDate new];

	NSDateFormatter *dateFormatter = [NSDateFormatter new];
	dateFormatter.locale = [NSLocale localeWithLocaleIdentifier:@"nl_NL"];
	dateFormatter.dateStyle = NSDateFormatterFullStyle;
	dateFormatter.formattingContext = NSFormattingContextDynamic; // this is the important setting

	NSString *dateString = [dateFormatter stringFromDate:date];

	NSString *s1 = [NSString stringWithFormat:@"Foo %@", dateString]; // "Foo dinsdag 13 december 2016"

	import Foundation

	final class SafeSyncQueue {

	struct QueueIdentity {
	let label: String
	}

	let queue: DispatchQueue

	// Just spending Saturday afternoon rewriting FP examples from Backus in Swift.
	// http://worrydream.com/refs/Backus-CanProgrammingBeLiberated.pdf
	//
	// Surprising fact: reasonably possible with only one major problem (tuples are not collections)
	// Unsurprising fact: you shouldn't write Swift this way
	// Other unsurprising fact: Rob should probably get out more
	// Don't judge me

	// For future research: is it possible to build this without arrayFormHack and the arity 2 ∘?
	// Basically, can we do this entirely with tuples or entirely with arrays? Currently almost everything

	// 1. Implement URLSession:task:didReceiveChallenge:completionHandler:. (Be sure to call the completion handler with NSURLSessionAuthChallengeDisposition.PerformDefaultHandling and a nil NSURLCredential.)
	// 2. Set a breakpoint in the callback + trigger an authentication challenge at runtime
	// 3. In the debugger, inspect the callback's `challenge.protectionSpace` argument for the values to put in the following lines of code...
	// 4. Put the following lines of code somewhere before your network requests (i.e. AppDelegate area, or other)

	let basicAuthCredentials = NSURLCredential(user: "username", password: "password", persistence: .Permanent)
	let foobarHttpsProtectionSpace = NSURLProtectionSpace(host: "foo.bar.com", port: 443, protocol: "https", realm: "This Probably Has A Value, Get It From The Delegate Callback", authenticationMethod: NSURLAuthenticationMethodHTTPBasic)
	NSURLCredentialStorage.sharedCredentialStorage().setDefaultCredential(basicAuthCredentials, forProtectionSpace: apidevHttpsProtectionSpace

	final class SampleViewController: StoryboardBackedViewController {
	// Unfortunately this must be an IUO var, so that we can set the value after super.init
	private var member: Foo!

	// Proper dependency injection in a storyboard backed VC!
	init(foo: Foo) {
	super.init(storyboardIdentifier: "SampleViewControllerIdentifier")

	// We have to set the members after calling super.init, since it changes the instance of `self`.
	self.member = foo

	//
	// ObservableType+Weak.swift
	//
	// Created by Ian Keen on 6/04/2016.
	// Copyright © 2016 Ian Keen. All rights reserved.
	//

	import Foundation
	import RxSwift

	// https://gist.github.com/JARinteractive/7fb33b6b0043f365ddfd
	import Foundation
	import XCTest

	@discardableResult
	public func AssertEventuallyTrue(file: StaticString = #file, line: UInt = #line, _ checkSuccess: @autoclosure () -> Bool) -> Bool {
	return AssertEventuallyTrue(10.0, file: file, line: line, checkSuccess)
	}

	@discardableResult

	final class CenteringView: UIView {
	// MARK: - Initialization

	init(contentView: UIView) {
	super.init(frame: .zero)

	addSubview(contentView)
	contentView.translatesAutoresizingMaskIntoConstraints = false

	NSLayoutConstraint.activateConstraints([