andymatuschak

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,

andymatuschak

//
//  NSObject+BlockObservation.h
//  Version 1.0
//
//  Andy Matuschak
//  andy@andymatuschak.org
//  Public domain because I love you. Let me know how you use it.
//

#import <Cocoa/Cocoa.h>

andymatuschak

import mdast from "mdast";
import remarkParse from "remark-parse";
import remarkStringify from "remark-stringify";
import unified from "unified";
import unist from "unist";
import { clozeNodeType, ClozePromptNode } from "./index";

// TODO: don't match clozes inside code and html blocks
const clozeRegexp = /^{(.+?)}/;
export default function clozePlugin(this: unified.Processor) {

andymatuschak

Andy Matuschak Contributor Agreement

This Andy Matuschak Contributor Agreement ("AMCA") applies to any contribution that you make to any product or project managed by us (the "project"), and sets out the intellectual property rights you grant us in the contributed materials. The term "us" shall mean Andy Matuschak. The term "you" shall mean the person or entity identified below. If you agree to be bound by these terms, fill in the information requested and sign the AMCA where indicated below. Read this agreement carefully before signing. These terms and conditions constitute a binding legal agreement.

1. The term "contribution" or "contributed materials" means any source code, object code, patch, tool, sample, graphic, specification, manual, documentation, or any other material posted or submitted by you to the project.

2. With respect to any worldwide copyrights, or copyright applications and registrations, in your contribution:

• you hereby assign to us joint ownership, and to the extent that such a

andymatuschak

//
//  CollectionViewDataSource.swift
//  Khan Academy
//
//  Created by Andy Matuschak on 10/14/14.
//  Copyright (c) 2014 Khan Academy. All rights reserved.
//

import UIKit

andymatuschak

function useWeakRef<T>(value: T): React.MutableRefObject<T> {
  const ref = useRef(value);
  useEffect(() => {
    ref.current = value;
  }, [value]);
  return ref;
}

function useByrefCallback<Args extends unknown[], Result>(
  callback: (...args: Args) => Result,

andymatuschak

# GDC Vault videos can't be watched on mobile devices and this is a very sad thing indeed!
# (Note: this has changed for GDC2013, which lets you watch raw MP4 streams. Kudos!)
# This script is designed to circumvent this by downloading the lecture and slideshow
# videos which can then be re-encoded into whatever format you wish. Obviously, you
# won't be able to do this without access to the Vault. This is strictly for the
# convenience of legitimate Vault users!

# Note: this code is rather flimsy and was written as fast as possible for my own personal use.
# The code only works for the most recent GDC Vault videos, since they all use the same player
# format. If the XML format used to run the player is changed (as it  has in the past), the code

andymatuschak

//
//  MultiDirectionAdjudicatingScrollView.swift
//  Khan Academy
//
//  Created by Andy Matuschak on 12/16/14.
//  Copyright (c) 2014 Khan Academy. All rights reserved.
//

import UIKit
import UIKit.UIGestureRecognizerSubclass

andymatuschak

struct User {
	let id: Int
	let name: String
	let email: String?
}

extension User: JSONDecodable {
	static func create(id: Int, name: String, email: String?) -> User {
		return User(id: id, name: name, email: email)
	}

andymatuschak

Day 0. Due:	100∑	[21,	79,	0,	0,	0,	0,	0,	0]		Reviewed: 100		States: [13,	18,	69,	0,	0,	0,	0,	0] (average 1.56)
Day 1. Due:	31∑	[13,	18,	0,	0,	0,	0,	0,	0]		Reviewed: 31		States: [5,	10,	85,	0,	0,	0,	0,	0] (average 1.8)
Day 2. Due:	15∑	[5,	10,	0,	0,	0,	0,	0,	0]		Reviewed: 15		States: [0,	5,	95,	0,	0,	0,	0,	0] (average 1.95)
Day 3. Due:	74∑	[0,	5,	69,	0,	0,	0,	0,	0]		Reviewed: 74		States: [0,	19,	31,	50,	0,	0,	0,	0] (average 2.31)
Day 4. Due:	48∑	[0,	19,	29,	0,	0,	0,	0,	0]		Reviewed: 48		States: [4,	9,	17,	70,	0,	0,	0,	0] (average 2.53)
Day 5. Due:	15∑	[4,	9,	2,	0,	0,	0,	0,	0]		Reviewed: 15		States: [4,	3,	21,	72,	0,	0,	0,	0] (average 2.61)
Day 6. Due:	7∑	[4,	3,	0,	0,	0,	0,	0,	0]		Reviewed: 0		States: [4,	3,	21,	72,	0,	0,	0,	0] (average 2.61)
Day 7. Due:	22∑	[4,	3,	15,	0,	0,	0,	0,	0]		Reviewed: 22		States: [1,	7,	8,	84,	0,	0,	0,	0] (average 2.75)
Day 8. Due:	16∑	[1,	7,	8,	0,	0,	0,	0,	0]		Reviewed: 16		States: [2,	2,	6,	90,	0,	0,	0,	0] (average 2.84)
Day 9. Due:	8∑	[2,	2,	4,	0,	0,	0,	0,	0]		Reviewed: 0		States: [