keehun

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,

keehun

import fcntl
import os

path = '/Applications/Safari.app/'

# MacOS-only fcntl command defined in xnu/bsd/sys/fcntl.h
F_GETPATH_NOFIRMLINK = 102

# The documented max length of an fcntl buffer in python documentation.
FCNTL_MAX_LENGTH = 1024

keehun

from time import sleep
from datetime import datetime, time

def date_diff(date1, date2):
  timedelta = date2 - date1
  return timedelta.days * 24 * 3600 + timedelta.seconds

def min_sec(seconds):
    minutes, seconds = divmod(seconds, 60)
    return minutes, seconds

keehun

import Foundation

let url = FileManager.default.temporaryDirectory.appendingPathComponent("test.plist")

func crashes() throws {
    try NSDictionary().write(to: url)
    let returned = try NSMutableDictionary(contentsOf: url, error: ())

    /// This will crash
    returned.setValue("Hello", forKey: "World")

keehun

func getMoviePackage(for movieID: String, completion: MoviePackageFetchCompletion? = nil) {
  var movieURL: URL?
  var subtitleURL: URL?

  var movieFile: File?
  var subtitleFile: File?

  /// The `DispatchGroup` for the parallel URL fetch operations
  let urlFetchGroup = DispatchGroup()

keehun

async function getMoviePackageFor(movie_id) {

    /// Get the URLs of the movie and subtitle file in parallel
    const movie_url = await MovieDatabase.getURLFor(movie_id)
    const subtitle_url = await SubtitleDatabase.getURLFor(movie_id)
    
    /*
    By the way, if you want those two async actions to run in
    parallel, you can do this:
    

keehun

import XCTest
@testable import AttributedDynamicLabelDemo

class AttributedDynamicLabelDemoTests: XCTestCase {

  var subject: AttributedStringView!

  override func setUp() {
    super.setUp()
    subject = AttributedStringView()

keehun

// MARK: Event Handlers

/// Update the sizes of all contained fonts whenever the Dynamic Type size changes.
@objc func uiContentSizeCategoryChanged() {

  /// Only continue if the `UILabel` has non-nil attributed text.
  guard let attributedString = display.attributedText else {
    return
  }

keehun

/// `FontSetterAttributeKey` is used to keep track of the custom Font assigned to a range within
/// an `AttributedString`.
static let FontSetterAttributeKey = NSAttributedString.Key.init("FontSetterAttributeKey")

/// A `UILabel` that will render the attributed string with Dynamic Type.
lazy var display: UILabel = {
  let fontSetter1: FontSetter = { UIFont.customFont(weight: .bold, points: 18.0) }

  let string1 = NSAttributedString(
    string: "Jived fox nymph grabs quick waltz. ",

keehun

@objc func uiContentSizeCategoryChanged() {
  
  /// Only continue if the `UILabel` has non-nil attributed text.
  guard let attributedString = display.attributedText else {
      return
  }

  /// Get the NSRange of the plaintext string.
  let fullTextRange = NSRange(location: 0, length: attributedString.string.count)