Learning iOS and Swift. Day 15: Optionals are syntax sugar for Rust-like enums; presenting case study

June 2, 2022

Abstract

Brief progress update on issues from the book. I present the mobile app I want to work on henceforth.

Optionals are Rust-like enums under the hood

Swift’s syntax made me believe that optionals are implemented as some special form rather than a two-case enum like Option<T> in Rust. I was wrong. Swift’s optionals are implemented in exactly the same way, and the commonly used syntax (literal value for .some(Wrapped) or nil for .none) is just syntax sugar. To test it, I wrote this snippet using Rust-like optional syntax:

// I typed this as Array<Optional<String>>,
// but swift-format converted it to this shorthand
let array: [String?] = [.some("Hello world!"), .none]

for elem in array {
  switch elem {
  case let .some(str):
    print("String found: \(str)")
  case .none:
    print("No string here!")
  }
}

And, lo and behold, it works exactly as expected:

$ swift optionals.swift
String found: Hello world!
No string here!

Progress update

Today I followed chapter 9 of SwiftUI Apprentice. The chapter dealt with persistent storage, saving current application data to property lists, and restoring data from property lists to state on boot. I also learned how to display alerts in SwiftUI, and even that is declarative.

TIL about exception handling

  • exceptions are enums
  • functions that may throw must be marked with throw
  • try? works like .ok() in Rust, i. e. converts a result to an optional
  • as? casts a value to another type; the result is optional

Case study

As my study project, I intend to develop an application which will basically recreate iMala - meditation tracker. This application is used to track a user’s progress in their Four Preliminary Practices, which are a set of four meditation practices used by practicioners of Diamond Way Buddhism. However, the app currently only supports iPhone and iPad, with no integration with Apple Watch. I could contact the developer and ask him for the source code, but the app is so old that it’s definitely written in UIKit, and possibly even in Objective-C. I will be working on this research project and the book about 50/50.

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