Using Swift’s Type System To Model Behaviour

Create fully immutable types for your domain using Swift

Photo by Luke Chesser on Unsplash

I want to demonstrate how we can use Swift’s type system to efficiently and effectively model and implement behavior. The emerging code is easy to read, as they resemble spoken English to a high degree. And the fact that each module exposes one method makes them simple to test and integrate. The code can be fully immutable — eliminating many sources for possible bugs. If it can’t change on purpose, it can’t change accidentally.

Let’s imagine a to-do list app.

A to-do task might contain a title, a longer description, or a creation date. It might be finished or unfinished. Conventionally, this would be modelled with mutable properties.

But we can create fully immutable types for our domain using Swift's sophisticated type system.

struct TodoTask {
    enum Change {
        case title(to:String)
        case description(to:String)
        case   finish
        case unfinish
    }
    enum State {
        case unfinished
        case finished
    }
    // members
    let id         : UUID
    let title      : String
    let description: String
    let state      : State
    let created    : Date
    // initialisers
    init(title:String) {…