The Ultimate Domain Language: Declarative Swift

Exploring declarative domain paradigm

Photo by Solaiman Hossen on Unsplash

I want to introduce you to Declarative Swift — a coding style that allows us to efficiently and effectively create domain-driven code that is easy to verify. Writing code like this has proven to be faster than conventional coding, while it offers less room for bugs to hide in.

Domain Examples

Let’s start with a simple example: a todo list.

The following code block contains a datatype for todo items and todo lists:

struct TodoItem:Identifiable {
    enum Change {
        case title   (to:String)
        case due     (to:TodoDate)
        case location(to:Location)
        case   finish
        case unfinish
    }
    // members
    let id       : UUID
    let title    : String
    let completed: Bool
    let created  : Date
    let due      : TodoDate
    let location : Location

    // initialisers
    init(title:String) { self.init(UUID(),title,false,.unknown, Date(),.unknown) }
    private 
    init(_ i:UUID,_ t:String,_ c:Bool,_ d:TodoDate,_ cd:Date,_ l:Location) { id=i; title=t; completed=c; due=d; created=cd; location=l }
    
    func alter(_ c:Change...) -> Self { c.reduce(self) { $0.alter($1) } }
    private 
    func alter(_ c:Change   ) -> Self {
        switch c {
        case…