.SWIFT File Format

Introduction

A .swift file holds source code in Swift, the compiled language Apple announced in 2014 and released as open source in 2015. It is the main language for iPhone, iPad and Mac apps, and also runs on Linux and Windows.

All the .swift files of a module are compiled together, so one file can use what another declares without importing it. A Package.swift file, itself written in Swift, describes a project to the Swift Package Manager.

Structure

A Swift file is UTF-8 text with no header. It usually opens with import lines, then declarations in any order: struct, class, enum, protocol, extension and func. Constants are declared with let and variables with var. Semicolons are optional, and a line break normally ends a statement. Comments are // to the end of the line and /* */, which can nest.

Statements at the top level of a file are only allowed in a file called main.swift or in a single file run as a script. Otherwise the program starts at a type marked @main.

import Foundation

struct Greeter {
    let name: String

    func greeting() -> String {
        return "Hello, \(name)"
    }
}

@main
struct App {
    static func main() {
        print(Greeter(name: "world").greeting())
    }
}

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