Experimental code for SwiftPM with Function Builders. Declarative package description for SwiftPM.

SwiftPM-Function-Builders.swift

import Foundation

@_functionBuilder
struct ContentListBuilder {

    static func buildBlock() -> ContentList {
        return EmptyContentList()
    }
    
    static func buildBlock(_ list: ContentList) -> ContentList {
        return list
    }
    
    static func buildBlock(_ lists: ContentList...) -> ContentList {
        return TupleContentList(lists)
    }
}

@_functionBuilder
struct ContentBuilder {
    
    static func buildBlock() -> Content {
        return EmptyContent()
    }
    
    static func buildBlock(_ content: Content) -> Content {
        return content
    }
    
    static func buildBlock(_ contents: Content...) -> Content {
        return TupleContent(contents)
    }
}

protocol Content: Encodable {}

extension Content {
    func getAll<T>() -> [T] {
        switch self {
        case let tuple as TupleContent:
            return (tuple.values as? [T]) ?? []
        case let content as T:
            return [content]
        default:
            return []
        }
    }
}

protocol ContentList: Content {
    var content: Content { get }
}

extension ContentList {
    var all: [ContentList] {
        switch self {
        case let tuple as TupleContentList:
            return tuple.values
        case let list as ContentList:
            return [list]
        default:
            return []
        }
    }
}

struct EmptyContent: Content {}

struct EmptyContentList: ContentList {
    var content: Content { EmptyContent() }
}

protocol Tuple {
    associatedtype Body
    
    var values: [Body] { get }
    
    init(_ values: [Body])
}

struct TupleContent: Tuple, Content {
    let values: [Content]
    
    init(_ values: [Content]) {
        self.values = values
    }
}

extension TupleContent {
    func encode(to encoder: Encoder) throws {}
}

struct TupleContentList: Tuple, ContentList {
    var content: Content {
        return self
    }
    
    let values: [ContentList]

    init(_ values: [ContentList]) {
        self.values = values
    }
}

extension TupleContentList {
    func encode(to encoder: Encoder) throws {}
}

struct Package: Content {
    enum CodingKeys: CodingKey {
        case name
        case products
        case dependencies
        case targets
    }
    
    let name: String
    let contentList: ContentList
    
    init(name: String, @ContentListBuilder builder: () -> ContentList) {
        self.name = name
        self.contentList = builder()
    }
}

extension Package {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(name, forKey: .name)
        let lists = contentList.all
        for list in lists {
            switch list {
            case let productList as ProductList:
                try container.encode(productList, forKey: .products)
            case let dependencyList as DependencyList:
                try container.encode(dependencyList, forKey: .dependencies)
            case let targetList as TargetList:
                try container.encode(targetList, forKey: .targets)
            default:
                break
            }
        }
    }
}

struct ProductList: ContentList {
    let content: Content
    
    init(@ContentBuilder builder: () -> Content) {
        self.content = builder()
    }
}

extension ProductList {
    func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        let products: [Product] = content.getAll()
        try container.encode(contentsOf: products)
    }
}

struct Product: Content {
    enum CodingKeys: CodingKey {
        case name
        case type
        case targets
    }
    
    enum `Type`: String, Codable {
        case executable, `static`, dynamic
    }
    
    let name: String
    let type: Type
    let targetList: ContentList
    
    init(name: String, type: Type = .executable, @ContentListBuilder builder: () -> ContentList = { EmptyContentList() }) {
        self.name = name
        self.type = type
        self.targetList = builder()
    }
}

extension Product {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(name, forKey: .name)
        try container.encode(type, forKey: .type)
        try container.encodeIfPresent(targetList as? TargetList, forKey: .targets)
    }
}

struct TargetList: ContentList {
    let content: Content
    
    init(@ContentBuilder builder: () -> Content) {
        self.content = builder()
    }
}

extension TargetList {
    func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        let targets: [Target] = content.getAll()
        try container.encode(contentsOf: targets)
    }
}

struct Target: Content {
    enum CodingKeys: CodingKey {
        case name
        case dependencies
    }
    
    let name: String
    let dependencyList: ContentList
    
    init(name: String, @ContentListBuilder builder: () -> ContentList = { EmptyContentList() }) {
        self.name = name
        self.dependencyList = builder()
    }
}

extension Target {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(name, forKey: .name)
        try container.encodeIfPresent(dependencyList as? TargetDependencyList, forKey: .dependencies)
    }
}

struct TargetDependencyList: ContentList {
    let content: Content
    
    init(@ContentBuilder builder: () -> Content) {
        self.content = builder()
    }
}

extension TargetDependencyList {
    func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        let dependencies: [Content] = content.getAll()
        for dependency in dependencies {
            switch dependency {
            case let value as TargetDependency:
                try container.encode(value)
            case let value as Target:
                try container.encode(value)
             case let value as Product:
                try container.encode(value)
            default:
                break
            }
        }
    }
}

struct TargetDependency: Content {
    let name: String
}

struct DependencyList: ContentList {
    let content: Content
    
    init(@ContentBuilder builder: () -> Content = { EmptyContent() }) {
        self.content = builder()
    }
}

extension DependencyList {
    func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        let dependencies: [Dependency] = content.getAll()
        try container.encode(contentsOf: dependencies)
    }
}

struct Dependency: Content {
    enum CodingKeys: CodingKey {
        case url
        case from
        case exact
    }
    
    var url: String?
    var from: String?
    var exact: String?
    
    init(url: String) {
        self.url = url
    }
    
    init(url: String, from: String) {
        self.url = url
        self.from = from
    }
    
    init(url: String, exact: String) {
        self.url = url
        self.exact = exact
    }
}

extension Dependency {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encodeIfPresent(url, forKey: .url)
        try container.encodeIfPresent(from, forKey: .from)
        try container.encodeIfPresent(exact, forKey: .exact)
    }
}