Playing around with KeyPaths in Swift

KeyPathDemo.swift

struct Paragraph {
    var words: String
}

struct Chapter {
    var title: String
    var content: [Paragraph]
}

struct Book {
    var title: String
    var author: String
    var preface: Chapter
    var chapters: [Chapter]
}

var introduction = Chapter(title: "Introduction",
                           content: [Paragraph(words: "Millions and billions...")])
var chapter1 = Chapter(title: "The Shores of the Cosmic Ocean",
                       content: [Paragraph(words: "paragraph 1"),
                                 Paragraph(words: "paragraph 2"),
                                 Paragraph(words: "paragraph 3")])
var chapter2 = Chapter(title: "One Voice in the Cosmic Fugue",
                       content: [Paragraph(words: "paragraph 1"),
                                 Paragraph(words: "paragraph 2"),
                                 Paragraph(words: "paragraph 3")])
var chapter3 = Chapter(title: "The Harmony of Worlds",
                       content: [Paragraph(words: "paragraph 1"),
                                 Paragraph(words: "paragraph 2"),
                                 Paragraph(words: "paragraph 3")])
var cosmos = Book(title: "Cosmos",
                  author: "Carl Sagan",
                  preface: introduction,
                  chapters: [chapter1, chapter2, chapter3])

cosmos[keyPath: \Book.author]
cosmos[keyPath: \.author]
cosmos[keyPath: \.preface.title]
cosmos[keyPath: \.chapters[1].title]

let path = \Book.author

cosmos[keyPath: path] = "Ray"

//func prefaceTitle(_ book: Book) -> String {
//    book.preface.title
//}

//var prefaceTitle: (Book) -> String = { book in book.preface.title }


dump(cosmos)

let interestingNumbers = ["prime": [2, 3, 5, 7, 11, 13, 17],
                          "triangular": [1, 3, 6, 10, 15, 21, 28],
                          "hexagonal": [1, 6, 15, 28, 45, 66, 91]]


interestingNumbers[keyPath: \[String: [Int]].["prime"]!.count]
interestingNumbers[keyPath: \.self["prime"]![0]]

var compond = (a: 1,b: 2)
compond[keyPath: \.self.a] = 100
compond