Smart key paths

Another important change introduced by Swift 4 is that of smarter key paths. Swift key paths are strongly typed and enforce a compile time check and remove a common runtime error.

You write a key path by starting with a backslash: `\Book.title`. Every type automatically gets a `[keyPath: …]` subscript to get or set the value at the specified key path:

struct Book {
   var title = ""
   let price : Float
}
let titleKeyPath = \Book.name
let mathsBook = Book(name: "Algebra", price: 10.50)
mathsBook[keyPath: titleKeyPath] 

The value in the earlier mentioned keyPath is "Algebra".

The titleKeyPath object defines a citation to the name property. Then, it can be used as a subscript on that object. You can store and manipulate key paths. For example, you can append additional segments to a key path to drill down further. Key paths are composed of a root, and then you can drill down by following a combination of properties and subscripts.

If you change the variable of mathsBook from let to var, a specific property can also be modified through the keyPath subscript syntax:

mathsBook[keyPath: titleKeyPath] = "Trigonometry"
let newTitle = mathsBook[keyPath: titleKeyPath]

The value in the mentioned keyPath is "Trigonometry".