blog-vapor4-routing-routes-routecontroller3.swift

func getBook(req: Request) throws -> Book {
    guard
        // 1
        let bookIDString = req.parameters.get("bookID"),
        // 2
        let bookID = Int(bookIDString),
        // 3
        let book = Self.books.first(where: { $0.id == bookID })
    else {
        // 4
        throw Abort(.notFound)
    }
    // 5
    return book
}