A Swift Tour: Explore Swift’s features and design

var x: Int = 1
var y: Int = x
x = 42
y

struct User {
    let username: String
    var isVisible: Bool = true
    var friends: [String] = []
}

var alice = User(username: "alice")
alice.friends = ["charlie"]

var bruno = User(username: "bruno")
bruno.friends = alice.friends

alice.friends.append("dash")
bruno.friends

struct User {
    let username: String
    var isVisible: Bool = true
    var friends: [String] = []

    mutating func addFriend(username: String) throws {
        guard username != self.username else {
            throw SocialError.befriendingSelf
        }
        guard !friends.contains(username) else {
            throw SocialError.duplicateFriend(username: username)
        }
        friends.append(username)
    }
}

enum SocialError: Error {
    case befriendingSelf
    case duplicateFriend(username: String)
}

var alice = User(username: "alice")
do {
    try alice.addFriend(username: "charlie")
    try alice.addFriend(username: "charlie")
} catch {
    error
}

var allUsers = [
    "alice": alice
]

func findUser(_ username: String) -> User? {
    allUsers[username]
}

if let charlie = findUser("charlie") {
    print("Found \(charlie)")
} else {
    print("charlie not found")
}

let dash = findUser("dash")!

// swift-tools-version: 6.0

import PackageDescription

let package = Package(
    name: "SocialGraph",
    products: [
        .library(
            name: "SocialGraph",
            targets: ["SocialGraph"]),
    ],
    dependencies: [
      .package(url: "https://github.com/apple/swift-testing.git", branch: "main"),
    ],
    targets: [
        .target(
            name: "SocialGraph"),
        .testTarget(
            name: "SocialGraphTests",
            dependencies: [
                "SocialGraph",
                .product(name: "Testing", package: "swift-testing"),
            ]),
    ]
)

/// Represents a user in the social graph.
public struct User: Equatable, Hashable {
    /// The user's username, which must be unique in the service.
    public let username: String

    /// Whether or not the user should be considered visible
    /// when performing queries.
    public var isVisible: Bool

    /// The usernames of the user's friends.
    public private(set) var friends: [String]

    public init(
        username: String,
        isVisible: Bool = true,
        friends: [String] = []
    ) {
        self.username = username
        self.isVisible = isVisible
        self.friends = friends
    }

    /// Adds a username to the user's list of friends. Throws an
    /// error if the username cannot be added.
    public mutating func addFriend(username: String) throws {
        guard username != self.username else {
            throw SocialError.befriendingSelf
        }
        guard !friends.contains(username) else {
            throw SocialError.alreadyFriend(username: username)
        }
        friends.append(username)
    }
}

class Pet {
    func speak() {}
}

class Cat: Pet {
    override func speak() { print("meow") }

    func purr() { print("purr") }
}

let pet: Pet = Cat()
pet.speak()

if let cat = pet as? Cat {
    cat.purr()
}

class Pet {
    var toy: Toy?
}

class Toy {}

let toy = Toy()
let pets = [Pet()]

// Give toy to pets
for pet in pets {
    pet.toy = toy
}

// Take toy from pets
for pet in pets {
    pet.toy = nil
}

class Pet {
    weak var owner: Owner?
}

class Owner {
    var pets: [Pet]
}

protocol StringIdentifiable {
    var identifier: String { get }
}

extension User: StringIdentifiable {
    var identifier: String {
        username
    }
}

let string = "🥚🐣🐥🐓"
for char in string {
    print(char)
}
// => "🥚" "🐣" "🐥" "🐓"

print(string[string.startIndex])
// => "🥚"

let numbers = [1, 4, 7, 10, 13]

let numberStrings = numbers.map { number in
    String(number)
}
// => ["1", "4", "7", "10", "13"]

let primeNumbers = numbers.filter { number in
    number.isPrime
}
// => [1, 7, 13]

let sum = numbers.reduce(0) { partial, number in
    partial + number
}
// => 35

let numbers = [1, 4, 7, 10, 13]

let numberStrings = numbers.map { String($0) }
// => ["1", "4", "7", "10", "13"]



let primeNumbers = numbers.filter { $0.isPrime }
// => [1, 7, 13]



let sum = numbers.reduce(0) { $0 + $1 }
// => 35

/// An in-memory store for users of the service.
public class UserStore {
    var allUsers: [String: User] = [:]
}

extension UserStore {
    /// If the username maps to a User and that user is visible,
    /// returns the User. Returns nil otherwise.
    public func lookUpUser(_ username: String) -> User? {
        guard let user = allUsers[username],
              user.isVisible else {
            return nil
        }
        return user
    }

    /// If the username maps to a User and that user is visible,
    /// returns the User. Otherwise, throws an error.
    public func user(for username: String) throws -> User {
        guard let user = lookUpUser(username) else {
            throw SocialError.userNotFound(username: username)
        }
        return user
    }

    public func friendsOfFriends(_ username: String) throws -> [String] {
        let user = try user(for: username)
        let excluded = Set(user.friends + [username])
        return user.friends
            .compactMap { lookUpUser($0) }      // [String] -> [User]
            .flatMap { $0.friends }             // [User] -> [String]
            .filter { !excluded.contains($0) }  // drop excluded
            .uniqued()
    }
}

extension Collection where Element: Hashable {
    func uniqued() -> [Element] {
        let unique = Set(self)
        return Array(unique)
    }
}

/// Makes a network request to download an image.
func fetchUserAvatar(for username: String) async -> Image {
    // ...
}

let avatar = await fetchUserAvatar(for: "alice")

import Hummingbird
import SocialGraph

let router = Router()

extension UserStore {
    static let shared = UserStore.makeSampleStore()
}

let app = Application(
    router: router,
    configuration: .init(address: .hostname("127.0.0.1", port: 8080))
)

print("Starting server...")
try await app.runService()

// Look up user
let user = allUsers[username]

// Store new user
allUsers[username] = user

// UserStore
var allUsers: [String: User]

import Hummingbird
import SocialGraph

let router = Router()

extension UserStore {
    static let shared = UserStore.makeSampleStore()
}

router.get("friendsOfFriends") { request, context -> [String] in
    let username = try request.queryArgument(for: "username")
    return try await UserStore.shared.friendsOfFriends(username)
}

let app = Application(
    router: router,
    configuration: .init(address: .hostname("127.0.0.1", port: 8080))
)

print("Starting server...")
try await app.runService()

struct FriendsOfFriends: AsyncParsableCommand {
    @Argument var username: String

    mutating func run() async throws {
        // ...
    }
}

import ArgumentParser
import SocialGraph

@main
struct SocialGraphClient: AsyncParsableCommand {
    static let configuration = CommandConfiguration(
        abstract: "A utility for querying the social graph",
        subcommands: [
            FriendsOfFriends.self,
        ])
}

struct FriendsOfFriends: AsyncParsableCommand {
    @Argument(help: "The username to look up friends of friends for")
    var username: String

    func run() async throws {
        var request = Request(command: "friendsOfFriends", returning: [String].self)
        request.arguments = ["username" : username]
        let result = try await request.get()
        print(result)
    }
}

import RegexBuilder

let dollarValueRegex = Regex {  // Equivalent to "\$[0-9]+\.[0-9][0-9]"
    "$"
    OneOrMore(.digit)
    "."
    Repeat(.digit, count: 2)
}