🅿️Hashable

code

// can't use tuple as key
let tuple = ("I want to be part of a key", "Me too!")

// ⛔ error: 
//       generic struct 'Dictionary' requires that 
//       '(String, String)' conform to 'Hashable'
let dict = [tuple: "I am a value"]

// ✅ fix (automatically synthesized Hashable)
struct Pair<T: Hashable, U: Hashable>: Hashable {
    let left: T
    let right: U
    // convenience init
    init(_ left: T, _ right: U){
        self.left = left
        self.right = right
    }
}

let pair = Pair("Joe", 20)      // ⭐️ 使用 Pair 時，不需特別指定 <T, U>
let dict = [pair: "friend"]

📘 手冊

• Standard Library ⟩ Basic Behaviors ⟩

  • Hashable

  • Hasher - universal hash function used by Set and Dictionary.

🗣 討論

• Developer Forum ⟩ How do I make (Int,Int) Hashable?

  nominal type 是「有名字的型別」(typealias 不算 nominal type), non-nominal type 是「沒有名字的型別」，如：(Int, Int) tuple。tuples 目前看起來還不會「自動」變成 Hashable。

• In Swift can I use a tuple as the key in a dictionary? - Ans: NO

• Swift Forum ⟩ Tuples Conform to Equatable, Comparable, and Hashable - Pitch

👥 相關

• Generics

• Hashing: concept behind Hashable.

Automatically Synthesized Hashable

• Declare Hashable conformance in the type’s original declaration.

• For a struct, all its stored properties must conform to Hashable.

• For an enum, all its associated values must conform to Hashable.

• An enum without associated values has Hashable conformance even without the declaration.

Implementing Hashable Manually

struct Pair<T: Hashable, U: Hashable>: Hashable {
    // properties ...
    // init ...
    
    // ⭐️ Hashable requirement
    hash(into hasher: inout Hasher){
        hasher.combine(left)
        hasher.combine(right)
    }
    
    // ⭐️ Equatable requirement
    static func == (p1: Pair<T,U>, p2: Pair<T,U>) -> Bool {
        return p1.left == p2.left && p1.right == p2.right
    }
}