operator (~=)

Swift ⟩ Patterns ⟩

⭐️ 注意：在自訂 ~= 運算時，一定要遵循 pattern ~= value 的模式。

func ~=<P, V>(pattern: P, value: V) -> Bool

Swift uses various overloads of the ~= operator to do pattern matching, which also lets us define our own overloads ... 👉 Sundell

⭐️ 重點

Ole 提及如何設計一個 generic operator：

func ~=<T>(pattern: (T) -> Bool, value: T) -> Bool {
    return pattern(value)
}

Sundell 提及如何設計一個 generic pattern：

// Sundell 的做法等於是將 Ole 的做法包裝在一個 struct 裡面，
// 但兩者的精神是一樣的。
struct Pattern<Value> {
    let match: (Value) -> Bool
}

func ~=<T>(pattern: Pattern<T>, value: T) -> Bool {
    pattern.match(value)
}

💈例一

💾 程式：replit

// -----------------------------
//     ⭐ custom ~= operator 
// -----------------------------

// matches a "boolean keypath into a type" (instance.booleanProperty)
// against an "instance of that type"      (instance)
// ⭐ matching pattern: "pattern" ~= "value"
func ~=<T>(keypath: KeyPath<T, Bool>, instance: T) -> Bool {
    // ⭐ \.isNumber ~= Character("3") returns Character("3").isNumber
    return instance[keyPath: keypath]
}

// extension
extension Character {
    func expressYourself() {
        switch self {

            // built-in ~=
            case "<"        : print("\(self) is an Element tag")
            case "#"        : print("\(self) is a Hash tag")

            // ⭐ custom ~= applied here
            case \.isNumber : print("\(self) is a Number")
            case \.isNewline: print("\(self) is a NewLine")

            default         : print("\(self) is something else")
        }
    }
}

// ------------
//     main 
// ------------

let chars: [Character] = [
    "<", "#", "8", "\n", "x"
]

chars.forEach { $0.expressYourself() }
// < is an Element tag
// # is a Hash tag
// 8 is a Number
//
//  is a NewLine
// x is something else

💈例二

💾 程式：replit

// ⭐ `str ~= person` returns `str == person.name`
func ~=(str: String, person: Person) -> Bool {
    return str == person.name
}

// custom type
struct Person {
    let name : String
}

extension Person {
    func areYou(_ name: String) {
        switch name {

            // ⭐ custom ~= applied here
            case self.name: print("Hey it's me!")

            default   : print("Not me")
        }
    }
}

let joe = Person(name: "Joe")
joe.areYou("Joe")               // Hey it's me!

📗 參考

• Ole ⟩ Pattern Matching in Swift (⭐️ 重點)
• Sundell ⟩

  • Using key paths in switch statements ⭐️ (💈例一)
  • Defining custom patterns in Swift - model the "pattern" concept into a generic custom type struct Pattern<Value>. (⭐️ 重點)

👥 相關

• operator - all operators

• enum case pattern

• filter cases

• generics - about generic functions/types.

• Key Path - pattern matching using keypath. (💈例一)
• CaseReflectable - custom protocol for enum case name & associated values.

• switch on other types - switch on custom types.

🗣 討論

• ~= operator in Swift ⭐️ (💈例二)

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