A type-erased hashable value.

public struct AnyHashable {

public init<H>(_ base: H) where H : Hashable

public var base: Any { get }

public static func == (lhs: AnyHashable, rhs: AnyHashable) -> Bool

}

let descriptions: [AnyHashable: Any] = [

AnyHashable("??"): "emoji",

AnyHashable(42): "an Int",

AnyHashable(Int8(43)): "an Int8",

AnyHashable(Set(["a", "b"])): "a set of strings"

]

public struct Dictionary<Key, Value> where Key : Hashable

Key通常要指定一個確定的類型，如果類型不統一，向上抽象；向上無法解決問題，使用AnyHashable

AnyHashable類型擦除的原因：set和dictory需要指定一個確定的類型