Equatable and Hashable — why? Why must a Dictionary key be Hashable?

Junior PRO

The idea

Equatable provides the == operator — "these two values are the same". Hashable refines Equatable and adds hash(into:) — folding the value into a number via Hasher. Dictionary and Set require Hashable from the key/element because they search by hash rather than by scanning: hash → bucket index, then == to resolve collisions. This gives O(1) on average (worst case, with massive collisions, O(n)).

struct User: Hashable {   // == and hash(into:) synthesized from all fields
    let id: Int
    let name: String
}

struct Tag: Hashable {           // custom semantics: case-insensitive
    let value: String
    static func == (l: Tag, r: Tag) -> Bool {
        l.value.lowercased() == r.value.lowercased()
    }
    func hash(into hasher: inout Hasher) {
        hasher.combine(value.lowercased())   // equal → equal hash
    }
}

let tags: Set<Tag> = [Tag(value: "Swift"), Tag(value: "swift")]
print(tags.count)                 // 1
var ages: [User: Int] = [:]
ages[User(id: 1, name: "Ann")] = 30