Hashable

Built-in trait · Adopted with with Hashable, which includes Equatable

A dictionary or a set finds a value quickly by a number computed from it, its hash. Values that are equal must have the same hash, or a set could hold two “equal” values. A struct that chooses its own equality with Equatable must therefore choose a matching hash too, by adopting Hashable, before it can be a key or a set member.

struct Name with Hashable {
    const text: String

    @override
    func equals(other: Name): Bool {
        return self.text.lower() == other.text.lower()
    }

    @override
    func hash(): Int {
        return self.text.lower().count
    }
}

const names: Set[Name] = [Name("Ada"), Name("ADA")]
print(names.count)   # prints 1

hash(): Int

A number for the value. Two values that are equal by equals must give the same number; two that are not equal may give the same number, but fewer such clashes make a set or a dictionary faster.

Hashable includes Equatable, so a type that adopts Hashable writes both methods. A struct that adopts only Equatable cannot be a key, and Emerald says why.

A struct that keeps the usual field-by-field equality needs neither trait: it can already be a key, as long as its fields can. An object of a class can never be a key, even with Hashable, since its fields could change while it is stored.