#66: where Clauses in Swift Generics

A where clause lets you describe extra generic constraints in a readable way.

func commonElements<S1: Sequence, S2: Sequence>(_ a: S1, _ b: S2) -> [S1.Element]
where S1.Element == S2.Element, S1.Element: Hashable {
    let setB = Set(b)
    return a.filter { setB.contains($0) }
}

Without where, signatures like this become much harder to parse.

It is one of the best tools for keeping generic code powerful without making it unreadable.