Left-view enumerations of Swift Strings

left-view

protocol Viewl {
    typealias El
    func uncons() -> (El, Self)?
}

func head<X:Viewl>(x: X) -> X.El? { return x.uncons()?.0 }
func tail<X:Viewl>(x: X) -> X?    { return x.uncons()?.1 }

extension String:Viewl {
    typealias El = Character
    func uncons() -> (Character, String)? {
        return isEmpty ? nil : (self[startIndex], dropFirst(self))
    }
}

extension Array:Viewl {
    typealias El = Element
    func uncons() -> (Array.Element, Array)? {
        return isEmpty ? nil : (self[0], Array(dropFirst(self)))
    }
}