RecursiveSubSequence is a protocol I've found useful.

RecursiveSubSequence.swift

protocol RecursiveSubSequence: Sequence where SubSequence: RecursiveSubSequence { }
extension String: RecursiveSubSequence { }
extension Substring: RecursiveSubSequence { }
extension Array: RecursiveSubSequence {}
extension ArraySlice: RecursiveSubSequence {}


func process<T>(_ value: T) -> T { return value }

func count<S: RecursiveSubSequence>(_ s: S)  -> Int {
    guard let head = (s.first { _ in true }) else { return 0 }
    process(head) // supress compiler warning
    let tail = s.dropFirst()
    return count(tail) + 1
}

count("aa") // -> 2

count([1,2,3]) // -> 3