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You can use the unbounded range operator (...) to create a slice of a collection that contains all of the collection's elements. Slicing with an unbounded range is essentially a conversion of a collection instance into its slice type.

For example, the following code declares countLetterChanges(_:_:), a function that finds the number of changes required to change one word or phrase into another. The function uses a recursive approach to perform the same comparisons on smaller and smaller pieces of the original strings. In order to use recursion without making copies of the strings at each step, countLetterChanges(_:_:) uses Substring, a string's slice type, for its parameters.

func countLetterChanges(_ s1: Substring, _ s2: Substring) -> Int {
    if s1.isEmpty { return s2.count }
    if s2.isEmpty { return s1.count }

    let cost = s1.first == s2.first ? 0 : 1

    return min(
        countLetterChanges(s1.dropFirst(), s2) + 1,
        countLetterChanges(s1, s2.dropFirst()) + 1,
        countLetterChanges(s1.dropFirst(), s2.dropFirst()) + cost)
}

To call countLetterChanges(_:_:) with two strings, use an unbounded range in each string's subscript.

let word1 = "grizzly"
let word2 = "grisly"
let changes = countLetterChanges(word1[...], word2[...])
// changes == 2