How to I get the first or last few characters of a string in a method chain?

Question

If I use functional-style method chains for string manipulation, I can not use the usual machinery for getting the first or last few characters: I do not have access to a reference to the current string, so I can not compute indices.

Example:

[some, nasty, objects]
    .map( { $0.asHex } )
    .joined()
    .<first 100>
    .uppercased()
    + "..."

for a truncated debug output.

So how to I implement <first 100>, or do I have to break the chain?

Answer 1

I don't know of any API that does this. Fortunately, writing our own is an easy exercise:

extension String {
    func taking(first: Int) -> String {
        if first <= 0 {
            return ""
        } else if let to = self.index(self.startIndex, 
                                      offsetBy: first, 
                                      limitedBy: self.endIndex) {
            return self.substring(to: to)
        } else {
            return self
        }
    }
}

Taking from the end is similar.

Find full code (including variants) and tests here.