As an example I want to parse a string that can either be 2 dot-separated words or a single word. Each “word” is a combination of alphanumeric and underscore characters, so I write the word-parser as a closure. In either case I want the parser-function to return a pair of words with one of them potentially being an empty string. There might be a better solution for this specific problem, but I simply want to motivate my problem here.