Argument Parser for Swift

Specifying the interface of a CLI command is a good example of a natural place for declarative programming. You just want to write WHAT the interface is, not HOW to do the parsing.

When I made a homegrown Lisp implementation, eventually being able to put a single macro "(usage ...)" in the standard library that, when utilised in a script, would invisibly generate the parsing and variable binding machinery, was one of the more rewarding parts.

This has a pretty minimal amount of code compared to many alternatives I’ve seen.

One minor nitpick; the default help output is a bit unlike most tools. For example, I’ve never seen a tool that says “OVERVIEW:”, they simply lead with a summary line; and, all-caps “USAGE:” doesn’t read as nicely to me as “Usage:” that almost any other command-line tool would print. (For some reason caps are common in man pages but not screen dumps.)