How to parse an argument without a name with Ruby's optparse

I need to parse a command line like

  script.rb <mandatory filename> [options]

with optparse.

Sure I can write some custom code to handle the filename, then pass ARGV to optparse, but maybe there's a simpler way to do it?

EDIT: there's another hacky way to parse such a command line, and that is pass ['--mandatory-filename'] + ARGV to optparse, then handle the --mandatory-filename option.