An example of `Ruby::Box` isolating globals (due to be released on Christmas)

options.rb

class Options < Ruby::Box
  def [](name) = instance_eval { eval "$#{name.to_s.delete_prefix('$')}" }
end

options = Options.new
options.instance_eval do
  require 'optparse'

  class Option
    def []=(key, value)
      key = key.to_s.tr('-', '_')
      raise ArgumentError, "Bad key: #{key}" unless key.match?(/\A\w+\z/)

      eval "$#{key} = value"
    end
  end

  OptionParser.new { it.on '--warn' }.parse!(into: Option.new)
end

warn 'oops' if options[:warn]