An example just to demonstrate how the new `Ruby::Box` sandboxes stuff including globals

options.rb

require 'optparse'

class Options < Ruby::Box
  class Into
    class BadKeyError < ArgumentError
      def initialize(key) = super("Bad key: #{key.inspect}")
    end

    def []=(key, value)
      global = "$#{key.to_s.delete_prefix('$').tr('-', '_')}"
      raise BadKeyError, global unless global.match?(/\A\$[[:alpha:]_][[:alnum:]_]*\z/)

      eval("#{global} = value", binding, __FILE__, __LINE__)
    end
  end

  def initialize(argv = ARGV, &block)
    super()
    parser = OptionParser.new
    block&.call(parser)
    parser.parse!(argv, into: Into.new)
  end

  def [](name)
    global = "$#{name.to_s.delete_prefix('$')}"
    Kernel.eval(global, binding, __FILE__, __LINE__)
  end
end

options = Options.new do |parser|
  parser.on '--warn'
end

warn 'oops' if options[