module Travis::CLI::Parser

def on(*args, &block)
  block ||= begin
    full_arg = args.detect { |a| a.start_with? '--' }
    name = full_arg.gsub(/^--(\[no-\])?(\S+).*$/, '\2').gsub('-', '_')
    attr_reader(name)               unless method_defined? name
    attr_writer(name)               unless method_defined? "#{name}="
    alias_method("#{name}?", name)  unless method_defined? "#{name}?"
    proc { |instance, value| instance.public_send("#{name}=", value) }
  end
  on_initialize do |instance|
    instance.parser.on(*args) do |value|
      block.call(instance, value)
    end
  end
end