Switch that can omit argument.

Methods
P
Instance Public methods
parse(arg, argv, &error)

Parses argument if given, or uses default value.

Source: show | on GitHub 

# File lib/optparse.rb, line 511
def parse(arg, argv, &error)
  if arg
    conv_arg(*parse_arg(arg, &error))
  else
    conv_arg(arg)
  end
end