From: Robert Klemme <shortcutter@...>
Date: 2010-02-10T07:15:12+09:00
Subject: Re: OptionParser does not enforce required argument

On 02/09/2010 08:22 PM, Sean Smitz wrote:
> I'm not seeing the difference between specifying a required argument and
> an option argument in OptionParser other than they used '<<' in the
> assignment of the former.

"Mandatory" refers to the argument to an option - not the presence of 
the option as such.

Quoting the docs

         # Mandatory argument.
         opts.on("-r", "--require LIBRARY",
                 "Require the LIBRARY before executing your script") do 
|lib|
           options.library << lib
         end

         # Optional argument; multi-line description.
         opts.on("-i", "--inplace [EXTENSION]",
                 "Edit ARGV files in place",
                 "  (make backup if EXTENSION supplied)") do |ext|
           options.inplace = true
           options.extension = ext || ''
           options.extension.sub!(/\A\.?(?=.)/, ".")  # Ensure extension 
begins with dot.
         end

Option -r needs an argument (the library) while option -i can go 
without.  Example:

irb(main):004:0> OptionParser.new do |o|
irb(main):005:1* o.on '-i NAME' do |x| p x end
irb(main):006:1> end.parse ["-i", "xx"]
"xx"
=> []
irb(main):007:0> OptionParser.new do |o|
irb(main):008:1* o.on '-i NAME' do |x| p x end
irb(main):009:1> end.parse ["-i"]
OptionParser::MissingArgument: missing argument: -i
	from (irb):7
	from /usr/local/bin/irb19:12:in `<main>'
irb(main):010:0> OptionParser.new do |o|
irb(main):011:1* o.on '-i [NAME]' do |x| p x end
irb(main):012:1> end.parse ["-i"]
nil
=> []
irb(main):013:0>

If you want to ensure that an option is present (which is somewhat 
contradictory, there is a reason they are called "options") you can 
assign the argument (or a value provided) to a variable and check that 
variable for nil after option processing.

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/