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OptionParser is a command-line argument parsing library for Ruby that provides several really nice features. Using OptionParser tends to be verbose, but it is also very flexible. One of it’s features that I really like is argument casting.

Argument casting allows you to validate a command-line option and convert it from the user-supplied String into whichever object you like. The ri for OptionParser has an example similar to this one for casting a floating-point argument into a Float value:

require 'optparse'

options = {}

opts = OptionParser.new do |opts|
  # Cast 'delay' argument to a Float.
  opts.on("--delay N", Float,
          "Delay N seconds before executing") do |n|
    options[:delay] = n
  end
end

opts.parse! ARGV

p options

The second argument to opts.on, Float, tells OptionParser to cast the option’s value to a Float before passing it to the handler block. When you run this example with a number as the argument, you’ll see the value in options is a Float:

<samp>$ ruby op_test.rb --delay 1
{:delay=>1.0}</samp>

If you pass a value that can’t be cast into a Float an OptionParser::InvalidArgument is raised:

<samp>$ ruby op_test.rb --delay X
[...]/optparse.rb:454:in `parse': invalid argument: --delay X (OptionParser::InvalidArgument)
    [...]
    from [...]/optparse.rb:1353:in `parse!'
    from op_test.rb:13</samp>

You can rescue this and provide an appropriate help message.

At the bottom you’ll find some tables of the various casts that are built-in to OptionParser. If none of those do what you want, writing your own is very easy.

In RubyGems, various arguments are automatically cast to the appropriate objects. For example, when you specify a version with `gem install—version ’= 1.2.3’`, the argument ’= 1.2.3’ is turned into a Gem::Requirement:

OptionParser.accept Gem::Requirement do |value|
  Gem::Requirement.new value
end

Gem::Requirement.new knows how to cast a String and raises an exception if it can’t, so we delegate to it to do the work.

If you only want to have a specially formatted string, you can provide a regular expression instead. The DecimalInteger cast is defined like this:

DecimalInteger = /\A[-+]?\d+(?:_\d+)*/io
accept(DecimalInteger) {|s,| s.to_i if s}

So the pattern referenced by the name is used to validate the argument.

You can also provide a pattern as the second argument. The Float cast is defined like this:

floatpat = %r"\A[-+]?[...]"io
accept(Float, floatpat) {|s,| s.to_f if s}

Notice that for each of these, you still need to turn the string argument into the appropriate object.

OptionParser Built-in Casts

With no extra requires, OptionParser can cast the following arguments for you:

If you require ‘optparse/date’:

If you require ‘optparse/shellwords’:

If you require ‘optparse/time’:

If you require ‘optparse/uri’: