bin_doubler.rb

#!/usr/bin/env ruby

def usage(msg = nil)
  puts "ERROR: #{msg}" if msg
  puts <<EOF
USAGE:
  doubler.rb num
EOF
  exit 1
end

require 'doubler'

num = ARGV.shift || usage("No target provided")

puts "#{num} doubled: #{Doubler.double(num)}"

lib_doubler.rb

module Doubler
  def self.double(num)
    # handle strings, convert to float or integer intelligently
    if num.is_a?(String)
      if num.index '.'
        num = num.to_f
      else
        num = num.to_i
      end
    end
    2 * num
  end
end