gistfile1.rb

# application.rb
class ApplicationDSL
  attr_accessor :applications

  def initialize
    self.applications = []
  end

  def application(name, &block)
    application = Application.new(name)
    applications << application
    application.instance_eval &block
    application
  end

  def self.execute(dsl)
    new.tap do |context|
      context.instance_eval(dsl)
    end
  end
end

class Application
  attr_accessor :name, :roles

  def initialize(name)
    self.name = name
    self.roles = []
  end

  def roles(*roles)
    if roles.empty?
      @roles
    else
      @roles = [] if @roles.nil?
      if roles[0].kind_of? Array
        @roles += roles[0]
      else
        @roles = []
        @roles << roles[0]
      end
      self
    end
  end
end

# provided_sample.rb
application "foo" do
  roles %w{a b c d}
end

# desired_sample.rb
application "foobear" do
  roles %w(polar grizzly black)
end

application "pokercat" do
  roles %w(lion tiger panther)
end

# parse.rb
#! /usr/bin/env ruby

require 'application.rb'

provided_sample = File.read(File.expand_path(File.join('.', 'provided_sample.rb')))
applications = ApplicationDSL.execute(provided_sample).applications

puts "Provided sample roles:"
puts "===================="
applications.each do |app|
  puts "Roles for #{app.name}"
  puts app.roles.join(', ')
end

puts

desired_sample = File.read(File.expand_path(File.join('.', 'desired_sample.rb')))
applications = ApplicationDSL.execute(desired_sample).applications

puts "Desired sample roles:"
puts "====================="
applications.each do |app|
  puts "Roles for #{app.name}"
  puts app.roles.join(', ')
end

# output
Provided sample roles:
====================
Roles for foo
a, b, c, d

Desired sample roles:
=====================
Roles for foobear
polar, grizzly, black
Roles for pokercat
lion, tiger, panther