How one might implement Chef's Recipe DSL

chef_dsl.rb

# Basics

module Chef
  class Resource
    attr_reader :name

    def initialize name
      @name = name
    end

    def execute
      raise 'To be implemented by subclass...'
    end
  end
end

module Chef
  class Recipe
    attr_reader :resources

    def initialize resources
      @resources = resources
      @executed  = false
    end

    def execute
      return if @executed
      @executed = true
      @resources.each(&:execute)
    end
  end
end



# Builders

module Chef
  class Resource
    class Builder
      attr_reader :resource

      def camelize s
        s.to_s.split('_').map(&:capitalize).join('')
      end

      def initialize kind, name, &block
        klass = Chef::Resource.const_get camelize(kind)
        @resource = klass.new name
        @resource.instance_eval &block if block
      end
    end
  end
end

module Chef
  class Recipe
    class Builder
      attr_reader :recipe

      def initialize &block
        @resources = []
        instance_eval &block if block
        @recipe = Chef::Recipe.new @resources
      end

      def method_missing resource, *args, &block
        @resources << Chef::Resource::Builder.new(resource, *args, &block).resource
      rescue NameError
        super
      end
    end
  end
end

module Chef
  def self.recipe &block
    Chef::Recipe::Builder.new(&block).recipe.execute
  end
end



# Resources

module Chef
  class Resource
    class Log < Resource
      require 'date'
      require 'time'

      def initialize name
        super name
        @level = 'INFO'
        @precision = 3
      end

      def level l
        @level = l
      end

      def precision n
        @precision = n
      end

      def now
        Time.now.utc.iso8601(@precision)
      end

      def execute
        puts '%s [%s] %s' % [ @level, now, @name ]
      end
    end
  end
end

module Chef
  class Resource
    class File < Resource
      require 'fileutils'
      require 'date'
      require 'time'

      def action a
        @action = a
      end

      def contents s
        @contents = s
      end

      def execute
        @action ||= :create
        case @action
        when :create
          ::File.open(@name, 'w') { |f| f.puts @contents }
          puts 'Wrote file at %s' % @name.inspect
        when :delete
          ::FileUtils.rm_rf @name
          puts 'Deleted file at %s' % @name.inspect
        else
          raise 'Invalid action provided for file resource'
        end
      end
    end
  end
end

module Chef
  class Resource
    class RubyBlock < Resource
      def block &block
        @block = block
      end

      def execute
        @block.call if @block
      end
    end
  end
end



# Using the DSL...

Chef.recipe do

  log 'hello, world!'

  file '/tmp/foo' do
    contents 'test'
  end

  ruby_block 'read_file' do
    puts 'inside the ruby_block, but outside the block itself'
    block do
      contents = File.read('/tmp/foo')
      puts "Read file: %s" % contents.inspect
    end
  end

  file '/tmp/foo' do
    action :delete
  end

  log 'goodbye, world!' do
    level 'DEBUG'
    precision 6
  end

  puts inspect

end