From: "Eric I." <rubytraining@...>
Date: 2008-08-10T23:52:47+09:00
Subject: Re: DSL challenge. Do you guys have any elegant ideas?

Hi Patrick,

Here's another solution.  You didn't specify the problems you were
having, so I can't be certain whether this solution avoids them.

Eric

====

Ruby training and Rails training available at http://LearnRuby.com .

====

module Farm
  class << self
    def create(&block)
      @scope_stack = []
      push_frame(:farm)
      class_eval(&block)

      "Farm contains\n[\n" + indent(pop_frame) + "\n]\n"
    end

    def barn(&block)
      push_frame(:place)
      class_eval(&block)

      append_to_frame "Barn contains\n[\n" + indent(pop_frame) +
        "\n]\n"
    end

    def pond(&block)
      push_frame(:place)
      class_eval(&block)

      append_to_frame "Pond contains\n[\n" + indent(pop_frame) +
        "\n]\n"
    end

    def animal(name)
      raise "IllegalMethodError -- animal can only be called " \
            "under a place" unless
        top_frame[0] == :place
      append_to_frame name + "\n"
    end

    private

    def push_frame(description)
      @scope_stack.push [description, ""]
    end

    def pop_frame
      @scope_stack.pop[1]
    end

    def top_frame
      @scope_stack[-1]
    end

    def append_to_frame(str)
      top_frame[1] << str
    end

    def indent(str, level = 2)
      str.split("\n").map { |s| " "*level + s }.join("\n")
    end
  end
end


string = Farm.create do
  barn do
    animal "dog"
    animal "cat"
  end
  pond do
    animal "whale"
    animal "shark"
  end
end

puts string