method_missingと内部DSL ref: http://qiita.com/ykst/items/5b7b2903c71b43797a8e

file0.txt

struct(:data) {
  int :x;
  int :y;
};

file1.txt

class Parser
  def initialize
    @env_stack = [[]]
  end

  def parse(program)
    eval(program)
    p @env_stack
  end

  private

    def int(name)
      var = { :id => name, :type => :int }
      @env_stack.last.push var
    end

    def struct(name)
      var = { :id => name, :type => :struct }
      @env_stack.push []
      yield if block_given?
      var.merge! :fields => @env_stack.pop
      @env_stack.last.push var
    end
end

file2.txt

Parser.new.parse(%Q(
struct(:data) {
  int :x;
  int :y;
}
))

=>
[[ 
  {
    :id => :data, :type => :struct,
    :fields => [
      { :id => :x, :type => :int },
      { :id => :y, :type => :int },
    ]
  }
]]

file3.txt

def method_missing(name, *args)
  name
end

file4.txt

struct(data) {
  int x;
  int y;
};