A DSL for protobuf in Ruby

proto.rb

require 'erb'
require 'binding_of_caller'

GENERATED_CLASS_FOR_MESSAGE = <<-CLS
  class <%= name %>
    attr_reader <%= message.fields.map {|name, _| ':' + name.to_s }.join(', ') %>
    def initialize
      <% message.fields.each do |name, field| %>
      <%= field.to_s %>
      <% end %>
    end
  end
CLS

class Parser
  def self.parse(source)
    proto = ParsedProto.new
    proto.instance_eval(source)
    proto
  end
end

module DSL
  def message(mess)
  end

  def method_missing(method_name, *args, &block)
    @messages[method_name] = Message.new(args[0], &block)
  end
end

class ParsedProto
  include DSL
  def initialize
    @messages = {}
  end

  def generate
    @messages.map do |name, message|
      ERB.new(GENERATED_CLASS_FOR_MESSAGE).result(binding).lines.reject {|line| line =~ /^\s*$/ }.join
    end
  end
end

class Field < Struct.new(:label, :type, :name, :position)
  TYPES = %w(double float int32 int64 uint32 uint64 sint32
      sint64 fixed32 fixed64 sfixed32 sfixed64 bool string bytes)
  LABELS = %w(optional required repeated)

  def initial_value
    case
    when label == :repeated then []
    when label == :optional then nil
    when type == :double then 0.0
    when type == :float then 0.0
    when type == :int32 then 0
    when type == :int64 then 0
    when type == :uint32 then 0
    when type == :uint64 then 0
    when type == :bool then false
    when type == :string then ""
    end
  end

  def to_s
    "@#{name} = #{initial_value}"
  end
end

class Message
  attr_reader :fields, :name
  def initialize(name, &definition)
    @name = name
    @fields = {}
    instance_eval(&definition)
  end

  def repeated(field)
    field.label = :repeated
  end

  def optional(field)
    field.label = :optional
  end

  Field::TYPES.each do |type_method_name|
    define_method type_method_name do |value|
      field_name = binding.of_caller(1).local_variables[fields.count]
      fields[field_name] = Field.new("", type_method_name.to_sym, field_name, value)
    end
  end
end

puts Parser.parse("
    message Thing {
      repeated string tags = 1;
      int64 id = 2;
      bool public = 3;
    }
  "
).generate