Obvious immutable entity with validation

immutable_entity.rb

module Obvious

  module EntityMixin 
    class << self
      def included(base)
        base.extend ClassMethods
      end
    end

    module ClassMethods
      attr_reader :shape
      attr_reader :validations

      def value name, type 
        name = name.to_sym
        @shape ||= {}
        @shape[name] = type   
        define_method(name) { @values[name] }
      end

      def validation name, method
        name = "#{name}_validation".to_sym
        @validations ||= [] 
        @validations << name 
        define_method(name) { instance_exec &method }
      end

    end
  end

  class Entity

    include EntityMixin

    def initialize input
      shape = self.class.shape
      validations = self.class.validations || []

      @values = {}
      input.each do |k, v|
        unless shape[k]
          raise StandardError.new "Invalid input field: #{k}" 
        else
          @values[k] = v
        end
      end 

      freeze
      
      shape.each do |k, v|
        unless @values[k].class == v
          msg = "Validation Error: Invalid value for #{k}, should be a #{v}"
          raise StandardError.new msg 
        end
      end

      validations.each { |method| send method } 
    end 

    def to_hash
      hash = {}
      @values.each do |k, v| 
        hash[k] = v
      end 
      hash
    end
  end
end

class Thing < Obvious::Entity

  value :foo, String
  value :test, Fixnum

  validation :something, Proc.new {
    if foo != "hello world"
      msg = "Validation Error: Invalid value for foo, should be 'hello world'"
      raise StandardError.new msg 
    end
  }

end

class Thing2 < Obvious::Entity
  value :something, String
  value :else, TrueClass 
end


describe Thing do
  it 'should be a thing' do
    thing = Thing.new foo: 'hello world', test: 1
    puts thing.inspect
    puts Thing.shape
    puts thing.foo
    puts thing.test
    puts thing.to_hash 

    thing2 = Thing2.new something: 'funny', else: true
    puts thing2.inspect
  end
end