.rb

module Model
  module ClassMethods
    TYPE_MAP = {}
    def attribute(attribute_name, type_restriction)
      TYPE_MAP[self] ||= {attribute_name => type_restriction}
    end
  end

  def self.included(mod)
    mod.extend ClassMethods
  end

  def initialize(attributes)
    @attributes = attributes
    @attributes.each do |k,v|
      ClassMethods::TYPE_MAP[self.class][k] == v.class or raise(TypeError, 'blah')
      instance_variable_set :"@#{k}", v
    end
  end
end

class SomeThing
  include Model
  attribute :foo, String

  def start
    puts "@foo is a string"
  end
end

SomeThing.new(foo: "hi").start # ok
SomeThing.new(foo: 1).start # error