stateful_builder.rb

module Yaks
  # State monad-ish thing.
  class StatefulBuilder < BasicObject
    def create(*args, &block)
      @state = @klass.create(*args)
      instance_eval(&block)
      @state
    end

    class StatefulMethods < ::Module
      def initialize(klass, methods)
        create_method_delegates(methods)
      end

      def assert_type(klass, object, method_name, args)
        unless object.is_a?(klass)
          ::Kernel.raise ::Yaks::IllegalState, "#{klass}##{method_name}(#{args.map(&:inspect).join(', ')}) returned #{object.inspect}. Expected instance of #{klass}"
        end
      end

      def create_method_delegates(methods)
        module_exec(self, methods) do |this, methods|
          methods.each do |method_name|
            define_method method_name do |*args, &block|
              @state = @state.public_send(method_name, *args, &block)
              this.assert_type(@klass, @state, method_name, args)
            end
          end
        end
      end
    end

    def initialize(klass, methods)
      @klass = klass

      mod = StatefulMethods.new(klass, methods)
      mod.__send__(:extend_object, self)
    end
  end
end