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

    def initialize(klass, methods)
      @klass = klass

      mod = ::Module.new
      mod.__send__(:extend_object, self)

      mod.module_exec(klass, methods) { |klass, methods|
        methods.each do |method_name|
          define_method method_name do |*args, &block|
            @state = @state.public_send(method_name, *args, &block)
            unless @state.is_a?(@klass)
              ::Kernel.raise ::Yaks::IllegalState, "#{@klass}##{method_name}(#{args.map(&:inspect).join(', ')}) returned #{@state.inspect}. Expected instance of #{@klass}"
            end
          end
        end
      }
    end
  end
end