[ruby-core:25226] Re: Module#prepend and Array#prepend

Indeed,
The existing in-ruby solutions only work if the original author takes pains
to supports extensions. In Rails 3, we do this. However, this is a large
burden on Ruby programmers, and we could provide an in-ruby solution
(prepend) that guarantees that any class can be extended.

Consider the following (extremely simplified) scenario:

class ActionController
  def render(template)
    find_template(name).render(self)
  end

  def find_template(name)
    return Template.new(name)
  end
end

Again, this is an extremely simplified example. Now what if I want to add
support for layouts to ActionController? Because render is defined on the
class itself, I cannot do so. As the creator of ActionController, I can do:

module Renderer
  def render(template)
    find_template(name).render(self)
  end

  def find_template(name)
    return Template.new(name)
  end
end

class ActionController
  include Renderer
end

And then to extend, I can do:

module Layouts
  def render(template, layout)
    find_template(layout) { super }
  end
end

class ActionController
  include Layouts
end

Because the original creator of ActionController put the implementation in a
module, another Ruby programmer can extend it. But frequently, Ruby code
does not do this. I would like to be able to do:

module Layouts
  def render(template, layout)
    find_template(layout) { super }
  end
end

class ActionController
  prepend Layouts
end

which would have the same effect as when the implementation was explicitly
inserted into a module, but it'll work on *any* class, not just ones
specially designed.

-- Yehuda
On Mon, Aug 31, 2009 at 9:41 AM, Carl Lerche <[e-mail]> wrote:

> Yusuke,
>
> The reason that something like #prepend would be needed is so that ruby can
> provide a good way to do AOP. Your example is not exactly a 1-1 mapping onto
> the prepend feature. Instead, you would need to do this:
>
>   module Speak
>>   def speak(words)
>>     puts words
>>   end
>>  end
>>  module Exclaim
>>   def speak(words)
>>     super("#{ words }!")
>>   end
>>  end
>>  class Person # Empty class that only includes the module
>>   include Speak
>>  end
>>  class Exclaimer < Person
>>   include Exclaim
>>  end
>>
>
>
> If your classes are NOT empty and only include modules, then it is
> impossible to include a module "under" methods defined directly on the
> class. This causes pain when you are using a ruby library that does not
> assume that you will be extending classes. Rails "solved" this problem using
> alias_method_chain, but this method can cause confusion.
>
> I hope I explained my thoughts clearly.
>
> Regards,
> Carl Lerche
>
>
> On Aug 31, 2009, at 5:39 AM, Yusuke ENDOH wrote:
>
>  Hi,
>>
>> 2009/8/31 Yehuda Katz <[e-mail]>:
>>
>>> Matz,
>>> As we discussed at LoneStar, I have the following proposal:
>>>
>> (snip)
>>
>>> Effectively, prepend would prepend a module to the class' ancestor chain.
>>>
>>
>>
>> I cannot guess the problem you want to solve.  I know this mail
>> is just discussion log or reminder, but I'd like you to elaborate
>> your proposal completely when sending to ruby-core.
>>
>> Why don't you define subclass explicitly?
>>
>>  class Person
>>   def speak(words)
>>     puts words
>>   end
>>  end
>>  module Exclaim
>>   def speak(words)
>>     super("#{ words }!")
>>   end
>>  end
>>  class Exclaimer < Person
>>   include Exclaim
>>  end
>>  Exclaimer.new.speak("matz") #=> matz!
>>
>> As far as I read your mail, I think this is the correct way.
>>
>> Do you want to define Person by including Exclaimer?  If so,
>> how about this?
>>
>>  class Person
>>   def speak(words)
>>     puts adjust_words(words)
>>   end
>>  end
>>  module Exclaimer
>>   def adjust_words(words)
>>     "#{ words }!"
>>   end
>>  end
>>  class Person
>>   include Exclaimer
>>  end
>>  Person.new.speak("matz") #=> matz!
>>
>> --
>> Yusuke ENDOH <[e-mail]>
>>
>>
>
>


-- 
Yehuda Katz
Developer | Engine Yard
(ph) 718.877.1325