From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-07-15T07:25:10+09:00
Subject: Re: Preferred monkeypatching technique

transfire@gmail.com wrote:
> Daniel Schierbeck wrote:
>> Tom Werner wrote:
>>> Allow me to present a scenario:
>>>
>>> class Firetruck
>>>    def put_out_fire(options = {})
>>>       # code
>>>    end
>>> end
>>>
>>> Pretend Firetruck is in a 3rd party application (like Rails) that is
>>> happy to allow plugins to modify core code. Now, let's say I want to
>>> write some code that always adds a certain attribute to the options
>>> hash. I could do this:
>>>
>>> class Firetruck
>>>    alias_method :__old_put_out_fire, :put_out_fire
>>>    def put_out_fire(options = {})
>>>       __old_put_out_fire(options.merge({:nozzle => :big}))
>>>    end
>>> end
>>>
>>> Which works just fine until someone else comes up with a plugin that
>>> wants to modify the same method (doing something similar to me) and just
>>> so happens to also use :__old_put_out_fire as THEIR alias. Now we've got
>>> my plugin's method as the alias calling itself, which leads to, you
>>> know, badness.
>>>
>>> So I'm wondering if there's a better way. Perhaps some way to turn
>>> Firetruck into an ancestor of itself, so to speak, so that my plugin
>>> would create a new Firetruck class, pushing the old Firetruck backward
>>> in the chain and allowing me to call super instead and preventing
>>> alias_method explosions. Or would that just end up causing more havoc?
>> Not a solution, but:
>>
>>    <http://rcrchive.net/rcr/show/321>
>>
>>    cut MyCut < Firetruck
>>      def put_out_fire(options = {})
>>        super(options.merge(:nozzle => :big))
>>      end
>>    end
>>
>> (Trans, can I omit the `MyCut <' part?)
> 
> Sure. Although, the name provides a handle if you need to dynamically
> effect the cut later --just like any Ruby class. But you can of course
> do it annonymously too:
> 
>   Cut.new(Firetruck) do
>     ...
>   end

Neat-o.

How about a method-level shorthand?

   class Firetruck
     redef put_out_fire(options = {})
       # maybe something other than "super"?
       super(options.merge(:nozzle => :big))
     end
   end

That way you can also easily cut a class from within itself, even 
conditionally.


Cheers,
Daniel