Indeed. I think we need both. In the same model.

On 12 Mar 2007, at 12 March/08:32, Andreas Raab wrote:
> Roel Wuyts wrote:
>> Andreas, I prefer refactoring, but refactoring assumes a closed  
>> world approach where you control all the pieces of the puzzle.
>
> I would call access to all pieces of the puzzle "open" not "closed"  
> but be that as it may...
>
>> It simply is not always possible to refactor (because you do not  
>> have the source, because you do not want to create a fork, because  
>> you have other programs that absolutely rely on the old behaviour,  
>> etc.). In that case extending the existing software from within  
>> your own package helps, but should be used sparingly for all the  
>> reasons you mention.
>> Note that a decent module system should support both.
>
> Which I have advocated earlier myself. I have never doubted that  
> there are situation in which patching another module is  
> advantageous and desirable. Which is why I don't understand why  
> some people are so opposed to the idea of having the other end of  
> the spectrum (isolation) available as well.
>
> Cheers,
>   - Andreas
>