[self-interest] Keeping several versions around

[Bergel, Alexandre]

> Didn't you have to change the Squeak VM to get Classboxes on
> it?
>
The first implementation yes. But this is a bazooka-like method that  
is recommendable only in a last development phase (for speed  
efficiency). For prototyping, I suggest to use reflection. I am not  
sure about Self's meta-programming capabilities, but I believe Self  
does better than Smalltalk on that respect.

> What did you do for Java?
>
In Java, when a redefined method is invoked, the whole method stack is  
introspected. The stack of classboxes involved in the computation is  
then built. You can then freely invoke the proper version of the  
method to invoke.
> One fundamental question I have is whether a Classbox is possible on a
> prototype based system or whether you need to go the whole hog to Us.
>
I do not know Us in details, but the key aspects of classboxes are:
	- a modular unit may define class extension (adding a method on a  
class that is defined in a different modular unit)
	- the method lookup takes modular unit's visibility into account

I believe Us has similar postulates. In Classboxes, I rely on the  
complete method call stack. After all these years, I am not completely  
sure this was the right decision. The reason is that you can easily  
end up with a complex situation where you do not know which method is  
gonna to be invoked. The method lookup algorithm should remain simple.

I wrote a short description of Selector Namespace. This mechanism is  
simpler than classboxes and should be easy to implement.
http://bergel.eu/download/SelectorNamespaceEssay.pdf

Cheers,
Alexandre

>
> Russell
>
> On 01/08/2009, at 12:29 AM, Bergel, Alexandre wrote:
>
> > Dear List,
> >
> > I like the discussion you're having regarding modularity and system
> > extension. We had similar discussion in the Smalltalk World.
> > It will be a pleasure to participate in discussion and provide  
> help if
> > necessary to have classboxes/changeboxes in Self.
> >
> >> I'm lumping the Changebox stuff you refer to with their earlier
> >> Classbox stuff because keeping multiple versions of an object  
> around
> >> is the same as keeping multiple implementations of compatible
> >> objects around - that is, versioning is really the same as
> >> modularisation.
> >>
> > Indeed
> >> The Classbox/Changebox etc stuff is a limited implementation of the
> >> type of stuff that 'Us' implemented: subjective, perspective based
> >> messaging systems (seehttp://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.56.7535)
> >> There isn't a publically available version of Us at the moment,
> >> though.
> >>
> >
> > Yes, the work done on Us is very similar to what have been done with
> > classbox/changeboxes.
> >
> > Regards,
> > Alexandre
> >
> >
> >> Niko Schwarz wrote:
> >>
> >>>
> >>> Hi!
> >>>
> >>> I am interested in keeping several versions of code living side by
> >>> side in the same image. There is an implementation in Smalltalk,  
> but
> >>> I'd like to know if this could be done more elegantly using Self.
> >>>
> >>> The Smalltalk implementation is called Changeboxes,
> >>> http://scg.unibe.ch/archive/papers/Denk07cChangeboxes.pdf. It
> >>> essentially works by hooking into the compiler and then, upon  
> every
> >>> method compilation, NOT pushing the Class onto the stack, but
> >>> rather a
> >>> level of indirection which finds the correct version of the class.
> >>>
> >>> The idea is essentially the following. Imagine you just built a
> >>> feature, and it works, let's say you just implemented  
> Exponentiation
> >>> using a while loop, that just multiplies a number with itself a
> >>> given
> >>> number of times. It works. But you find it slow. Now, you fork,  
> and
> >>> keep the old version around. You change the method, right there,  
> you
> >>> don't make a new method, you CHANGE the old one. But still, you  
> keep
> >>> the old one around.
> >>>
> >>> And then you run them side by side. The currently active branch
> >>> might
> >>> be the sped up algorithm, but you can still access the main trunk,
> >>> so
> >>> now you benchmark the two, and see if they yield the same results:
> >>>
> >>> [2 raisedTo: 100 ] timeToRunAndResult
> >>> [[2 raisedTo:100] inBranch: #main] timeToRunAndResult
> >>>
> >>> So, how would you implement this version change with only little
> >>> performance penalty? How does the lookup process work anyway and  
> how
> >>> can I hook into it?
> >>>
> >>> All the best,
> >>>
> >>> Niko
> >>>
> >>> raisedToThe: anInteger
> >>> |res|
> >>> res := 1.
> >>> anInteger timesRepeat: [ res = res * self]
> >>> ^
> >>>
> >>> but it is slow. Now you fork, and keep the working version around,
> >>> both for examination
> >>>
> >>
> >>
> >>
> >
> > --
> > _,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:
> > Alexandre Bergel http://www.bergel.eu
> > ^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;.
> >
> >
> >
> >
> >
> >
> >
> > ------------------------------------
> >
> > Yahoo! Groups Links
> >
> >
> >
>
>
> 

-- 
_,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:
Alexandre Bergel  http://www.bergel.eu
^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;.