I don't know about Envy, but DVS style class extension practically do
support modifying existing methods - you change a method's category to
be "*myextending-system-category", and modify away. This means that for
methods, only removals aren't supported.

About unloading, it is possible (in principle, not yet in code) to
unload class extensions that modify existing methods, because the
history know which is the latest version that wasn't in this class
extension.

Of course this means that there's no guarantee of behavior preservation.
Though with the caveats, there wasn't much commitment behind that
guarantee anyway ;-)

For a practical example of this, load DVS, then load SpaghettiTracer,
and look at PackageInfo. You'll see that SpaghettiTrace extends and
modifies DVS - a little light refactoring, and widening some protocol so
I can optimize some operations. (BTW, Avi, if you're reading this, you
might want to review those and consider taking them/part upstream into
DVS)

Daniel Vainsencher

Doug Way <dway at riskmetrics.com> wrote:
> 
> Ned Konz wrote:
> 
> >On Friday 18 October 2002 07:36 am, Roel Wuyts wrote:
> >  
> >
> >>Stef has this kind of setup in mind, and in that setup there is no
> >>difference between modules and delta-modules. A module is something
> >>which has definitions and which you can apply on a system.
> >>
> >But that (a simple declarative system) not sufficient to do what we 
> >want to do with the system. There has to be a way to remove an 
> >existing method (at least for your code).
> >
> >So unless you effectively define an entire module which consists of 
> >the original module MINUS the method, you don't get the behavior you 
> >want.
> >
> >I _think_ the DM's are a way to do this kind of specification: "this 
> >is a module just like module X but with this removed, this added, and 
> >this modified".
> >
> Okay, this is getting into a difference between these two approaches 
> which needs to be fleshed out some more (and which I'm somewhat familiar 
> with, at least :-) ).
> 
> With Envy/Ginsu-style modules (which I assume are similar to what Roel & 
> Steph are talking about), modules can contain classes, and can also 
> contain "Class Extensions" (a.k.a. loose methods).  I would define Class 
> Extensions as methods which are additions to classes in other modules, 
> but they are ONLY additions, NOT changes to existing methods (and not 
> removals either).
> 
> DeltaModules, on the other hand, can contain method additions, but also 
> changes to existing methods and method removals too.
> 
> So, to generalize a bit, DeltaModules are more powerful, but Class 
> Extensions are quite a bit simpler.
> 
> Let's think about a couple of useful properties of modules:
> 
> 1. Can they be cleanly unloaded?
> 2. Are they behavior-preserving?  In other words, if you load (and 
> activate) a module, will the rest of the system still run without 
> breaking or behaving differently?
> 
> Regarding #1, 3.3a-style Modules and DeltaModules can be unloaded, and 
> Envy/Ginsu-style modules can too, so no problems there.  Changesets 
> cannot be unloaded, which is their big limitation.
> 
> Regarding #2, this is one reason for the split between Modules and 
> DeltaModules in 3.3a.  If you load a Module (with no DeltaModules), the 
> rest of the system will be unaffected, so they are behavior-preserving. 
>  DeltaModules are not behavior-preserving, but that's the point of 
> them... they can be used if you do need to change something else in the 
> system.
> 
> Because DeltaModules are not behavior-preserving, they naturally have to 
> change version numbers of the Modules that they modify.  So for example 
> if a WebBrowser Module has a DeltaModule which adds the method 
> #asEncodedHtml or changes a method in class String, in the Module 
> "Kernel 2.7", the Kernel module will need to be assigned a new version 
> number such as 2.8.  (I believe this is how DeltaModules are supposed to 
> work, correct me if I'm wrong.)  This may have big implications for the 
> rest of the modules in the system, which might expect to run on Kernel 
> 2.7 but not know about 2.8.
> 
> But the interesting thing is, Envy/Ginsu-style modules w/Class 
> Extensions *are* behavior-preserving, with a few caveats* (see bottom of 
> message).  This is because, in general, if you add a new method to an 
> existing class, none of the rest of the system would ever send that new 
> method, so the behavior of the rest of the system is unchanged.  So, 
> reusing the example above, if a WebBrowser module has a Class Extension 
> which adds the method #asEncodedHtml (but does not change any method) in 
> class String, in the module "Kernel 2.7", the Kernel module will still 
> remain 2.7.  (This is exactly what Envy does.  I don't remember now if 
> Ginsu supports version numbers.)
> 
> The other question is then, if we're using Class Extensions, is it good 
> enough to only be able to *add* methods to classes in other modules, not 
> modify/remove them (as DeltaModules can do)?
> 
> Well, I guess that's a big question. :-)  I would say that most of the 
> time, yes, it's good enough.  A module does not often need to 
> change/remove methods in other modules, unless we're directly fixing a 
> bug in that other module.  Adding methods is much more useful, and can 
> be specific to the "outer" module (such as the example WebBrowser module 
> adding #asEncodedHtml to String).  People who've used Envy know that it 
> can be very handy... although you don't want to abuse it to an extreme.
> 
> With Envy/Ginsu, if your module *really* needs to change/remove a method 
> in Kernel 2.7, you make your own private version of Kernel 2.8 with the 
> change, and/or you lobby the maintainer of Kernel to make the change. 
>  But this should be relatively rare aside from bug fixes.
> 
> On the other hand, the ability of DeltaModules to handle method 
> changes/removals could be quite nice.  And they seem a bit more 
> rigorous, no "caveats" as with Class Extensions below.  And namespaces 
> are already built-in.  But there is extra complexity cost...  For 
> example, DeltaModules have to worry about activation/deactivation, with 
> Class Extensions it is irrelevant.  The Class Extensions are always 
> active when the module is active, since they don't affect anything else.
> 
> One last thing: As far as splitting up the current Squeak image into 
> modules goes, my hunch is that it would be easier to do with 
> Envy/Ginsu-style modules.  This is because you would not need the 
> DeltaModule capability of specifying changed/removed methods when 
> divvying things up, only the ability to specify added methods, which the 
> lighter-weight Class Extensions can also handle without version number 
> worries.  For example, String>>asMorph could be quickly dumped into some 
> Morphic module.
> 
> (Avi, are the DVS "logical modules" similar to Envy/Ginsu with Class 
> Extensions?)
> 
> Am I making sense here?  This message is getting too long, sorry.  Let 
> me know if I've characterized one of the approaches incorrectly.  And I 
> know this is only one aspect of the differences between these two module 
> systems.  Anyway, I've often found that I learn more by see two things 
> compared against each other, than by seeing them described separately.
> 
> - Doug Way
>   dway at riskmetrics.com
> 
> 
> 
> *Caveat #1: There could be namespace conflicts with Class Extensions, 
> which Envy/Ginsu don't handle.  I'm guessing that adding namespace 
> support is one thing that Roel & Steph were working on?
> 
> *Caveat #2: Ideally, to preserve behavior, you would not allow a Class 
> Extension to override a method in a superclass.  This would be a simple 
> restriction to add.  Otherwise, you could bring an image to its knees by 
> adding a new method #size to SortedCollection which returns nil. :-)
> 
> *Caveat #3: If a base module class is doing something funky with 
> reflection such that adding a new method changes its behavior, then the 
> Class Extension might not be behavior-preserving.  But this falls under 
> the category of Very Rare, IMHO.  (I guess an example might be adding a 
> testXXX method to an SUnit test suite class.)