SystemNavigation and deprecated methods
andreas.raab at gmx.de
Fri Aug 15 16:19:58 UTC 2003
> Because you then have base-level code that directly accesses
> meta-level methods. If ever you want to remove reflection from
> your image you risk running into big problems, because it is
> very hard to determine all the places in the code where the
> reflection is actually used.
I don't quite understand what you mean here. Assuming that I use reflection
code at all, isn't it _always_ the case that I will have to check the places
in which that code is actually used? How would (for example) checking for
the senders of Behavior>>allCallsOn: be different from checking for the
senders of SystemNavigation>>allCallsOn:in:?
So the only situation in which having the functionality in SystemNavigation
would be advantageous is when you don't use reflection at all and can just
remove that class. But it seems quite natural to me that assuming we use
extension methods we remove those methods along with the class. Again, I see
no recognizable difference here.
> Sure. Every problem in computer science can be solved by
> introducing an extra level of indirection :-)
> No, serious, you run into problems because if you introduce a simple
> proxy (let's call it 'aProxy', instance of class MyProxy) for
> an object a (instance of A), then what happens if you evaluate
> 'aProxy class selectors'? That is the point I wanted to make. No
> object-orientedness will save you there, I'm afraid :-(
Oh, but absolutely. The only reason why the above wouldn't work in Squeak is
that #class is not a message send. The point has been made various times by
various people that this is a significant problem for proxies. And for the
most part I agree. So if we were to remove that restriction then you could
trivially answer a ClassProxy if you wanted to. Let's not confuse certain
shortcomings of Squeak (which could be trivially fixed) with the larger
> Yes, that's why I like to have responses like yours; no
> question about it. Note that we are having two discussions at
> the same time in this thread, I think. One is the general
> overall discussion (all the stuff above and the stratification
> below, etc.). The other one is the concrete problems that started
> the whole thread in the first place. For the latter, I thought
> that you only objected to some of the methods being gone from
> class. Is that right? If so, could you give a concrete list?
That's correct. For the list, let's see:
Those are the ones that I would want to ask a class directly (mostly because
they're not immediately accessible from the tools for various of the queries
that come up often).
> The clear division between base level code and meta level code, as I
> tried to made clear in the paragraphs at the beginning of
> this mail. By itself stratification is not that new; for
> example, I need it in Soul, since I have to marry two
> different languages. You clearly see the difference between
> the logic (meta) code and the Smalltalk (base) code.
I see what you're saying. In some respects I agree (though maybe not for the
same reasons ;) that having some sort of fence between the base and the meta
level representations can be advantageous. But I think that (for me) this
fence shouldn't really appear as a stratification which sounds like lots of
extra indirections but rather by some sort of "context switch" (think:
perspectives) where you have to be explicit about when you are meta and when
you aren't but can use the same convient way of thinking about and referring
to objects and their methods once you really are meta.
Let me try to illustrate this in a practical example: I think we both agree
that when I write a class-side (utilitiy) method I should not be able to
access the meta level directly, if only for preventing mistakes, e.g.,
writing something like "name := something" (this actually happened to me).
But I don't think that this problem should be addressed by requiring an
extra indirection if I _do_ want in fact to be meta such as in "(ClassMirror
on: self) setNameTo: #Mumble" but rather something where I perhaps have an
explicit annotation for this method that means, yes, I am now in the meta
level of the system and I (hopefully ;) know what I'm doing.
> So there you have to make a clear separation or suffer the
> consequences. When the meta language and the base language are the
> same, and the representation of the language are objects just as base
> level objects (the Array object, for example), it is very
> easy to write statements that jump between base and meta level
> code all the time ('myObject class selectors', for example).
> By itself this is not a problem, until you suddenly need to
> change some part of the representation (because your things
> do not live in your image anymore).
> Then all thse seemingly ok and very clear statements become quite
> difficult. Again, this is not to say that I want such a
> complete system in Squeak: that would be a research goal
> (and you'll end up with Strongtalk or something very similar).
> But that is part of the motivation why we make the separation,
> implementing it on a separate class. Then we can still add
> class extensions to point to these things,
> or make a global variable to hold the instance, etc.
Okay, now I understand where you are coming from. Thanks for the
explanation. Though I have to say that it feels to me like the wrong
solution to the right problem ;-) This sounds a lot like a problem which
might be much better addressed by using something like Traits in combination
with selector namespaces.
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