[squeak-dev] Re: What is equivalence? (was: The Trunk:
ma.chris.m at gmail.com
Thu Feb 5 02:38:44 UTC 2015
> IMO that's a bug. And the bug is that WeakArray species = WeakArray,
> whereas it should be Array. If one selects/rejects/copies a WeakArray the
> result is more useful if it is strong. It would be interesting to make the
> change and see what effect it has on the standard test suite.
>> I think the answer is because #= (not any other method, just #=) needs
>> to care about which _implementation_ of the argument which is passed
>> when considering true equivalence to another object.
> IMO the problem is because coming up with general purpose #= implementations
> is hard. You must have seen the controversy my definition of
> CompiledMethod>>#= has caused over recent years. #= has evolved a lot since
> Smalltalk-80 as its been more affordable to be cleverer, but that evolution
> also shows that its not absolutely obvious what #= should do; it depends on
> context, and current definitions of #= are general agreements of useful
> behaviour, some of these agreements having been made a long time ago.
Being cleverer in #= feels right but general agreements also feels
right in terms of having dependable contracts. For example, its
really nice that everyone has "agreed" to include a valid #hash method
when they write an #= method. We just do it regardless whether we
actually think we will put them in a HashedCollection.
>> I agree we human developers can consider unequal but same-quacking
>> objects interchangeable in our fuzzy minds when we design our
>> applications, and that is powerful, however, *within the system*, I
>> just it needs #= to make implementation-specific distinctions,
>> especially for as base-classes as Array and Dictionary.
> One thing #= should do is accept any object as an argument without error.
Another very good general agreement for equivalence testing. I agree! :)
> This wasn't always the case, but Smalltalk was not and never will be
> perfect. Apart from that, what #= means is something we have to negotiate.
> I don't think my recent changes broke anything.
I don't think anything will be broken. None of my outboard
SpecialDictionary's implement isDictionary. It gave me pause to
wonder what it should answer..
>> This seems to be reflected by most of the #= implementations in the
>> system, which check either #class, #species, or #isKindOf:.
> Few of them use isKindOf:. Some use class or species. Some use #isFoo. I
> don't see that #isFoo is worse than #class or #species. Do you think it is?
Hm. In any other method besides #=, #isFoo is always better. In
Dictionary>>#=, it will perform better, it probably won't break
anything even external. So, definitely not worse. :) Just making
sure when changing such a method as Dictionary>>#=...
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