[squeak-dev] The Trunk: Collections-ul.668.mcz

Chris Muller ma.chris.m at gmail.com
Tue Oct 13 19:19:00 UTC 2015

Okay if you do would you please use a species rather than class check?
 #class checks create problems for Magma or any systems which use
proxies, because such tests produce a different result depending on
whether the proxy is materialized or not because #class is a inlined
message which cannot be detected by the proxy.

On Tue, Oct 13, 2015 at 1:05 PM, Levente Uzonyi <leves at elte.hu> wrote:
> What we can do is to use class (or species) checks. That way we can be
> consistent with the current hash implementation, and ensure symmetry, but
> different dictionaries holding the same pairs will not be equivalent.
> PluggableDictionary needs its own implementation of course.
> And Sets can use the same kind of check as well.
> Levente
> On Tue, 13 Oct 2015, Chris Muller wrote:
>>>> Exactly. It can happen with other kind of dictionaries as well. And
>>>> class checks would not enough:
>>>> d1 := PluggableDictionary new.
>>>> d2 := PluggableDictionary new equalBlock: #==.
>>>> d1 at: 'test' copy put: 4.
>>>> d2 at: 'test' put: 4.
>>>> {d1 = d2.
>>>> d2 = d1}
>>>> So, performance goes down in some cases, but I guess we agree on #= has
>>>> to be symmetric.
>> For simple value types, yes, we should be able to make that
>> assumption, but symmetry of equivalence starts to break down for
>> objects as complex as a Dictionary..
>>> We can speed up some cases if the dictionaries can tell if #= is
>>> guaranteed to be symmetric
>>> when they have the same class.
>>> But #hash still uses species, that means, we can have a Dictionary and an
>>> IdentityDictionary to be equal, but with different hash.
>>> s := 'a'
>>> ed := Dictionary with: s -> $a.
>>> id := IdentityDictionary with: s -> $a.
>>> { ed = id. id = ed. ed hash = id hash } "->  #(true true false)"
>> Equivalence of complex objects is always a "loose" definition, of
>> which there can be several.  Additional API like #hasEqualElements:
>> (of SequenceableCollection) could be added as needed to accomodate
>> other application needs, but we only get ONE particular implementation
>> which can also be suitable for use in HashedCollections, because they
>> send #=.  Changing it should be really scrutinized.
>> I'm wondering if Levente could get by with an app-specific equivalence
>> check without needing it for use in HashedCllections?  Seems like a
>> lot of the #= in the system require class or species equivalence, a
>> few allow isKindOf:, but only one or two allow #is message response.
>> I'm wondering now if those last kind are ill-advised...
>> BTW, with this change, I guess we're now somewhat inconsistent with Set,
>> | ed id |
>> ed := Set with: 'a' copy.
>> id := IdentitySet with: 'a'.
>> { ed = id. id = ed }   " #(false true)"
>> My production app is going to be punished by Levente's change,
>> unnecessarily.  I think we should consider an alternative like adding
>> #hasEqualElements: for your app...

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