[armedbear-devel] eql for java objects

[Tobias C. Rittweiler]

Alessio Stalla <alessiostalla at gmail.com>
writes:

> On Sun, Apr 25, 2010 at 8:37 PM, Erik Huelsmann <ehuels at gmail.com> wrote:
>> Hi all,
>>
>> On Sun, Apr 25, 2010 at 7:26 PM, Alan Ruttenberg
>> <alanruttenberg at gmail.com> wrote:
>>>
>>> On Apr 25, 2010, at 6:55 AM, Tobias C. Rittweiler wrote:
>>>
>>>> I think that's past Alan's point which, from my understanding, is simply
>>>> "Java enums should be comparable by EQL." Makes perfect sense as EQL is
>>>> for object identity.
>>>
>>> Yes, and Java "objects" too.
>>
>> I've read this thread, the thread on c.l.l regarding EQUAL and the
>> definitions of EQ and EQL. The following is my interpretation of it
>> all:
>>
>> 1. Our Java side JavaObject class is merely a box for a Java instance
>> value (a pointer to a Java object, if you will)
>> 2. The definitions of EQ and EQL talk about Objects, but I interpret
>> them to refer to the first meaning in [1], not to instances of lisp
>> classes
>> 3. Our JavaObjects have not been defined in the (Common) Lisp spec;
>> they don't adhere to the object-instantiation protocol nor are they
>> any of any one of the predefined built-in classes and therefore can't
>> be taken to be "Lisp Objects" in that sense of the word
>> 4. From point (1) and the definitions of EQ and EQL, I concur with
>> Alan that "raison d'etre" of EQL should equally apply to JavaObjects
>> 5. From ponits (2) and (3) and the discussion on #lisp and c.l.l, I
>> conclude that we're basically free to extend the meaning of EQL here:
>> the JavaObject values were not in the spec to begin with: they're not
>> Lisp class instances, they're not symbols, numbers nor characters
>>
>> The only objection there is from both #lisp and c.l.l is that EQ, EQL
>> and EQUAL shouldn't start to behave unpredictably regarding defined
>> behaviour. For all the spec cares, we would have generated an error
>> when EQ-comparing 2 java objects...
>>
>>
>> Agreed?
>
> I agree almost completely, but I'd like to add that:
>
> 1. if EQL is modified, then EQUAL must be modified as well because
> (eql x y) should always imply (equal x y)

EQUAL does not internally call EQL? If that's indeed not the case,
perhaps it should be made so?

> 2. if we have freedom to modify EQL and EQUAL, then I'd say that EQUAL
> should call Object.equals(), but I know this is controversial

No that would be false reasoning IMHO. The only reason why it's
acceptable to extend EQL that way is because the Notes section in EQL
kind of implies (albeit does not spell it out explicitly) certain
implementational freedom in the interplay of EQ and EQL.

  -T.