[armedbear-devel] eql for java objects

Alessio Stalla alessiostalla at gmail.com
Sun Apr 25 20:50:18 UTC 2010

On Sun, Apr 25, 2010 at 10:25 PM, Alan Ruttenberg
<alanruttenberg at gmail.com> wrote:
> On Sun, Apr 25, 2010 at 4:05 PM, Alessio Stalla <alessiostalla at gmail.com> wrote:
>> On Sun, Apr 25, 2010 at 9:49 PM, Tobias C. Rittweiler <tcr at freebits.de> wrote:
>>> 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?
>> Maybe it does, I was reasoning in the abstract, I meant: let's ensure
>> that EQUAL works consistently.
>>>> 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.
>> I don't read more implementational freedom in EQL than in EQUAL.
> I concur, though the kinds of freedoms are spelled out. Java .equals()
> doesn't seem to have the same constraint that lisp equals has - namely
> that the comparison is structural.
> My read is that == on the java side is eql on the lisp side, that
> equals on the java side is it's own thing, and that we would be on the
> right side to define lisp equal and equalp on java objects such as
> arraylists in such a way that the comparison is elementwise as would
> be done on the lisp side.

Ok, I understand your view. However, if you fear equals() is too much
of a black box, and might make cl:equal work in unexpected ways, then
I'd prefer always using ==. Traversing stuff in Lisp isn't wise imho,
since collections are not just ArrayLists and LinkedLists, but also
Sets (for which you need another algorithm than traversing it one
element at a time, since sets are unordered in general), and more
hairy things like PersistentCollection (Hibernate) which potentially
are not easily compared elementwise. equals() exists also to provide
an unified entry point for all those diverse algorithms.


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