Re: BLD issue: non-disjointness of internal and external predicate symbols

Jos de Bruijn wrote:
> Michael,
> 
> My argument is not about error-checking, but rather about the principle 
> that the same constant you use in different places should mean the same 
> thing.
> 
> But I'm also willing to compromise for the case of external frames, so 
> the compromise would be:
> 
> - we create additional sets for external functions and predicates that 
> are disjoint from the sets of "internal" symbols, and impose the 
> condition that internal function and predicate symbols may not be used 
> in external terms
> - we do not impose restrictions on symbols used in external frames
> 
> Let's see what the rest of the working group thinks about this.

Why do we need to make this restriction? It doesn't seem to buy us 
anything, as semantically, the external funcs and preds are well 
separated from the internal ones, so, no danger.

Axel


> Best, Jos
> 
> Michael Kifer wrote:
>>
>> On Thu, 10 Jul 2008 17:03:25 +0200
>> Jos de Bruijn <debruijn@inf.unibz.it> wrote:
>>
>>>
>>> Michael Kifer wrote:
>>>> This was not an omission, but I am fine with separating external from
>>>> non-external symbols for functions and predicates. 
>>> ok, good
>>>
>>>  > As to the frames, I do not think any of the symbols
>>>> should be required to be external.
>>> But then the same constant used in different contexts has a different 
>>> meaning, which I think was something we were trying to avoid in BLD.
>>
>> Frames are reflexive by nature. So, in some other statement you may 
>> want to
>> say that some object (even external one) has a particular set of 
>> properties and
>> list them).
>>
>> Frankly, I do not understand why is it a deal to allow the same, say 
>> predicate,
>> to appear inside External(...) and outside of it. The reason for 
>> separating the
>> symbols was to ease the interface with FOL. But separating external and
>> non-external symbols does not affect that.
>>
>> Syntactically it is clear whether a symbol is used as external or 
>> internal, and
>> I see no reason to reinforce this with an additional syntactic kludge (I
>> conceded it just in the interests of peace :-).  If your argument is
>> error-checking then it is not our business. Systems that care about it 
>> would
>> build the appropriate error checkers.
>>
>>
>>     --michael 
>>> Best, Jos
>>>
>>>>
>>>>
>>>>     --michael 
>>>>
>>>> On Thu, 10 Jul 2008 12:57:45 +0200
>>>> Jos de Bruijn <debruijn@inf.unibz.it> wrote:
>>>>
>>>>> There is an issue in BLD, which I unfortunately did not catch 
>>>>> before.  I think it is probably an omission in the definition, but 
>>>>> it is a substantive one.
>>>>> If we all agree that it is indeed an omission, we can probably 
>>>>> address the problem, create a new frozen version, and vote about 
>>>>> publication in the next phone conference on Tuesday.
>>>>> Personally, I am not ready to sign off on publication before this 
>>>>> issue is resolved.
>>>>>
>>>>> The issue is the following: in the definition of well-formed terms, 
>>>>> the set of all symbols is partitioned into predicate symbols, 
>>>>> function symbols, etc. however, no distinction is made between 
>>>>> external and "internal" symbols.  The consequence is that the same 
>>>>> function or predicate symbol can be used both in an external term 
>>>>> and an internal term, and these two terms have different meanings, 
>>>>> i.e., the same constant is interpreted differently based on the 
>>>>> context, which is something we explicitly wanted to avoid in BLD.  
>>>>> So, a built-in function may be used outside an external term and 
>>>>> will be uninterpreted.
>>>>>
>>>>> The problem is easy to fix by defining additional sets of external 
>>>>> predicate function symbols that are disjoint from the other sets of 
>>>>> symbols and defining appropriate restrictions on external terms 
>>>>> (i.e., the first function/predicate symbol in an external term must 
>>>>> be an external symbol).
>>>>> It becomes a bit more tricky when considering external frames, but 
>>>>> probably all constants used in an external frame should be external 
>>>>> individuals/functions/predicates.
>>>>>
>>>>> Best, Jos
> 


-- 
Dr. Axel Polleres, Digital Enterprise Research Institute (DERI)
email: axel.polleres@deri.org  url: http://www.polleres.net/

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Received on Thursday, 10 July 2008 16:11:35 UTC