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

From: Jos de Bruijn <debruijn@inf.unibz.it>
Date: Mon, 14 Jul 2008 10:06:51 +0200
Message-ID: <487B091B.7060606@inf.unibz.it>
To: Chris Welty <cawelty@gmail.com>
CC: Axel Polleres <axel.polleres@deri.org>, kifer@cs.sunysb.edu, RIF <public-rif-wg@w3.org>

Chris Welty wrote:
> This appears to be unresolved.  I see the problem Jos points out, that 
> you can have the same symbol (and most problematically I guess if its 
> the same URI) that *could* syntactically denote both an external 
> predicate and an internal one. This *seems* to violate the design of the 
> semantic web (URIs are rigid designators).
> </chair>
> Personally, I think the internal/external distinction is outside the 
> scope of the semantic web anyway, so I don't think this matters.  I am 
> happy to leave it as is.  It merely *allows* one to use the same symbol 
> in different RIF contexts, it doesn't require it.
> <chair>
> Anyway, we need to wrap this up.  Were you OK with things as they are, Jos?

I am OK with the way things are *currently*.

Best, Jos

> -Chris
> Axel Polleres wrote:
>> 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


Jos de Bruijn,        http://www.debruijn.net/
One man that has a mind and knows it can
always beat ten men who haven't and don't.
   -- George Bernard Shaw

Received on Monday, 14 July 2008 08:07:47 UTC

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