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Re: Denotation of datatype values

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Fri, 19 Apr 2002 11:34:06 +0300
To: Pat Hayes <phayes@ai.uwf.edu>
CC: RDF Core <w3c-rdfcore-wg@w3.org>
Message-ID: <B8E5B12E.1377A%patrick.stickler@nokia.com>
On 2002-04-19 2:40, "ext Pat Hayes" <phayes@ai.uwf.edu> wrote:


>> The purpose of *all* datatyping idioms is to identify datatype values.
> 
> No, really, it isn't. Dont believe me, check with Graham about DC
> uses of datatype checking. Not everyone is interested in values.

With all due respect to Graham and anyone else, I think that anyone
who says they are interested in constraining a literal to the lexical
space of a datatype but not interested in datatype values has a screw
loose (or several).

>> If an idiom doesn't have that purpose, then it is not a datatyping
>> idiom. If the inline idiom doesn't have that purpose, then it
>> should not be called a datatyping idiom.
> 
> That is a narrow view which not everyone shares. I started off with
> that position, but being on this WG kind of knocked it out of me.

I never got the impression that the WG did not share that view. The
only issue of contention I recall is whether users should be forced
to give explicit denotation in the graph for all values, or whether
one could express the value solely by its lexical form in the graph.

In all cases, it is the datatype value that is central, otherwise,
there's no point to datatyping.

If those proponents of lexical checking weren't concerned about
datatyping, then they shouldn't be concerned about lexical checking.

>> And BTW, I consider an idiom that places a literal in the lexical
>> space of a datatype to have identified a datatype value. How could
>> it not?! So I consider the inline idiom to be a fully valid datatyping
>> idiom.
>> 
>> That *doesn't* mean that the literal node denotes the datatype value,
>> but that ultimately, some application should understand at some level
>> that the RDF graph is talking about the actual datatype value in relation
>> to that particular property
> 
> BUt look what you just said. The MT says A, and the MT is the RDF
> meaning spec, but we should say that an application 'at some level'
> should understand this to mean (not A). Thats not a coherent position
> to take, even if you think A is wrong.

Again, you think I'm saying we should change the meaning of the
single, specific literal node itself. I'm not.

Consider it as *extra* knowledge. Such that, in addition to
saying e.g. that Judy's ex:age is "10", it's also true that,
based on the presence of an rdfd:datatype assertion and the
datatyping MT, Judy's ex:age is understood to be
[ xsd:integer "10" ].

Now, we don't actually need to assert that additional
knowledge in the graph in order to provide it to some
application, do we?
   
>> , even if that datatype value is not denoted
>> in the graph itself.
> 
> No, I insist, we should NOT say that, because it isn't true. An
> application MIGHT do that , but it is not OBLIGED to do that.

The whole point of a datatyping standard for the global interchange
of datatyped knowledge is to *OBLIGE* every application to
interpret the RDF graph and agree on the specific datatype value
identified therein.

If the present MT doesn't do that, then it is IMO insufficient.

> RDF 
> allows an application to make up its own mind on that point. And, I
> might add, if it does do that *and then tries to draw RDF inferences
> on that basis*, it will likely get wrong results. In other words,
> once it has done that, it's on its own, it's out of our sphere, it's
> obeying its own rules, we wash our RDF hands of it. Not our problem.

I both agree and disagree. I agree that we are not responsible for
anything not asserted by the MT. But I disagree that the datatyping
MT should not assert a specific datatype value for each idiom.

>> I.e.
>> 
>>    literal node all by itself          = literal
>>    literal node combined with datatype = datatype value
>> 
>> the latter does *not* change the meaning of the literal
>> itself, no more than 2 + 1 = 3 changes the meaning of 2
>> just because if combined with 1 we get 3.
>> 
>> The datatyping idioms have a meaning that is the sum of their
>> parts -- the literal and the datatype -- and that total meaning
>> does not change the meaning of their component parts.
>> 
>> I tried to ask a question a week ago: does a datatype value have to
>> have an explicit denotation in the graph, by a single node, in order
>> for it to be expressed/identified/communicated by the graph?
>> 
>> I was told, by a combination of silence and limited comments, that
>> no, it does not.
>> 
>> You seem to be arguing that it does.
> 
> I am saying that what an RDF expresses/identified/communicates, in
> some grander scheme of things, is none of our business. Our business
> is to be very clear what exactly it is that the various parts of an
> RDF graph actually *mean*, and to pin that down as accurately and
> unambiguously as we can. Then users can choose to conform to our spoc
> or not, up to them. If they don't, its not our fault if they get into
> a pickle.

Well, that may be your business, but it's certainly not mine, and
I don't think is that of the WG as suggested by the charter.

We clearly have diffferent scopes of focus.

>> That if an RDF graph is going
>> to express/identify/communicate a particular datatype value, that
>> value must have denotation in the graph, or at least explicit
>> definition in the MT.
> 
> Sure. And if the graph is NOT going to express/whatever that value,
> then it need not have any such denotation. And that we allow BOTH
> options, and that we should say so clearly.

But I consider/expect that every complete idiom *does* express
a specific datatype value, even if the inline idiom does not
provide explicit denotation for it in the graph.

Is the value 5 explicitly denoted in the equation 2 + 3?
Is 5 identified/expressed by that equation?
Could you sanely assume any other value is identified/expressed?

I say no, yes, and no.

It's the same with the inline idiom + rdfd:datatype.

>> I considered it to be an important and pivotal question then,
>> and I still do. And the answer to that question seems to be
>> at the root of all of the misunderstanding and disagreement
>> about what meaning the idioms capture and how to define the
>> MT to that end.
>> 
>> My answer to this question is no, it does not. In that a literal
>> node and a datatype associated with that literal node *together*
>> can represent/express/identify/communicate a specific datatype
>> value even if that value has no explicit denotation in the graph
>> by a single node. And I consider that to be the ultimate goal of RDF
>> Datatyping, to communicate datatype values.
> 
> Then Im sorry, but you ought to be writing a different document, with
> a different title: something like "Guide to using RDF in the Stickler
> way", or something. But it shouldn't be the spec., because the spec
> does not mandate only that kind of usage or restrict users to that
> particular goal.

I disagree. And would like the input of the WG to resolve this.

>> 
>> Here's an analogy:
>> 
>> Datatype Idiom:       value = foo("x");
>> Lexical Form Idiom:   function = foo; value = *function("x");
>> Inline Idiom:         function = foo; *function("x");
>> 
>> Now, in the first two cases, the result of executing the
>> function 'foo' with the argument "x" is stored in the variable
>> 'value'. In the last case, the result is not stored, but it
>> is still expressed.
>> 
>> In all cases, the function is executed and the result is
>> obtained.
>> 
>> It is exactly the case with the datatyping idioms. It it
>> analogous to
>> 
>> Datatype Idiom:       bnode = xsd:integer("10");
>> Lexical Form Idiom:   datatype = xsd:integer; bnode = *datatype("10");
>> Inline Idiom:         datatype = xsd:integer; *datatype("10");
>> 
>> In all cases, the lexical to value mapping is defined and the
>> datatype value is identified. That's the whole point of
>> the datatyping idioms.
> 
> Its one point, but not the only point possible. I might only care
> that something is a numeral, and not be interested in its numerical
> value. Perhaps I want to check that it is a valid part of a date in a
> certain format. Im not interested (right now) in the actual date,
> only that this piece of text could be a date (and then I'll send it
> on to the date-checker, and let him worry about the value. We will
> both use datatyping, but probably use different properties, or maybe
> he will use more information about my properties than I am using.)

FOR CRYING OUT LOUD! You *CAN'T* assert that a literal is a valid
lexical form of a datatype without executing the lexical to value
mapping to test if a value is actually represented by that lexical
form!

And the *ONLY* reason why you would care about having a valid lexical
representation of a date is so that some application could *USE*
it as a date, obtain an actual date value from it.

You are talking absolute blithering nonsense here! Forgive my
being so blunt.

Who in their right mind would actually want/need such a twisted,
ridiculous use of RDF datatyping?!

>> 
>> You seem to say that only the bnode idioms actually identify
>> the datatype value, because the value has explicit denotation;
>> and that the inline idiom does not identify a datatype value
>> but only constrains the literal to the lexical space of a datatype.
> 
> Right.
> 
>> But in order to test if the literal is a member of the datatype,
>> you must -- and this is crucial -- attempt to map it to a datatype
>> value, and see if you get an actual value!
> 
> Not at all. How the testing is done depends entirely on code on the
> other side of the API for the datatype. Some might offer a lexical
> check that just does a string-parse, without ever computing a value.
> (For example, you don't need arithmetic to check that a numeral is a
> numeral.) But in any case, that is all external to RDF and irrelevant
> to the issue.

To say that a literal is a valid lexical form of a datatype, by
whatever means, is to say that a lexical to value mapping exists,
and is to identify the value represented by that lexical form.

And the only point in saying that a literal is a valid lexical
form of a datatype is to say that a datatype value is obtainable
via that lexical form.

>> And thus, to say a
>> literal is a lexical form of a datatype is to identify the value
>> that it represents, presuming it is a valid lexical form.
> 
> That is just obviously wrong. The BNF
> 
> <digit> ::= 0|1|2|3|4|5|6|7|8|9
> <numeral> ::= <digit>|<numeral><digit>
> 
> identifies lexical forms without providing any way to even refer to
> their values.

I am not wrong. To say that "10" is a valid lexical form
of xsd:integer, such as by a succesful parse according to
the above BNF, and based on the defined N:1 mapping from
lexical forms to values, we know that "10" represents a
single specific value and have thus identified that
value.

We don't necessarily say how that value will be represented
by a given system, or even if it is representable by a given
system, but we have narrowed down the known universe to a
single specific value.

>> 
>> Thus, I find your intepretation of the inline idiom as *only*
>> constraining literals to valid lexical forms to be both
>> useless insofar as the practical needs of datatyping are concerned
>> (which is to communicate datatype values
> 
> You keep saying that, and it sounds more and more like someone saying
> that Islam is the only true religion. Sorry, but many people disagree.

Well, then let the WG call me a heretic.

>> The expression L2V(I(ddd))("LLL") identifies a specific
>> datatype value, no?
> 
> Yes, so what? The condition on the inline idiom is only that this
> expression is *defined*; it does not refer to its value. If you
> prefer, we could rephrase this differently: the condition is that
> "LLL" is in the lexical space of I(ddd). I didn't phrase it that way
> purely for the sake of mathematical elegance, since the MT only
> assumes the existence of the L2V map, but we could say that "LLL" is
> in the domain of L2V(I(ddd)).

Again, the only practical, rational reason for saying such a thing
is to identify the datatype value represented by the lexical form.

>> 
>> And all three idioms define L2V(I(ddd))("LLL"), no?
>> 
>> And the bnode idioms further make the assignment
>> I(ccc) = L2V(I(ddd))("LLL"), i.e. they fix the datatype
>> value to the bnode, no?
> 
> The bnode idioms do, yes.
>
>> 
>> Thus when I say that the datatyping
>> MT provides a datatype value interpretation for all
>> three idioms, I just don't get why you say that's not
>> correct. If that's not what the MT is saying, then
>> that's what the MT *should* be saying, IMHO, and
>> that is certainly what I thought the MT was saying
>> when I voted in favor of the "stake in the ground".
> 
> Well then evidently there was some misunderstanding. Look, if your
> interpretation were correct, what use would the bnode idioms have?
> They would only say the same as the inline idiom, but using more
> nodes. The whole point of all these idioms is that each says a
> different thing, conveys a different piece of information, but they
> all refer in one way or another to datatypes. They all make use of
> datatyping information in various ways. They all work together
> smoothly and are mutually consistent, but they provide a variety of
> ways of using the information and a variety of degrees of commitment
> to what is being said about ranges and properties.

The only reason for the different idioms is practicality, whether
datatyping is asserted locally or globally, and whether one has to
denote the value in the graph or not (and not having to being
motivated simply by convenience).

Your interpretation that not all of the idioms are intended to
communicate datatype values seems contrary to the whole point
of datatyping (and in a practical sense, bizarre, since if it
is clear what datatype value the idiom identifies, then even
if the graph doesn't explicitly denote it with a single node,
you've still unambiguously communiated that datatype value).

Patrick

--
               
Patrick Stickler              Phone: +358 50 483 9453
Senior Research Scientist     Fax:   +358 7180 35409
Nokia Research Center         Email: patrick.stickler@nokia.com
Received on Friday, 19 April 2002 04:31:16 EDT

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