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Re: 2001-09-07#5 Literals (use cases/test inputs, please?)

From: Graham Klyne <Graham.Klyne@MIMEsweeper.com>
Date: Fri, 19 Oct 2001 23:51:06 +0100
Message-Id: <5.1.0.14.2.20011019230533.00ab58d0@joy.songbird.com>
To: jos.deroo.jd@belgium.agfa.com
Cc: w3c-rdfcore-wg@w3.org
At 10:43 PM 10/19/01 +0100, jos.deroo.jd@belgium.agfa.com wrote:
> > I'm starting to think this is worth changing the charter over:
> > if the xml:lang stuff is to be significant, it must be
> > in triple form.
>
>and Sergey's proposal S3
>[[
>   (S3) use bNodes [M&S,TBL]
>
>       Examples: John_Smith weight [units Pounds, rdf:value "10"], or
>                 John_Smith weight [pounds [decimal "10"]]
>]] -- http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Oct/0343.html
>[TBL] http://www.w3.org/DesignIssues/InterpretationProperties.html
>
>I find that a very reasonable path to follow!

Yes, but I have concerns that it could break some reasonable existing 
interpretations.
I like the general idea of capturing the language/type information in RDF 
triples, but I can see some possible difficulties.  Let's see if I can 
build this by example:

(1)
    <ex:Subj>
      <ex:prop>chat</ex:prop>
    </ex:Subj>
-->
    ex:Subj ex:prop "chat" .

I think this is pretty uncontroversial.  But let's see what happens when 
language tagging (or data typing) information is introduced:

(2)
    <ex:Subj>
      <ex:prop xml:lang="en">chat</ex:prop>
    </ex:Subj>
-->
    ?
I feel that it is important that the triple:

    ex:Subj ex:prop "chat" .

should still be generated;  i.e. having the presence of xml:lang force an 
"interpretation property" form, such as:

(2a)
    ex:Subj ex:prop _:a .
    _:a lang:en "chat" .

would be confusing, because adding the xml:lang property (compared with 
(1)) makes a triple disappear.  That seems to me like a landmine for unwary 
developers.  Also, it presumes that the extra information is available at 
triple-generation time (which is true for xml:lang, but maybe not for data 
typing).

Similar considerations would apply to an translation like:
(2b)
    ex:Subj ex:prop _:a .
    _:a rdf:value "chat" .
    _:a xml:lang "en" .

which is what the example cited above from Sergey's (S3) seems to suggest.

Another approach, overcoming this objection, is to interpret (2) as:
(2c)
    ex:Subj ex:prop "chat" .
    "chat" xml:lang "en" .

but this introduces a different set of considerations:

- we have to admit literals as subjects, at least in the abstract 
syntax.  (I don't see this as a problem, just something to be faced.)

- each *instance* of a given literal may have a different model theoretic 
interpretation;  e.g. the "chat"s in:

    "chat" xml:lang "en" .
    "chat" xml:lang "fr" .

must denote different things, I think.  In turn, I think this means that 
each *instance* of a literal must correspond to a different graph node 
(which have distinct identities to hand the different interpretations onto).

Yet another approach is suggested by DanCs technical note on using XML 
schema data types:
(2d)
    ex:Subj ex:prop "chat" .
    ex:Subj _:prop _:a .
    _:a rdf:value "chat" .
    _:a xml:lang "en" .

This avoids all the above issues, but requires the introduction of an 
unidentified property (_:prop).  I have a really hard time to see how that 
would work in practice, especiially if ex:Subj has multiple qualified 
properties:

    ex:Subj ex:does "chat" .
    ex:Subj ex:about "chat" .
    ex:Subj _:prop _:a .
    _:a rdf:value "chat" .
    _:a xml:lang "en" .
    ex:Subj _:prop _:b .
    _:b rdf:value "chat" .
    _:b xml:lang "fr" .

 From this, not knowing the association between the unknown properties and 
the given properties, it's not possible to tell which literal is to be 
interpreted in English, and which in French.

...

Having got this far, I find myself wondering if a combination of Dan's 
approach PLUS interpretation properties could work:
(2e)
    ex:Subj ex:prop "chat" .
    ex:Subj ex:prop _:a .
    _:a lang:en "chat" .

In this case, the presence of xml:lang qualifier forces the generation of 
two extra triples, having the form of blank node and an interpretation 
property, but leaves the original property intact.  But this too seems to 
have some subtle problems:  suppose it were used as an element of a 
daml:Collection type of list -- would the duplication of a listhead 
property not result in an ill-formed list when subjected to further analysis?

...

If one accepts that language tagging can be viewed as kind of type 
information, I think many of the issues raised by xml:lang are also raised 
when one wants to use XML schema data types (or any other data types).  A 
clean solution to this problem will address both sets of issues.

I find all of the above feel somewhat unsatisfactory, except (2c), and that 
approach requires that some aspects of the model theory need to be 
fundamentally revisited, which appears to me like a big bite to take in RDF 
1.0.  Earlier today, I was thinking that we should do as little as possible 
about data typing because it seems to be such a difficult area, but having 
xml:lang to deal with seems to require us to face the issues now.

I have ignored the issues of treating the language information as part of 
the literal itself, but as we have seen that seems to introduce its own set 
of difficulties, and I don't think it really helps with the data typing 
case (e.g. we can't infer this extra information from schema data types 
when available).  (Maybe this is what Jan meant when he talked today about 
two levels of typing?)

Still undecided,

#g


------------------------------------------------------------
Graham Klyne                    MIMEsweeper Group
Strategic Research              <http://www.mimesweeper.com>
<Graham.Klyne@MIMEsweeper.com>
------------------------------------------------------------
Received on Friday, 19 October 2001 18:57:44 EDT

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