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Re: Alternatives to containers/collections (was Re: Requirements for a possible "RDF 2.0")

From: Pat Hayes <phayes@ihmc.us>
Date: Thu, 14 Jan 2010 23:29:25 -0600
Cc: Geoff Chappell <geoff@sover.net>, 'Dan Brickley' <danbri@danbri.org>, 'Danny Ayers' <danny.ayers@gmail.com>, 'Steve Harris' <steve.harris@garlik.com>, 'Semantic Web' <semantic-web@w3.org>
Message-Id: <537E9963-3CCF-45FB-9E3E-ECA08A0A360A@ihmc.us>
To: Jiří Procházka <ojirio@gmail.com>

On Jan 14, 2010, at 10:50 PM, Jiří Procházka wrote:

> On 01/15/2010 04:17 AM, Pat Hayes wrote:
>>
>> On Jan 14, 2010, at 3:12 PM, Geoff Chappell wrote:
>>
>>> -----Original Message-----
>>> From: semantic-web-request@w3.org [mailto:semantic-web-request@w3.org 
>>> ] On
>>> Behalf Of Pat Hayes
>>> Sent: Thursday, January 14, 2010 3:19 PM
>>> To: Dan Brickley
>>> Cc: Danny Ayers; Steve Harris; Semantic Web
>>> Subject: Re: Alternatives to containers/collections (was Re:  
>>> Requirements
>>> for a possible "RDF 2.0")
>>>
>>>
>>> On Jan 14, 2010, at 11:31 AM, Dan Brickley wrote:
>>>
>>>>> On Thu, Jan 14, 2010 at 4:20 PM, Pat Hayes <phayes@ihmc.us> wrote:
>>>>>> A lot, perhaps all, of this hair could be avoided if RDF allowed
>>>>>> general
>>>>>> tuples as well as triples. All that is needed is some way to  
>>>>>> put N
>>>>>> things
>>>>>> into a sequence: so, put N things into a sequence. The 'graph
>>>>>> model' would
>>>>>> be a hyperlink, drawn as a polygon (eg triangle for N=3) rather
>>>>>> than a line.
>>>>>> In triples-style syntax, it would just be moving a dot.
>>>>>
>>>>> I periodically wonder what an RDF without the binary restriction  
>>>>> would
>>>>> look like.
>>>
>>>> My 2c suggestions, as answers to the questions.
>>>>>
>>>>> Would each property/relation have a fixed arity, eg. dc:source  
>>>>> might
>>>>> 'be a 4', 'foaf:knows' a 7?
>>>
>>>> No. But it might be useful to distinguish 'really binary' ones,  
>>>> which
>>>> only fit in triples, from the others, which can take any number of
>>>> things in the sequence. Or maybe not, whatever. But the default  
>>>> should
>>>> be, any number (even though most of them will be 2 in practice, ie
>>>> triples.)
>>>
>>>>> That doesn't make a lot of sense to me. So
>>>>> presumably they'd vary freely. In which case, we have a lot of
>>>>> figuring out to do when wondering whether   livesWith(alice, bob,
>>>>> 2007, 'y') implies livesWith(alice,bob) or livesWith(alice, bob,  
>>>>> 'y',
>>>>> 'foo.html'). The binary straightjacket makes some of these  
>>>>> questions
>>>>> impossible, albeit maddeningly...
>>>
>>>> The Common Logic answer is very puritan: each number of arguments  
>>>> is a
>>>> separate assertion, and they are all independent (unless you write
>>>> axioms connecting them.) So liveswith(a b c) has nothing to do with
>>>> liveswith(a b) or with liveswith(a b c d), etc., as far as the  
>>>> logic
>>>> itself is concerned.  This is actually quite elegant and works well
>>>> WHEN you can write the axioms you might need. So maybe we would  
>>>> need,
>>>> for RDF, some way to attach some common inference patterns to  
>>>> these by
>>>> giving properties to the property of the tuple. For example, one
>>>> useful and common pattern allows ends of argument lists to be  
>>>> lopped
>>>> off, so that
>>>>
>>>> liveswith(alice, bob, <address>, <maritalstatus>)
>>>> entails
>>>> liveswith(alice, bob)
>>>>
>>>> and we might specify this pattern (in a semantically extended  
>>>> RDF) by
>>>> asserting
>>>>
>>>> liveswith rdf:type rdf:ExtendableProperty .
>>>>
>>>> But this is very much off the top of my head. Whaddayathink?
>>>>
>>>> Pat
>>>
>>> I wonder how much of this needs to be in RDF itself vs. in query/ 
>>> rule
>>> languages that operate over RDF.
>>>
>>> E.g. we support rules in our sparql extensions and while we of  
>>> course
>>> support rules with triples at the head, we also support ones that  
>>> have
>>> n-ary
>>> relations at the head. I find the non-triple variety useful for of  
>>> course
>>> dealing with inferring relations that have a natural arity greater  
>>> than
>>> three but also for just performing transformations without  
>>> polluting the
>>> triple space. Similarly, we have a native list type which is  
>>> useful for
>>> things like accumulating values -- something that would be  
>>> extremely ugly
>>> with a pure triple syntax. In both cases I find the extensions
>>> useful/necessary for processing RDF efficiently, but I never really
>>> feel the
>>> need to push the extensions into RDF storage/graph layer.
>>
>> Well, fair enough. After all, RDF does already have what are in  
>> effect
>> LISP S-expressions embeddable in it, so this is always *possible*.  
>> But I
>> thought the idea of this thread was to find a way to avoid all this
>> pseudo-Lisp list hacking using triples.
>>
>> <rant>
>> This brings up a broader issue. Everyone agrees its good to keep RDF
>> simple. But keeping RDF simple by making it in effect into a
>> general-purpose construction kit, and then expecting that as a  
>> matter of
>> routine people will use the constructions, isn't really being honest
>> about 'simple'. OWL/RDF is a lot less simple than RDF itself largely
>> because its written in what we might more honestly call
>> OWL-syntax-coded-in-lists-described-using-RDF, which IMO isn't really
>> RDF any more. If we want to expect RDF to do this kind of thing,  
>> then it
>> ought to have a whole datastructuring facility built in explicitly,
>> perhaps along JSON lines, rather than prostituting the triple store  
>> to
>> be a data structuring tool.
>> </rant>
>
> Wasn't this the whole idea of RDF? Simple language for expressing any
> information? Doesn't it use triples, simple relations between two
> objects, specifically for the reason that it is universal, and any  
> kind
> of structure can be expressed by it?

There are two notions of universal here, and we shouldn't get them  
confused. One is a universal programming language, which can be used  
to build arbitrary data structures and define operations over them. In  
other words, in fact, a programming language. The other sense is a  
universal *descriptive* language, which RDF is supposed to be , but  
isn't because it is too simple. (Thats why we need OWL, etc.; though  
RDF is closer than I used to think, see my ISWC talk.)

My point was only that if RDF needs to *use* sequences (like, as data  
structures to encode OWL syntax, for example, or as a way of hacking N- 
ary relations using only binary links) then this is more like building  
data structures than describing anything, and maybe it would be better  
to admit this up front and deal with it with a real datatstructuring  
notation. It wasn't the whole idea of RDF to have it be a kind of  
awkward version of simplified LISP.

> Why should we give preferential
> treatment to relations describing structure than those describing  
> time,
> measures, events, organizations, people, pizzas...?

Because they aren't *descriptions* of structure, they are structures  
being used to describe other things. Just like triples are.

> This is what I always thought what RDF was supposed to be and things
> like containers, blank nodes, literal data types, language tags got in
> because of pragmatical reasons, mainly to give users some kind of
> direction, examples how to get started.

Well, not really. All of these things are needed in order to describe  
things that people wanted to describe, and couldn't be described any  
other way. If you don't have literals, how are you going to use  
numbers in a description? Without language tags, how are you going to  
say that something is written in French? Well, OK, maybe containers...

>
> Reality that even such thing as RDFS was made separate of the RDF  
> itself
> support my view (even though I suspect one of reasons if that  
> separation
> was the process of it's development).

Yes, most the latter. I would have preferred to have one namespace,  
but this was set in stone very early on.

>
> On the other hand I am not against creating community developed
> framework for making common uses of Semantic Web technologies easier,
> defining useful structures and design patterns... but build that *on
> top* of RDF as separate framework.
>
> I would like RDF to be as lightweight (low level) as possible - if I
> were to make mine RDF, there would be no containers, bnodes, literal
> data types, language tags, maybe even no literals - using data URI
> scheme. Anyway I value compatibility and adoption more than sticking  
> to
> my view of the world so I use RDF as it is.
>
> Other alternative would be to have the low level core, build modules  
> on
> top of it, for example like some programming languages do (Python,
> Java...) and call the whole thing RDF - I guess that is what would  
> make
> most people happy.

But this whole idea of 'building modules on top' is a programming  
idea, not a description language idea. It muddles up the notion of a  
description. Which 'level' defines the semantics you should be using  
in a query? There's no way to know.

Pat

>
> Best,
> Jiri Prochazka
>
>>
>> Pat
>>
>>
>>
>>>
>>> Rgds,
>>>
>>> -Geoff
>>>
>>>
>>>
>>>
>>>
>>
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>>
>>
>>
>>
>>
>>
>

------------------------------------------------------------
IHMC                                     (850)434 8903 or (650)494 3973
40 South Alcaniz St.           (850)202 4416   office
Pensacola                            (850)202 4440   fax
FL 32502                              (850)291 0667   mobile
phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
Received on Friday, 15 January 2010 05:30:44 UTC

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