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Re: ISSUE-12: xs:string VS plain literals: proposed resolution

From: Lee Feigenbaum <lee@thefigtrees.net>
Date: Wed, 04 May 2011 14:43:47 -0400
Message-ID: <4DC19E63.3090803@thefigtrees.net>
To: Eric Prud'hommeaux <eric@w3.org>
CC: Alex Hall <alexhall@revelytix.com>, Pat Hayes <phayes@ihmc.us>, Antoine Zimmermann <antoine.zimmermann@insa-lyon.fr>, public-rdf-wg <public-rdf-wg@w3.org>
On 5/4/2011 2:29 PM, Eric Prud'hommeaux wrote:
> * Alex Hall<alexhall@revelytix.com>  [2011-05-04 14:08-0400]
>> On Wed, May 4, 2011 at 1:36 PM, Lee Feigenbaum<lee@thefigtrees.net>  wrote:
>>
>>> On 5/4/2011 1:17 PM, Pat Hayes wrote:
>>>
>>>>
>>>> On May 4, 2011, at 9:08 AM, Lee Feigenbaum wrote:
>>>>
>>>>   I'd like to understand if the proposed resolution of this issue is
>>>>> ("merely") a recommendation, or is a change to RDF syntactic equality. In
>>>>> particular, will we be changing
>>>>> http://www.w3.org/TR/rdf-concepts/#section-Literal-Equality such that
>>>>> "foo" and "foo"^^xsd:string are equal literals?
>>>>>
>>>>> Looking at this through SPARQL's eyes (as I am wont to do), one of the
>>>>> goals of this change is so that I can write:
>>>>>
>>>>> SELECT ... { ?s :p "foo" }
>>>>>
>>>>> and have that match whether the data that was loaded into the store was
>>>>> "foo" or "foo"^^xsd:string.
>>>>>
>>>>> Recommending that stores canonicalize to "foo" would be one way to
>>>>> accomplish this, but only for new data. (And even then, is only a
>>>>> recommendation.) If we changed (or made a SHOULD-style change) literal
>>>>> equality, then the above query would match against :s :p "foo"^^xsd:string
>>>>> as well as :s :p "foo", which -- for me -- is the goal of this issue.
>>>>>
>>>>
>>>> Well, have SPARQL decide that the appropriate entailment is
>>>> {xsd:string}-entailment (that is, D-entailment where D={xsd:string}), and
>>>> that fixes the necessary matching. Seems to me that this is not RDF
>>>> business, in fact. RDF already provides the machinery for doing this, all
>>>> SPARQL has to do is use the existing RDF specs appropriately.
>>>>
>>>
>>> Then maybe I don't understand the original motivation behind ISSUE-12 in
>>> this working group at all.
>>>
>>> *shrug*
>>>
>>>
>> > From what I can tell based on looking at the charter, the original
>> motivation was exactly what you stated: to make querying for string data
>> simpler in SPARQL.
>>
>> Unfortunately, the only ways I can see of making that work transparently in
>> SPARQL are:
>> 1. Follow Pat's suggestion and define SPARQL BGP matching in terms of
>> {xsd:string}-entailment.
>> 2. Modify the abstract syntax specified in RDF Concepts so that there's only
>> one way of expressing string data in an RDF literal, which seems to be what
>> you're asking for.
>
> 3. Add a little text saying that plain literals are preferred to
> literals of type xsd:string.
>
> The RDB2RDF WG faced this in defining the Direct Mapping of relational
> databases to RDF. The ISO SQL committee provides a mapping of SQL
> types to XSD types, and naturally SQL's string types (STRING, CHAR(n),
> VARCHAR(n)) map to xsd:string. Because we didn't want to needlessly
> encumber users with a typed literal when a plain literal would do, we
> overrode the mapping for strings (ints, etc. still map per ISO). A
> little guidance text could encourage others to do the same and
> unification will get that much easier.

This isn't a new suggestion; this is apparently what this WG is already 
doing. It's also what I (and Alex) are saying seems like not very 
effective. And what I'm saying is potentially not worth the time.

Lee

>
>> I'm not fundamentally opposed to either of those approaches, but they both
>> would require significant changes to deployed code.  Given a choice, I would
>> go with the second one because I don't think the problem is confined to
>> SPARQL.  I personally think that making a breaking change to the abstract
>> syntax would be worthwhile in this case because string data is so pervasive,
>> but I wouldn't be surprised if there's backlash from the community over
>> that.
>>
>> The proposed resolution for ISSUE-12 appears to me to be avoiding making any
>> breaking changes by recommending that data producers prefer one form
>> syntactic form over another.  I share your skepticism over how well that
>> will work in the long run.
>>
>> -Alex
>>
>>
>>
>>> Lee
>>>
>>>
>>>
>>>> Pat
>>>>
>>>>
>>>>> (SPARQL defines matching based on subgraphs, which in terms is based on
>>>>> RDF graph equivalence.)
>>>>>
>>>>> I'm not an expert on the RDF standards documents, admittedly, so I might
>>>>> be missing something.
>>>>>
>>>>> thanks,
>>>>> Lee
>>>>>
>>>>> On 5/4/2011 6:04 AM, Antoine Zimmermann wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>>
>>>>>> With respect to ISSUE-12, I propose that we reformulate the resolution
>>>>>> as follows:
>>>>>>
>>>>>> "PROPOSED: Recommend that data publishers use plain literals instead of
>>>>>> xs:string typed literals and tell systems to silently convert xs:string
>>>>>> literals to plain literals without language tag."
>>>>>>
>>>>>> In the text of the spec, we may want to add some more details, saying:
>>>>>>
>>>>>> "In XSD-interpretations, any xs:string-typed literal "aaa"^^xs:string is
>>>>>> interpreted as the character string "aaa", that is, it is the same as
>>>>>> the plain literal "aaa". Thus, to ensure a canonical form of character
>>>>>> strings and better interoperability, we recommend that data publishers
>>>>>> always use plain literals instead of xs:string typed literals and tell
>>>>>> systems to silently convert xs:string literals to plain literals without
>>>>>> language tag whenever they occur in an RDF graph."
>>>>>>
>>>>>>
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>
>>>>>
>>>>>
>>>> ------------------------------------------------------------
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>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>
Received on Wednesday, 4 May 2011 18:44:19 GMT

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