W3C home > Mailing lists > Public > public-data-shapes-wg@w3.org > May 2016

Re: ISSUE-105: Prefixes in SPARQL fragments

From: Holger Knublauch <holger@topquadrant.com>
Date: Thu, 5 May 2016 13:15:14 +1000
To: public-data-shapes-wg <public-data-shapes-wg@w3.org>
Message-ID: <efefa80f-e5e4-0fce-9fc7-561400bdcbca@topquadrant.com>
Just to give further input of why SHACL should include something like 
sh:prefix.

I collected some statistics about stored SPARQL queries in our product. 
These queries are not only constraints, but most of them are part of 
user interface code (SPARQL Web Pages). We have 899 SELECT queries, 240 
MODIFY, 173 ASK and 43 CONSTRUCT queries. We have 741 SPIN functions, 
101 magic properties. And we have thousands (!) of small SPARQL snippets 
(stand-alone FILTER/BIND expressions) in IF/LET statements that drive 
the UI. It would be nice to migrate to SHACL syntax in the future, but 
having to repeat all used prefix declarations thousands of times will 
lead to excessive bloat. Note that these statistics only include the 
standard UI modules in our product - we have many more cases (such as 
SPIN rules and SPARQLMotion scripts) in custom consulting projects and 
customer deployments.

I acknowledge that all points raised by others here (such as difficulty 
of copy and paste, potential confusion with duplicate prefixes and the 
extra overhead of managing sh:prefix triples) are valid. But these 
points do need to be weighed against the cost that also will be there if 
we don't provide anything.

My strong preference remains to support simple ?namespace sh:prefix 
?prefix triples. This is a predicate that would have been useful to 
standardize a long time ago, for use cases that go beyond SHACL. For 
example it is a perfectly valid scenario that people upload and edit 
resources to a database (that may not manage prefixes) yet want to track 
how these will look like when serialized back to files. It is also a 
common requirement to query the prefix in SPARQL, e.g. to build display 
strings or abbreviations.

In SPIN we did not have this issue because SPIN uses an RDF data model 
to represent queries. This avoids the prefix question because proper 
references to URIs are used, but has the drawbacks that it leads to 
almost unmaintainable Turtle files and that the verbatim syntax as 
entered by the user is lost (e.g. indentation, line breaks formatting 
and # comments). For people who are using SPARQL heavily (such as in 
TQ's products), these aspects are very important.

Holger



On 24/04/2016 17:41, Eric Prud'hommeaux wrote:
>
> On Apr 24, 2016 7:15 AM, "Holger Knublauch" <holger@topquadrant.com 
> <mailto:holger@topquadrant.com>> wrote:
> >
> > That's an interesting thought, Peter. We could extend and generalize 
> this further, basically eliminating the problem of possible prefix 
> conflicts: Each sh:sparql triple is already wrapped into an object 
> (such as sh:SPARQLValuesDeriver, sh:SPARQLSelectValidator, subclasses 
> of sh:SPARQLExecutable) in the current draft. These wrapper objects 
> could all have another property sh:prefixes, making it explicit which 
> prefixes are to be used:
> >
> > sh:DefaultPrefixes
> >     a sh:PrefixDeclaration ;
> >     sh:prefix [ sh:prefix "rdf" ; sh:namespace "http://..." ] ;
> >     sh:prefix [ sh:prefix "owl" ; sh:namespace "http://..." ] ;
> >     ...
> >
> > ex:MyPrefixes
> >     a sh:PrefixDeclaration ;
> >     sh:extends sh:DefaultPrefixes ;
> >     sh:prefix [ sh:prefix "ex" ;  sh:namespace 
> "http://example.com/ns#" ] .
> >
> > and then
> >
> >
> > ex:MyShape
> >     a sh:Shape ;
> >     sh:property [
> >         sh:predicate ex:grandParent ;
> >         sh:derivedValues [
> >             a sh:SPARQLValuesDeriver ;
> >             sh:prefixes ex:MyPrefixes ;
> >
> >             sh:sparql "$this ex:parent/ex:parent ?value" ;
> >         ]
> >     ] .
> >
> > Very chatty syntax, but hopefully closer to acceptable to those who 
> are currently against the whole idea.
>
> This doesn't address Mark's comment about cut/pastability but very 
> nicely reduces the uncertainty associated with default prefixes.
>
> > Holger
> >
> >
> > On 23/04/2016 22:52, Peter F. Patel-Schneider wrote:
> >>
> >> Another possible approach would be to have a property on these 
> constructs that
> >> provided prefixes for the construct only, as in
> >>
> >> ex:MyShape
> >>      a sh:Shape ;
> >>      sh:property [
> >>          sh:predicate ex:grandParent ;
> >>          sh:derivedValues [
> >>              a sh:SPARQLValuesDeriver ;
> >>              sh:prefixHere ( "ex" "http://example.com" ) ;
> >>              sh:sparql "$this ex:parent/ex:parent ?value" ;
> >>          ]
> >>      ] .
> >>
> >> On 04/22/2016 04:35 PM, Holger Knublauch wrote:
> >>>
> >>> I just thought about another problem with prefixes. Currently we 
> have one
> >>> construct in the spec that operates on SPARQL fragments:
> >>>
> >>> ex:MyShape
> >>>      a sh:Shape ;
> >>>      sh:property [
> >>>          sh:predicate ex:grandParent ;
> >>>          sh:derivedValues [
> >>>              a sh:SPARQLValuesDeriver ;
> >>>              sh:sparql "$this ex:parent/ex:parent ?value" ;
> >>>          ]
> >>>      ] .
> >>>
> >>> Without something like sh:prefix, people would need to always 
> spell out the
> >>> full URIs, because they cannot create PREFIX declarations in a SPARQL
> >>> fragment. We already do have other use cases of such fragments in 
> our tool
> >>> suite (for expressions that constraint the applicability of menu 
> items etc),
> >>> and it is quite plausible that the WG may want to adopt the 
> following syntax
> >>> for path-based constraints in the future:
> >>>
> >>> ex:MyShape
> >>>      a sh:Shape ;
> >>>      sh:constraint [
> >>>          a sh:PathConstraint ;
> >>>          sh:path "ex:parent/ex:parent" ;
> >>>          sh:minCount 2 ;
> >>>          sh:maxCount 4 ;
> >>>      ] .
> >>>
> >>> Without sh:prefix we would essentially rule out these syntactic 
> choices,
> >>> unless we can get users enthusiastic about always using full URIs.
> >>>
> >>> As always, there are work-arounds, e.g. we could force everything 
> to be SELECT
> >>> or ASK queries. But at some stage I find the pain points are 
> really becoming
> >>> too big.
> >>>
> >>> Thanks,
> >>> Holger
> >>>
> >>>
> >
> >
>
Received on Thursday, 5 May 2016 04:23:43 UTC

This archive was generated by hypermail 2.4.0 : Friday, 17 January 2020 19:30:33 UTC