Hi Andy,
> &n is illegal SPARQL. Use ?n if you want to use the more robust rewiring of
> the AST (abstract syntax tree) or algebra.
>
I think that should be defined a special char (or char sequence) to make
reference to outer variables, variables from rule terms for instance, in a
Sparql command declared in a rule. In this way, it will be clear when we want
to make reference to outer variables. If we use the same char as variables (?),
inadvertently can be made reference to a outer variable and the system
undesirably replace the variable by a value.
Furthermore, and in the best of my knowledge (correct me if I’m wrong), in
SPARQL command, a subquery cannot make reference to outer variables in the
query pattern. If I use the same symbol of variables to make reference to outer
variables, seems that my sparql command has a "special behaviour". In my point
of view, a command SPARQL in a rule should have the same behaviour as a
subquery in a sparql command, except that we can introduce a way to make
reference to outer variables.
I hope that I have clearly exposed my idea. Otherwise, I will try to explain
better.
have you any suggestion for the special char (or char sequence)?
Miguel
On 19 Aug 2014, at 16:15, Andy Seaborne <a...@apache.org> wrote:
> On 19/08/14 16:03, Miguel Bento Alves wrote:
>> Andy,
>>
>> "Another approach is to use a named variable, parse the SPARQL query, which
>> identifies variables, then rewrite the parsed structure to replace
>> occurrences of variable 'n' by a specific value.”
>>
>> in a first glance, it's what I expect to do. If I have a variable ?n
>> instantiated, I will lookup for &n and replace.
>
> That's string processing.
>
> &n is illegal SPARQL. Use ?n if you want to use the more robust rewiring of
> the AST (abstract syntax tree) or algebra.
>
> Andy
>
>>
>> I read carefully your suggestions but I need to start to have a well founded
>> opinion. I will start only in next week.
>>
>> Miguel
>>
>>
>> On 19 Aug 2014, at 14:36, Andy Seaborne <a...@apache.org> wrote:
>>
>>> On 18/08/14 12:17, Miguel Bento Alves wrote:
>>>> Dear all,
>>>>
>>>> I will start to develop an engine to evaluate rules that combines SPARQL
>>>> commands with rule terms. The purpose is to be possible to define rules
>>>> like:
>>>>
>>>> ex:SportsMan ex:minimumSportsPlayed 1 .
>>>> ex:IronMan ex:minimumSportsPlayed 3 .
>>>>
>>>>
>>>> (?x rdf:type ?t) <-
>>>> ?t ex:minimumSportsPlayed ?n .
>>>> (\\\SPARQL
>>>> (Select ?x
>>>> Where {
>>>> ?x ex:playSport ?y .
>>>> }
>>>> group by ?x
>>>> having (count(1) >=&n)
>>>> \\\SPARQL).
>>>>
>>>> Note: in a first approach syntax "&n" is to make reference to ?n .
>>>>
>>>> my question is: how we should make reference to external variables in a
>>>> Sparql command?
>>>>
>>>> To start the discussion, I have two proposals:
>>>> the symbol "&";
>>>> keep the symbol "?", where some variables must be instantiated before
>>>> the execution of the sparql command. Something like:
>>>> (?x rdf:type ?t) <-
>>>> ?t ex:minimumSportsPlayed ?n .
>>>> (\\\SPARQL
>>>> (Select ?x
>>>> Where {
>>>> ?x ex:playSport ?y .
>>>> }
>>>> group by ?x
>>>> having (count(1) >=?n)
>>>> \\\SPARQL).
>>>>
>>>>
>>>> Miguel
>>>>
>>>
>>> Using & means you need to process the query string and so need to be
>>> careful of & used elsewhere (e.g. in a string, in the '&&' operator).
>>>
>>> Another approach is to use a named variable, parse the SPARQL query, which
>>> identifies variables, then rewrite the parsed structure to replace
>>> occurrences of variable 'n' by a specific value.
>>>
>>> If "?t ex:minimumSportsPlayed ?n" has multiple matches, it would need to
>>> loop on ?n.
>>>
>>> I do have some code that rewrites the abstract syntax tree:
>>>
>>> https://github.com/afs/AFS-Dev/tree/master/src/main/java/element
>>>
>>> which you're free to copy and use.
>>>
>>> Or you can do it on the Algebra -- see
>>> com.hp.hpl.jena.sparql.core.Substitute(Op, Binding)
>>>
>>> If you know "?t ex:minimumSportsPlayed ?n" is from the data, you may be
>>> able to convert that part and the SPARQL query into a single algebra
>>> expression bu joining the two parts, and execute the whole right-hand side
>>> as SPARQL.
>>>
>>> Andy
>>>
>>
>
