Hi Andy,
&n is illegal SPARQL, ok, but that does not means that can be used to refer a
outer variable? The symbol “&” can appear in a SPARQL command (excepting in a
String)?
and the symbol “$”?
BTW, I almost finished the development of an engine to evaluate rules that
combines rules terms with sparql commands. To finish, I only need to define the
special char to make reference to outer variables (for now, and to develop the
engine, I'm using a non-valid char).
Miguel
On 27 Aug 2014, at 18:51, Miguel Bento Alves <mbentoal...@gmail.com> wrote:
> 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
>>>>
>>>
>>
>
