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sparql_client.pl -- SPARQL client library
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This module provides a SPARQL client. For example:

?- sparql_query('select * where { ?x rdfs:label "Amsterdam" }', Row,
                [ host('dbpedia.org'), path('/sparql/')]).

Row = row('http://www.ontologyportal.org/WordNet#WN30-108949737') ;
false.

Or, querying a local server using an ASK query:

?- sparql_query('ask { owl:Class rdfs:label "Class" }', Row,
                [ host('localhost'), port(3020), path('/sparql/')]).
Row = true.

HTTPS servers are supported using the scheme(https) option:

?- sparql_query('select * where { ?x rdfs:label "Amsterdam"@nl }',
                Row,
                [ scheme(https),
                  host('query.wikidata.org'),
                  path('/sparql')
                ]).
Source sparql_query(+Query, -Result, +Options) is nondet
Execute a SPARQL query on an HTTP SPARQL endpoint. Query is an atom that denotes the query. Result is unified to a term rdf(S,P,O) for CONSTRUCT and DESCRIBE queries, row(...) for SELECT queries and true or false for ASK queries. Options are

Variables that are unbound in SPARQL (e.g., due to SPARQL optional clauses), are bound in Prolog to the atom '$null$'.

endpoint(+URL)
May be used as alternative to Scheme, Host, Port and Path to specify the endpoint in a single option.
host(+Host)
port(+Port)
path(+Path)
scheme(+Scheme)
The above four options set the location of the server.
search(+ListOfParams)
Provide additional query parameters, such as the graph.
variable_names(-ListOfNames)
Unifies ListOfNames with a list of atoms that describe the names of the variables in a SELECT query.

Remaining options are passed to http_open/3. The defaults for Host, Port and Path can be set using sparql_set_server/1. The initial default for port is 80 and path is /sparql/.

For example, the ClioPatria server understands the parameter entailment. The code below queries for all triples using _rdfs_entailment.

?- sparql_query('select * where { ?s ?p ?o }',
                Row,
                [ search([entailment=rdfs])
                ]).
Source sparql_extra_headers(-List)[private]
Send extra headers with the request. Note that, although we also process RDF embedded in HTML, we do not explicitely ask for it. Doing so causes some (e.g., http://w3.org/2004/02/skos/core to reply with the HTML description rather than the RDF).
Source ssl_verify(+SSL, +ProblemCert, +AllCerts, +FirstCert, +Error)[private]
Currently we accept all certificates.
Source read_reply(+Status, +ContentType, +In, -Close, -Row)[private]
Source sparql_set_server(+OptionOrList)
Set sparql server default options. Provided defaults are: host, port and repository. For example:
    sparql_set_server([ host(localhost),
                        port(8080)
                        path(world)
                      ])

The default for port is 80 and path is /sparql/.

Source sparql_read_xml_result(+Input, -Result)
Specs from http://www.w3.org/TR/rdf-sparql-XMLres/. The returned Result term is of the format:
select(VarNames, Rows)
Where VarNames is a term v(Name, ...) and Rows is a list of row(....) containing the column values in the same order as the variable names.
ask(Bool)
Where Bool is either true or false
Source variables(+DOM, -Varnames)[private]
Deals with <variable name=Name>. Head also may contain <link href="..."/>. This points to additional meta-data. Not really clear what we can do with that.
Source sub_element(+DOM, +Name, -Atttribs, -Content)[private]
Source sparql_read_json_result(+Input, -Result) is det
The returned Result term is of the format:
select(VarNames, Rows)
Where VarNames is a term v(Name, ...) and Rows is a list of row(....) containing the column values in the same order as the variable names.
ask(Bool)
Where Bool is either true or false
See also
- http://www.w3.org/TR/rdf-sparql-json-res/