sparql_client.pl -- SPARQL client library

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')
                ]).

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

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])
                ]).

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).

ssl_verify(+SSL, +ProblemCert, +AllCerts, +FirstCert, +Error)[private]

Currently we accept all certificates.

read_reply(+Status, +ContentType, +In, -Close, -Row)[private]

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/.

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

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.

sub_element(+DOM, +Name, -Atttribs, -Content)[private]

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/