HTTP POST to SPARQL server, get empty HTTP 200 back - ios

Question

I'm sending an HTTP POST request to http://api.artsholland.com/sparql?apiKey=27b6abd4129fc9ced3aa5390fd4fb15b
with this query:
string query =
"PREFIX ah: <http://purl.org/artsholland/1.0/> "
"PREFIX data: <http://data.artsholland.com/> "
"PREFIX nub: <http://resources.uitburo.nl/> "
"PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns/> "
"PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema/> "
"PREFIX owl: <http://www.w3.org/2002/07/owl/> "
"PREFIX dc: <http://purl.org/dc/terms/> "
"PREFIX foaf: <http://xmlns.com/foaf/0.1/> "
"PREFIX xsd: <http://www.w3.org/2001/XMLSchema/> "
"PREFIX time: <http://www.w3.org/2006/time/> "
"PREFIX geo: <http://www.w3.org/2003/01/geo/wgs84_pos/> "
"PREFIX vcard: <http://www.w3.org/2006/vcard/ns/> "
"PREFIX osgeo: <http://rdf.opensahara.com/type/geo/> "
"PREFIX bd: <http://www.bigdata.com/rdf/search/> "
"PREFIX search: <http://rdf.opensahara.com/search/> "
"PREFIX fn: <http://www.w3.org/2005/xpath-functions/> "
"PREFIX gr: <http://purl.org/goodrelations/v1/> "
"SELECT ?v ?p ?date ?title ?desc ?modified ?type ?genre "
"WHERE { "
"?v a ah:Venue . "
"?p dc:modified ?modified . "
"?e ah:venue ?v . "
"?e ah:production ?p . "
"OPTIONAL {{ "
"?p dc:title ?title . "
"}} "
"OPTIONAL {{ "
"?p ah:shortDescription ?desc . "
"}} "
"OPTIONAL {{ "
"?p ah:productionType ?type . "
"}} "
"OPTIONAL {{ "
"?p ah:genre ?genre . "
"}} "
"?v geo:geometry ?geometry . "
"FILTER (search:distance(?geometry, \"POINT(4.890 52.3764)\"^^<http://rdf.opensahara.com/type/geo/wkt>) < 2000 * " "0.00000898315284) . "
"?e time:hasBeginning ?date . "
"FILTER (?date >= \"2012-09-06T00:00:00Z\"^^xsd:dateTime) . "
"FILTER (?date <= \"2012-09-06T23:59:59Z\"^^xsd:dateTime) . "
"} ORDER BY ?v "
"LIMIT 100 ";
with these form fields:
ofxHttpForm form;
form.action = action_url;
form.method = OFX_HTTP_POST;
form.addFormField("query", query);
form.addFormField("output", "json");
httpUtils.addForm(form);
I get an 200 OK back, but no body. I've adapted the query several times, but still no valid return :(
The library used is https://github.com/arturoc/ofxHttpUtils

Answer 1

I tried your SPARQL query on the same endpoint (using the endpoint's web interface) with both the demo key and your key, but got HTTP 500 Internal server errors (instead of empty HTTP 200 responses).
I did get results when I used the correct namespaces and replaced "{{" by "{" in OPTIONAL blocks. The namespaces that the web interface predefines are
PREFIX ah: <http://purl.org/artsholland/1.0/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX owl: <http://www.w3.org/2002/07/owl#>
PREFIX dc: <http://purl.org/dc/terms/>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX time: <http://www.w3.org/2006/time#>
PREFIX geo: <http://www.w3.org/2003/01/geo/wgs84_pos#>
PREFIX vcard: <http://www.w3.org/2006/vcard/ns#>
PREFIX osgeo: <http://rdf.opensahara.com/type/geo/>
PREFIX bd: <http://www.bigdata.com/rdf/search#>
PREFIX search: <http://rdf.opensahara.com/search#>
PREFIX fn: <http://www.w3.org/2005/xpath-functions#>
PREFIX gr: <http://purl.org/goodrelations/v1#>
PREFIX gn: <http://www.geonames.org/ontology#>
You have to use these (note the # instead of / in some of them, it matters).