ruby-rdf / sparql-client / 164

96%

Pull #66

travis-ci

raise error when no suitable rdf reader is found

Currently sparql-client will silently fail with a nil if no suitable rdf reader is found. This can lead to the awkward situation where a nil is returned for the query and the user has to find out what is causing it. This pull request tries to quicken this debugging proces by at least indicating the point of failure by raising a descriptive error.
```
[1] pry(main)> require 'sparql/client'
=> true
[2] pry(main)> s = SPARQL::Client.new('http://localhost:8890/sparql')
=> #<SPARQL::Client:0x8fc(http://localhost:8890/sparql)>
[3] pry(main)> s.query("CONSTRUCT  {?s a ?type}  WHERE {?s a ?type}").first
NoMethodError: undefined method `first' for nil:NilClass
from (pry):3:in `<eval>'
[4] pry(main)> require 'rdf/turtle'
=> true
[5] pry(main)> s = SPARQL::Client.new('http://localhost:8890/sparql')
=> #<SPARQL::Client:0x902(http://localhost:8890/sparql)>
[6] pry(main)> s.query("CONSTRUCT  {?s a ?type}  WHERE {?s a ?type}").first
=> [#<RDF::URI:0x906 URI:http://www.openlinksw.com/virtrdf-data-formats#default-iid>, #<RDF::URI:0x908 URI:http://www.w3.org/1999/02/22-rdf-syntax-ns#type>, #<RDF::URI:0x90a URI:http://www.openlinksw.com/schemas/virtrdf#QuadMapFormat>]
```

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