xsd prefix is not recognized in Turtle file when parsing RDF literals (hashtag2790)

[vikramsubramanian]

I was doing some debugging to double check that the data types of RDF literals are parsed correctly. I have the following file:

 kz: < .
 xsd: < .

kz:Waterloo a kz:City ;
            kz:name "Waterloo" ;
            kz:population 10000.0 ;
            kz:population2 "20000.0"^^xsd:decimal ;
            kz:foundedIn "30000"^^< .

Copying this into an RDFGraph will result in the following set of triples:

---------------------------------------------------------------------------------------------------
| a.iri                          | p.iri                                           | o.val        |
---------------------------------------------------------------------------------------------------
|  |                       | Waterloo     |
---------------------------------------------------------------------------------------------------
|  |                 | 10000.000000 |
---------------------------------------------------------------------------------------------------
|  |                | 20000.0      |
---------------------------------------------------------------------------------------------------
|  |                  | 30000.000000 |
---------------------------------------------------------------------------------------------------
|  |  |              |
---------------------------------------------------------------------------------------------------

So the behavior is this:

• 10000.0 is automatically, by the parser, recognized and tagged as xsd:decimal (or xsd:double).
• 20000.0 is recognized as string (this is the bug, it should be recognized as xsd:decimal). This is because the RDFUtils::getLogicalTypeID function sees the string xsd:decimal instead of which is what it should be expanded to because I have the xsd: < . prefix at the top.
• 30000.0 is recognized as xsd:decimal

First we should recognize prefix namespaces in literal datatype tags as well. Second, I think we should recognize xsd in datatype tags even if the prefix xsd is completely missing. I tested this in GraphDB and they do recognize xsd even if its missing (though they do recognize rdf, rdfs, and owl too).

If you prefer: do not recognize xsd without the prefix for now and instead open an issue to have a configuration to support common namespaces including this. If you choose this, make sure you take a note reminding not to forget the xsd prefix and to add tests for it when parsing literal data types. )

[mayil-ai[bot]]

Summary: The Turtle file parser does not recognize the xsd prefix for RDF literals, leading to incorrect parsing of data types.

Possible Solution

Based on the provided information, the issue is with the incorrect parsing of xsd:decimal datatype in RDF literals. The relevant code snippet is from rdf_utils.cpp where the addRdfLiteral function is defined. The function checks the datatype of the literal and attempts to cast it to the appropriate C++ type.

To resolve the issue:

• Ensure that the XSD prefix is correctly defined in rdf_keyword.h and matches the expected URI for XML Schema datatypes (http://www.w3.org/2001/XMLSchema#).
• In rdf_utils.cpp, within the addRdfLiteral function, verify that the type.ends_with(XSD_decimal) condition correctly identifies the xsd:decimal datatype. If necessary, adjust the string comparison to match the full URI of the xsd:decimal datatype.
• Add unit tests to validate the parsing of xsd:decimal and xsd:double literals to ensure that they are correctly recognized and cast to the appropriate C++ types.

Here is a concise solution:

• Confirm that rdf_keyword.h contains the correct URI for XSD:

  static constexpr std::string_view XSD = "http://www.w3.org/2001/XMLSchema#";

• In rdf_utils.cpp, ensure that the addRdfLiteral function correctly identifies xsd:decimal:

  if (type.ends_with(XSD_decimal)) {
    // Handle xsd:decimal
    // ...
  }

• Implement unit tests to check the parsing of xsd:decimal and xsd:double literals.

Code snippets to check

• src/processor/operator/persistent/reader/rdf/rdf_utils.cpp

  This file contains logic for parsing RDF literals and recognizing data types, which is directly related to the issue.

• third_party/serd/src/n3.c

  This file is part of the Serd library used for parsing RDF data, and it contains functions for reading literals and IRIs which may need to be modified to recognize xsd prefixes correctly.

• src/include/common/keyword/rdf_keyword.h

  This header defines common RDF keywords and may be relevant for ensuring the xsd prefix is recognized correctly.

• src/processor/operator/persistent/reader/rdf/rdf_reader.cpp

  This file handles RDF reading and may be relevant for how prefixes are handled during the parsing process.