has_datatype? returns false when datatype is XMLSchema#string

[elrayle]

Description

I want to apply ldpath rules that limit returned results to objects with type xsd:string. But I am not able to create a graph with literals that have datatype xsd:string that will return true for datatype?.

The check I want to use is...

type = RDF::URI.new('http://www.w3.org/2001/XMLSchema#string')
rdf_literal if rdf_literal.has_datatype? && rdf_literal.datatype == type

There are multiple literals that have datatype http://www.w3.org/2001/XMLSchema#string, but none match because in all cases rdf_literal.has_datatype? returns false.

Examples

Numeric is auto-assigned a datatype.

>> y = RDF::Literal.new(2)
#<RDF::Literal::Integer:0x3fc1ae1605d0("2"^^<http://www.w3.org/2001/XMLSchema#integer>)>
>> y.has_datatype?
true
>> y.datatype
#<RDF::URI:0x3fc1ab871944 URI:http://www.w3.org/2001/XMLSchema#integer>

String is not auto-assigned a datatype... sort of?

>> x = RDF::Literal.new('hello world')
#<RDF::Literal:0x3fc1ae171484("hello world")>
>> x.has_datatype?
false
>> x.datatype
#<RDF::URI:0x3fc1ae1713a8 URI:http://www.w3.org/2001/XMLSchema#string>

Cannot explicitly assign a datatype by setting a language.

>> value = RDF::Literal.new("Hello!", language: :en)
#<RDF::Literal:0x3fc1ae148d2c("Hello!"@en)>
>> value.has_datatype?
false
>>value.datatype
#<RDF::Vocabulary::Term:0x3fc1aac306bc ID:http://www.w3.org/1999/02/22-rdf-syntax-ns#langString>

Cannot explicitly assign a datatype to a string literal.

>> value = RDF::Literal.new("hello world", datatype: RDF::XSD.string)
#<RDF::Literal:0x3fc1ae1314b0("hello world")>
>> value.has_datatype?
false
>> value.datatype
#<RDF::Vocabulary::Term:0x3fc1aad352d8 ID:http://www.w3.org/2001/XMLSchema#string>

[elrayle]

https://github.com/ruby-rdf/rdf/blob/develop/lib/rdf/model/literal.rb#L342-L351

States that strings are excluded for historical reasons. Can someone elaborate?

[gkellogg]

The notion that a literal with datatype xsd:string somewhat predates RDF 1.1, and there are a number of different gems that look to see if a literal has a datatype. With RDF 1.1 all literals have a datatype, so xsd:string is treated as if there is no datatype, same with rdf:langString. Otherwise, it's not useful to ask if a literal has a datatype.

As you note, "foo" and "foo"^^xsd:string are equivalent, which was decided as part of RDF 1.1. There is no way create a literal without a datatype anymore.

In RDF 1.0, and normatively, but not practically, in SPARQL 1.1, "foo" and "foo"^^xsd:string are different terms, but with RDF 1.1, pretty much all processors now treat them as being the same term. The datatype rdf:langString was created to ensure that all literals have a datatype.

Much 1.0 code that looked to distinguish between datatyped literals, and literals with no datatype with or without a language would make use of this. And, serializations of literals datatype xsd:string or rdf:langString will omit the datatype during serialization. The Changes since RDF 1.0 document may be useful in seeing these differences.

[no-reply]

proposing to close with no changes, per @gkellogg's comment.

the main alternative seems to be to deprecate and remove has_datatype?