Re: #undef resource in Andy's ResultSet vocabulary

Seaborne, Andy wrote:
> Arjohn,
> 
> There wasn't a strong reason for doing this except that in RDF the absence
> of something isn't the same as undefined.  Having an undefined value means
> that a result set can explicitly say that the the query did not bind the
> variable x.  As RDF allows merging of graphs, it is possible (although just
> a little strange!) to put different parts of the result in different RDF
> models, giving rows URIs and doing a graph merge.
> 
> Having rs:undef does complicate things though because, like nulls in
> programming langugaes or nil lists, the value is in the space of legal
> values and has a meaning outside that space.  Doing a query on a result set,
> or on the vocabulary, could legitimately bind a variable to rs:undef.  
> 
> I preferred the explicit recording of the fact that the variable was
> undefined (a more complex structure could differentiate the value-as-unbound
> case but it didn't seem worth it).  In conjunctive graph pattern query
> languages, unbound only occurs if the query asks for a variable (e.g. in the
> SELECT) and does not reference it in the graph pattern.

Thanks for the explanation. I had the same line of thought so I was
wondering about the reasons for choosing one over the other.

Arjohn
-- 
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