Ordered List

How do we specify an ordering over a collection of resources?

Context

Sometimes ordering is an important aspect of some collection of data. E.g. the list of authors associated with an academic paper, or the placings on a scoreboard. RDF offers a several different ways to capture ordering across some portion of a graph.

Solution

Use an rdf:List to describe a ordered list of resources.

Example(s)


<http://www.example.com/docs/1> ex:authors (
   <http://www.example.com/author/joe>
   <http://www.example.com/author/bob>
  ).

<http://www.example.com/author/joe> 
  foaf:name "Joe".

<http://www.example.com/author/bob> 
  foaf:name "Bob".


Discussion

RDF offers several modelling options for defining collections of resources. Formally these are the RDF Containers (Sequence, Bag, and Alternates) and the RDF Collections (List). For this purposes of this pattern an RDF Sequence and an RDF List are very similar: both describe an ordered list of resources. Semantically the two structures differ in that a sequence is open ended (i.e. other members may exist, but aren't itemized) while a list is closed (i.e. the members are complete).

In practice though there is no real difference between the structures as data cannot be easily merged into an existing sequence, e.g. to append values. Both also suffer from being poorly handled in SPARQL 1.0: there is no way to query or construct an arbitrary sized list or sequence without extensions; SPARQL 1.1 property paths will remedy the querying aspects.

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