[Jscheme-user] Scheme implementation of Jena RDQL

[Ken A. <kan...@bb...>]

Here's an example of how powerful Scheme can be.
You can replace over 5,000 lines of Java with less than 100 lines of Scheme!

I've been working with Jena, a Java framework for building Semantic
Web applications, with RDF and OWL, http://jena.sourceforge.net.

RDF is a XML way of representing Statements of the form of triples
like (subject predicate object), but uglier.  A set of such statements
is called a Model.

Jena provides RDQL, an RDF query language.  Here's two examples:

SELECT ?property
WHERE (?person, <vCard:FN>, "John Smith") ,
      (?person, ?property, ?obj)
USING 
   vCard FOR <http://www.w3.org/2001/vcard-rdf/3.0#> ,
   rdf   FOR <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

This says "find the person who's full name is "John Smith" and list
the names of the properties he has".

SELECT ?resource
WHERE (?resource, <info:age>, ?age)
AND ?age >= 24
USING info FOR <http://somewhere/peopleInfo#>

RDF has a primitive for querying a model to return a sequence of
statements that match the given subject predicate and object, which
#null meaning "don't care".

(.listStatements model subject predicate object)

I figured it wouldn't be hard to implement this kind of query language
in JScheme.  Here are two examples using
"http://jena.sourceforge.net/tutorial/RDQL/vc-db-2.rdf" as a model.

(select (?property)
        (join (?person VCARD.FN$ "John Smith")
              (?person ?property ?object)
              (from db)))

produces:
(http://www.w3.org/2001/vcard-rdf/3.0#N)
(http://www.w3.org/2001/vcard-rdf/3.0#FN)
(http://somewhere/peopleInfo#age)

Its macroexpansion is:
(map* (lambda (r)
        (let ((?person (.getSubject r))
              (?property (.getPredicate r))
              (?object (obj r)))
          (list ?property)))
      (let ((source (from db)))
        (apply append
               (map* (lambda (r)
                       (let ((?person (.getSubject r)))
                         (jenaq source ?person #null #null)))
                     (jenaq source #null VCARD.FN$ "John Smith")))))

;;; Select any resource who's age is >= 24.
(select (?resource)
        (filter (>= (string->number ?age) 24)
                (where (?resource INFO:AGE ?age)
                       (from
"http://jena.sourceforge.net/tutorial/RDQL/vc-db-2.rdf"))))

produces:

((http://somewhere/JohnSmith/))

Its macroexpansion is:
(map* (lambda (r)
        (let ((?resource (.getSubject r))
              (?age (obj r)))
          (list ?resource)))
      (filter
       (lambda (r)
         (let ((?resource (.getSubject r))
               (?age (obj r)))
           (>= (string->number ?age) 24)))
       (jenaq (from "http://jena.sourceforge.net/tutorial/RDQL/vc-db-2.rdf")
              #null
              INFO:AGE
              #null)))

One might argue that i must have cut some corners, which i have.  For example, there is no syntax checking and the query could be written more efficiently (by combining the map* and filter into a single pass).  But i can't imagine this taking up much more code.

k