02 March 2011

Updating RDF Lists with SPARQL

Something the SPARQL Working Group has been thinking about recently is updates to RDF lists.

RDF lists are hard to deal with because they are not first class objects in the RDF data model. Instead they are "encoded" in triples. The encoding using a cons cell like structure whereby each element of the list is a blank node (not necessary a blank node but it nearly always is).

RDF lists are correctly called "RDF collections" but as it's the list-nature (elements in order) that matters, I'll call them lists in this blog.

Turtle and SPARQL has syntax for lists, but it's only surface syntax, and there are really triples in the RDF graph:

@prefix :  .
:x :p (1 2 3) .

is the RDF:

:x    :p         _:b0 .
_:b0  rdf:first  1 .
_:b0  rdf:rest   _:b1 .
_:b1  rdf:first  2 .
_:b1  rdf:rest   _:b2 .
_:b2  rdf:first  3 .
_:b2  rdf:rest   rdf:nil

RDF toolkits help by presenting lists as progamming language lists. This also helps in keeping the lists well formed. In all those triples, there is one rdf:rest and one rdf:first per list element - but it's legal RDF to have several uses of the properties, or none, on one subject.

As an addition quirk, the empty list isn't any RDF triples, so looking for lists isn't just looking for rdf:rest properties.

@prefix :  .
:x :p () .

is the RDF:

:x :p rdf:nil .

Lists, Property Paths and Update

SPARQL 1.1 Query adds property paths, which make working with lists a bit easier, but it's not perfect. List elements do not necessarily come out in order.

{ :list rdf:rest*/rdf:next ?element }

But what about SPARQL 1.1 Update? How can we work with RDF lists? Here are some scripts for list operations. By using property paths they work on arbitrary length lists.

All the scripts are self-contained - they include tests data.

They are examples - they aren't necessarily fully general, for example, if lists are badly formed or the property :p is also used to relate the subject to things that aren't lists. The last example shows a way to address that by finding and marking relavent points in the graph, doing some work and going back and tidying up. The graph updated is also being used as a scratch pad.

Add an element to the start of a list

PREFIX :    <http://example/> 
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 

INSERT DATA {
  :x0 :p () .
  :x1 :p (1) .
  :x2 :p (1 2) .
  :x3 :p (1 2 3) .
} ;

DELETE { ?x :p ?list }
INSERT { ?x :p [ rdf:first 0 ; 
                 rdf:rest ?list ]
       }
WHERE
{
  ?x :p ?list .
}

This one is relatively easy. Find the list start ?x :p ?list, which works whether the list is zero length or already has elements, delete the old triple that connected to the start of the list, put in a new cons cell (the [...]) at the start, and link to it.

Add an element to the end of a list

PREFIX :    <http://example/> 
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 

INSERT DATA {
  :x0 :p () .
  :x1 :p (1) .
  :x2 :p (1 2) .
  :x3 :p (1 2 3) .
} ;

# The order here is important.
# Must do list >= 1 first.

# List of length >= 1
DELETE { ?elt rdf:rest rdf:nil }
INSERT { ?elt rdf:rest [ rdf:first 98 ; rdf:rest rdf:nil ] }
WHERE
{
  ?x :p ?list .
  # List of length >= 1
  ?list rdf:rest+ ?elt .
  ?elt rdf:rest rdf:nil .
  # ?elt is last cons cell
} ;

# List of length = 0
DELETE { ?x :p rdf:nil . }
INSERT { ?x :p [ rdf:first 99 ; rdf:rest rdf:nil ] }
WHERE
{
   ?x :p rdf:nil .
}

This is a bit harder - there are two cases, lists of length 0 and lists of length one or more. The element before the insertion point needs changing and that can be a cons cell (list length >= 1) or the empty list (the triple pointing to it).

Do the lists of length one or more first, otherwise the adding to a list of length zero will be caught again by the adding to a list of length one.

For a list of length 1 or more: find the last element. The WHERE finds ?elt by finding all elements of the list rdf:rest+, and checking it's the last element by looking for ?elt rdf:rest rdf:nil.

Then delete the rdf:rest, and insert the new cons cell [ rdf:first 98 ; rdf:rest rdf:nil ].

For a list of length 0, the style is the same but the finding the triple to delete-insert to attch the cons cell is different.

Delete the element at the start of a list

PREFIX :      <http://example/> 
PREFIX rdf:   <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 

INSERT DATA {
  :x3 :p (1 2 3) .
  :x2 :p (1 2) .
  :x1 :p (1) .
  :x0 :p () .
} ;

DELETE { 
   ?x :p ?list .
   ?list rdf:first ?first ;
         rdf:rest  ?rest }
INSERT { ?x :p ?rest }
WHERE
{
  ?x :p ?list .
  ?list rdf:first ?first ;
        rdf:rest ?rest .
}

This can be done in one step - we are not interested in lists of length 0 because they have no element to delete. So find the pattern at the start of the list, delete it (note the WHERE pattern and DELETE template are the same), and insert the new triple that links the list directly to the previous rdf:rest.

Delete the element at the end of a list

PREFIX :     <http://example/> 
PREFIX rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 

INSERT DATA {
  :x3 :p (1 2 3) .
  :x2 :p (1 2) .
  :x1 :p (1) .
  :x0 :p () .
} ;

# List of length 1
# Do before other lists.

DELETE { ?x :p ?elt .
         ?elt  rdf:first ?v .
         ?elt  rdf:rest  rdf:nil .
       }
INSERT { ?x :p rdf:nil . }
WHERE
{
  ?x :p ?elt .
  ?elt rdf:first ?v ;
       rdf:rest rdf:nil .
} ;

# List of length >= 2
DELETE { ?elt1 rdf:rest ?elt .
         ?elt  rdf:first ?v .
         ?elt  rdf:rest  rdf:nil .
       }
INSERT { ?elt1 rdf:rest rdf:nil }
WHERE
{
  ?x :p ?list .
  ?list rdf:rest* ?elt1 .

  # Second to end.
  ?elt1 rdf:rest ?elt .
  # End.
  ?elt rdf:first ?v ;
       rdf:rest rdf:nil .
}

The cases to consider are lists of exactly one and lists of two or more elements. It's the treatment of the element before the element we're deleteing that is different.

The style is the same though - find the place before the deleting, and the delete that cons cell.

For the list of length 2 or more, rdf:rest* is used which, is all elements including the ?list case of zero steps - then the structure beyond that is tested for being the end. There are 2 rdf:rest uses in the test for the end, hence list of length 2 or more.

Delete the whole list (common case)

PREFIX rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
PREFIX :     <http://example/> 

INSERT DATA {
:x0 :p () .
:x0 :q "abc" .

:x1 :p (1) .
:x1 :q "def" .

:x2 :p (1 2) .
:x2 :q "ghi" .
} ;

# Delete the cons cells.
DELETE
    { ?z rdf:first ?head ; rdf:rest ?tail . }
WHERE { 
      [] :p ?list .
      ?list rdf:rest* ?z .
      ?z rdf:first ?head ;
         rdf:rest ?tail .
      } ;

# Delete the triples that connect the lists.
DELETE WHERE { ?x :p ?z . }

This version is not fully general because it assume that :p is a link to the list and not also to any other RDF terms (non-lists) which we would want to keep.

The first DELETE finds and removes all cons cells. The second DELETE removes the triple with :p connecting the list to the subject.

Delete the whole list (general case)

PREFIX rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
PREFIX :     <http://example/> 

INSERT DATA {
:x0 :p () .
:x0 :p "String 0" .
:x0 :p [] .

:x1 :p (1) .
:x1 :p "String 1" .
:x1 :p [] .

:x2 :p (1 2) .
:x2 :p "String 2" .
:x2 :p [] .

# A list not connected.
(1 2) .

# Not legal RDF.
# () .

} ;

INSERT { ?list :deleteMe true . }
WHERE {
   ?x :p ?list . 
   FILTER (?list = rdf:nil || EXISTS{?list rdf:rest ?z} )
} ;

# Delete the cons cells.
DELETE
    { ?z rdf:first ?head ; rdf:rest ?tail . }
WHERE { 
      [] :p ?list .
      ?list rdf:rest* ?z .
      ?z rdf:first ?head ;
         rdf:rest ?tail .
      } ;

# Delete the marked nodes
DELETE 
WHERE { ?x :p ?z . 
        ?z :deleteMe true . 
} ;

## ------
## Unconnected lists.

DELETE
    { ?z rdf:first ?head ; rdf:rest ?tail . }
WHERE { 
      ?list rdf:rest ?z2 .
      FILTER NOT EXISTS { ?s ?p ?list }
      ?list rdf:rest* ?z .
      ?z rdf:first ?head ;
         rdf:rest ?tail .
      } 

Deep breath.

This one is quite long.

The first step is to find and mark all the triples from a subject to a list via :p. We will need to delete at the end of the process but the property might also be used for non-lists and after the middle DELETE step all evidence of the lists is lost. The test:

    FILTER (?list = rdf:nil || EXISTS{?list rdf:rest ?z} )

catches both zero length lists and lists with elements.

Second step: delete all list elements, any subjects with properties rdf:first and rdf:rest.

Third step: remove the connecting triples and the markers.

Finally, we delete any lists where the start isn't connected to anything, which is the

    FILTER NOT EXISTS { ?s ?p ?list }

test.

License and Copyright

This page and the SPARQL 1.1 Update scripts are (c) Epimorphics Ltd and licensed under a Creative Commons Attribution 3.0 License.

1 comment:

  1. Typo? In { :list rdf:rest*/rdf:next ?element }, shouldn't rdf:next be rdf:first?

    ReplyDelete