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Re: Proposal to resolve ISSUE-102 (well-formed lists)

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 12 Nov 2012 04:20:25 -0600
Cc: RDF Working Group WG <public-rdf-wg@w3.org>
Message-Id: <35FF42D9-985B-4A99-8E70-D821CF2BF51D@ihmc.us>
To: Richard Cyganiak <richard@cyganiak.de>
OK, after reading the email trail and thinking harder about this, some more comments.

1. RDF only describes lists, it does not construct or process them. So there is no such thing as a "well-formed RDF list". What there might be is something like a well-formed RDF *description* of a list, but that raises a number of issues.

2. What counts as a well-formed RDF description of a list? If this means the list which is described by the description is "well-formed" (which I take to mean, it is a LISP S-expression), then almost any description is well-formed. For example:

_:x rdf:first :a
_:x rdf:first :b

is true if :a owl:sameAs :b and _:x is the list (a), or indeed the list (a fred somethingelse); any list with a as its first element will do. 

3. If the idea is to require that RDF graphs must give complete information about lists, so that every rdf:next assertion is given all the way to rdf:nil, then this would break the basic entailment between graphs and subgraphs, so a strong -1 on that idea. In general, RDF is designed to allow partial information and to be open-world in its treatment of data, and this kind of a completeness assumption violates that basic design.

4. If the idea is to encourage RDF authors to use the RDF list vocabulary in a certain obvious way, then make it part of a non-normative best practice guide. The current specs already say this, however, and even say that systems may require adherence to this and treat "ill-formed" descriptions as an error. So I don't see that any major change is needed.

Pat



On Nov 9, 2012, at 2:48 AM, Richard Cyganiak wrote:

> ISSUE-102: Shall we highlight Turtle's list structures as "Well-Formed Lists" in one of our Recs?
> http://www.w3.org/2011/rdf-wg/track/issues/102
> 
> 
> PROPOSAL: Define the concept “well-formed list” in detail in RDF Schema, including a nice diagram. State that any use of terms from the collections vocabulary SHOULD be part of a well-formed list. Update Semantics to remove discussion of collections in 3.3.3. Update Turtle and RDF/XML to refer to well-formed lists when introducing the respective syntax shorthands. Send an email to OWL WG comments list informing them of this and suggest that future versions of OWL do the same.
> 

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Received on Monday, 12 November 2012 10:20:54 GMT

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