RE: Hello, n3

Hi Doerthe, all,

 

Thanks very much for helping to initiate this work, and your initial list of issues with N3! It looks like you’ve done a lot of very interesting work. Some initial two cents from my end:

 

* As an aside, I was wondering in what cases rules in consequences would be useful from a logical point of view (don’t have access to [1] right now). Isn’t the following just as effective:

 

{ ?C1 rdfs:subClassOf  ?C2, ?x a ?C1 } => ?x a ?C2

 

But I understand why it could be (syntactically) useful from a pre-processing point of view—since only the initial rule body needs to be instantiated with TBox terms (?).

 

For me personally, the interesting part about N3 is the fact that it supports a rule layer, making it much more than a “readable RDF syntax”. It makes passing rules together with the dataset very easy (although SPIN also supports this). 

 

* As mentioned by Doerthe, and unbeknownst to me previously, N3 allows to describe statements directly using “formulae”, without having to explicitly “s-p-o-encode” the described statements, i.e., 

 

{ :william dc:wrote :Moby_Dick } a n3:falsehood .

 

As also referenced by Doerthe, this very much reminded me of the work by Hartig et al., who introduced an extended semantics (RDF* and SPARQL*) [1] for reification support. Doerthe, it’s not clear what you mean by saying that reification is excluded by RDF semantics (then again, there are whole parts on that page that are unclear to me ..). 

 

*         From a “developability” point of view, it’s great that N3 can represent lists in a non-triple form, but it doesn’t seem to be mentioned in the Team Submission [2] (it is mentioned on the N3 logic page [3]). 

 

*         I believe there could be a semantic layer, aside from a syntactical one, that facilitates working with lists (i.e., RDF collections). For instance, when computerizing the OWL2 RL axiomatization, one often runs into the following types of rules:

 

{ ?c owl:unionOf ?l, ?l x:member ?cl } => ?cl owl:subClassOf ?c

 

Where the x:member can be supported by adding an extra few rules [4]. Using rdfs:member seems to do something similar for RDF(S) containers, but that’s much easier since containers do not represent linked lists.

 

Even in case all list elements need to be referenced:

 

{ ?c owl:intersectionOf ?l, @forall :cl (?l x:member :cl, :cl a ?t, ?y a ?t) } => ?y a ?c

 

As mentioned by Reynolds [5], such a rule could be replaced by 3 auxiliary rules for support by general-purpose rule engines. 

 

Maybe such rule-based representations could form the basis for an extended semantics (?) while N3-supporting reasoners could optimize support for them. 

 

 

Looking forward to your thoughts. Note that I tend to work more on the application side of things ([4] was a rare venture for me, even though it is already a bit applied).

 

 

William

 

 

[1] Hartig O, Thompson B. Foundations of an Alternative Approach to Reification in RDF. CoRR 2014;abs/1406.3.

[2] https://www.w3.org/TeamSubmission/n3 

[3] https://www.w3.org/DesignIssues/Notation3.html 

[4] http://www.semantic-web-journal.net/system/files/swj1881.pdf (code 7)

[5] https://www.w3.org/TR/rif-owl-rl/#Forward_intersectionOf_rule 

 

 

From: Doerthe Arndt <doerthe.arndt@ugent.be> 
Sent: November-30-18 7:47 AM
To: public-n3-dev@w3.org
Subject: Re: Hello, n3

 

Dear Nathan, all,

Thank you for pushing this forward!  I am really happy that you are as enthusiastic about this topic as I am. I always wanted to push forward the formalisation of N3, so I guess, now is the time.  

A little bit more about my and our background: 

I have a master in Mathematics and I am currently finishing my PhD in Computer Science which has N3 and its applications as a topic. I work at imec in Gent (Belgium) and there, we had many research projects using N3 and the EYE reasoner (I am sure Jos will tell us more about his implementation in another mail, till then you can already check http://eulersharp.sourceforge.net/ and http://n3.restdesc.org/). 

Some example projects :

* We implemented owl-rl rules in N3 and used it for ontology reasoning (maybe an idea to also test it on the work of you, William, but that is just a side note :) ). What was special about N3 here was that N3 allows rules in the consequence of rules ( like {?C1 rdfs:subClassOf  ?C2}=>{ {?x a ?C1}=>{?x a ?C2} }.). We used this for preprocessing. [1]
* We also had several projects in which we used the fact that Cwm and EYE both can produce proofs. We created a format to describe possible restful Web API calls in forms of rules with existential variables in the consequence (another nice feature of N3), RESTdesc. The user can then give a goal he or she wants to reach and by producing a proof the computer provides a plan  how this goal can be reached. [2,2a]
* We used N3 for data validation. Here, we used a lot of the built-ins of N3, but also  the RIF built-ins as they are implemented in EYE. [3] 

We also already worked on the formalisation of N3 and identified several problems which need to be fixed:

* Implicit quantification: When we used the reasoners Cwm and EYE, we discovered, that they differed in their way of handling implicit universal quantification. To give a concrete example, consider the rule:

{{?x :p :o} => {?x :p2 :o2}}=> {:a :b :c}.

Cwm interprets that as:

(∀x: p(x,o) → p2(x,o2)) → b(a,c) 

EYE interprets it as:

∀x: (p(x,o) → p2(x,o2)) → b(a,c)

The reason for that the W3C team submission is not really clear here, it says that universals are quantified "on the parent formula" (see https://www.w3.org/TeamSubmission/n3/#Quantifica), but it does not say what exactly this "parent" is.

* Explicit quantification: N3 provides a way to express explicit quantification (by using @forall and @forsome). An example formula is:

 @forAll <#h>. @forSome <#g>. <#g> <#loves> <#h> . 

which means in FOL: 

∀h: ∃g: loves(h,g)

But the spec also says that "If both universal and existential quantification are specified for the same formula, then the scope of the universal quantification is outside the scope of the existentials" (see https://www.w3.org/TeamSubmission/n3/#Quantifica). So the formula

@forSome <#g>. @forAll <#h>. <#g> <#loves> <#h> . 

also means

∀h: ∃g: loves(h,g)

I at least think that this is counter intuitive (but maybe there are good reasons to do it that way?).

* No distinction between variables and constants:

Related to the previous topic, it can be a problem that variables and constants are not clearly distinguished. Look for example at the first occurence of :h in this formula:

@forSome :g. :g :p :h.  @forAll :h. :g :loves :h . 

Given the reversal of quantifiers when interpreting it (∀h: ∃g: ...) it is not clear to me whether this :h is a universally quantified variable or a constant.

 

* Citation of Formulas:
We need to agree how to interpret cited graphs like for example

:x :says {:s :p :o}.

I think this topic is quite challenging (for TriG, for example, there was no agreement https://www.w3.org/TR/rdf11-datasets/). RDF* [4] reduces such constructs to rdf reification which  unfortunately doesn't solve the problem of defining a semantics since reification is excluded in rdf semantics (see: https://www.w3.org/TR/rdf11-mt/#reification). There are many different approaches we could follow like for example KIF [5], Common Logic [6] or also make it more complex like they do in SUMO [7].

I worked on the quantification part and published a paper in 2015 about that [8] (it is only a first draft which especially does not yet take the position of a quantifier into account) begin our discussions, till then I can refer to my presentation about this topic here: https://docs.google.com/presentation/d/1Tkh3JLuayBft63ltoeCCD68Zfa9wMFzjOz-h_xYX0PU/edit?usp=sharing (I enabled commenting in case you want to ask questions). I furthermore also did some work on formalising the proof calculus for N3, this part can be found in our Paper Pragmatic Proof [2a].

As you see. I am really interested in writing the spec, but I am also willing to invest time in working on and testing implementation(s) and I am looking forward to having detailed discussions. 

I am looking forward to working with you.

Kind regards,
Dörthe

 

 

[1]  D. Arndt, B. De Meester, P. Bonte, J. Schaballie, J. Bhatti, W. Dereuddre, R. Verborgh, F. Ongenae, F. De Turck, R. Van deWalle, E. Mannens, Improving OWL RL reasoning in N3 by using specialized rules, in: V. Tamma, M. Dragoni, R. Gonçalves, A. Ławrynowicz (Eds.), Ontology Engineering: 12th International Experiences and Directions Workshop on OWL, Vol. 9557 of Lecture Notes in Computer Science, Springer, 2016, pp. 93–104. doi:10.1007/978-3-319-33245-1_10. URL http://dx.doi.org/10.1007/978-3-319-33245-1_10

[2] http://restdesc.org/

[2a] R. Verborgh, D. Arndt, S. Van Hoecke, J. De Roo, G. Mels, T. Steiner, J. Gabarró Vallés, The pragmatic proof: Hypermedia API composition and execution, Theory and Practice of Logic Programming 17 (1) (2017) 1–48. doi:10.1017/S1471068416000016. URL http://arxiv.org/pdf/1512.07780v1.pdf

[3] D. Arndt, B. De Meester, A. Dimou, R. Verborgh, E. Mannens, Using rule-based reasoning for RDF validation, in: S. Costantini, E. Franconi, W. Van Woensel, R. Kontchakov, F. Sadri,
D. Roman (Eds.), Proceedings of the International Joint Conference on Rules and Reasoning, Vol. 10364 of Lecture Notes in Computer Science, Springer, 2017, pp. 22–36. doi:10.1007/978-3-319-61252-2_3. Available at: https://biblio.ugent.be/publication/8540876/file/8540882.pdf

[4] O. Hartig, B. Thompson, Foundations of an alternative approach to reification in RDF, CoRR abs/1406.3399. arXiv:1406.3399. URL http://arxiv.org/abs/1406.3399

[5] P. Hayes, C. Menzel, A semantics for the knowledge interchange format, in: IJCAI 2001 Workshop on the IEEE Standard Upper Ontology, Vol. 1, 2001, p. 145.

[6] ISO/IEC 24707:2007 Information technology – Common Logic (CL), standards.iso.org/ittf/PubliclyAvailableStandards/c039175_ISO_IEC_24707_2007%28E%29.zip (2007).

[7] C. Benzmüller, A. Pease, Higher-order aspects and context in sumo, Web Semantics: Science, Services and Agents on the World Wide Web 12 (2012) 104–117.

[8] D. Arndt, R. Verborgh, J. De Roo, H. Sun, E. Mannens, R. Van de Walle, Semantics of Notation3 logic: A solution for implicit quantification, in: N. Bassiliades, G. Gottlob,F. Sadri, A. Paschke, D. Roman (Eds.), Rule Technologies: Foundations, Tools, and Applications, Vol. 9202 of Lecture Notes in Computer Science, Springer, 2015, pp. 127–143. URL http://link.springer.com/chapter/10.1007/978-3-319-21542-6_9

Hi all, thanks for joining the n3-dev CG.

 

Since the holiday season is fast approaching, we should perhaps aim to start ramping up communications and efforts from the new year.

 

If you know anybody else who may be interested in joining, or should be involved, please encourage them to join the CG via https://www.w3.org/community/n3-dev/ 

 

In the interim, please do share anything you think is pertinent, including related background reading and references, or anything you'd like to discuss. It may also be useful to start collating implementations, any nuances or bugs that have been found after working with the current specification [1] (see also [2]), also suggestions on what could be improved / simplified / expanded / removed.

 

Let's all get on the same page, agree what to do, and get it done.

 

Personally I'll commit time, discussion, spec writing, mistakes, and working reference implementation(s). Throughout the current decade I've frequently thought or said I/you/we could do this with n3, so before the turn of the next decade, I want to be using n3+rules daily, at scale, and will do my utmost to facilitate you all in using it for what you want or need too.

 

Thanks all, and I look forward to working you all in the coming months,

 

Nathan

 

[1] https://www.w3.org/TeamSubmission/n3/

[2] https://www.w3.org/DesignIssues/Notation3.html

-- 
Dörthe Arndt
Researcher Semantic Web
imec - Ghent University - IDLab | Faculty of Engineering and Architecture | Department of Electronics and Information Systems
Technologiepark-Zwijnaarde 19, 9052 Ghent, Belgium
t: +32 9 331 49 59 | e: doerthe.arndt@ugent.be <mailto:doerthe.arndt@ugent.be>