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Re: How would option b) on the last straw poll of 12 March work?

From: Eric Prud'hommeaux <eric@w3.org>
Date: Sun, 15 Mar 2015 10:59:02 -0400
To: Karen Coyle <kcoyle@kcoyle.net>
Cc: public-data-shapes-wg@w3.org
Message-ID: <20150315145851.GC8719@w3.org>
* Karen Coyle <kcoyle@kcoyle.net> [2015-03-15 07:09-0700]
> On 3/14/15 5:34 PM, Eric Prud'hommeaux wrote:
> >As a down-payment, I offer<http://w3c.github.io/data-shapes/semantics/>.
> >I hope to produce a start on an axiomatic semantics and a SPARQL semantics
> >tomorrow.
> Eric, I think there are some basics that we need to settle on before
> getting more deeply into the writing of documents. Reading the
> introduction to this, I have the following questions/exceptions:
> >SHACL (Shapes Constraint Language) provides structural constraints
> for RDF graphs.
> *structural only? I don't consider all of the validation rules
> (which I prefer to "constraints") to be structural, e.g. the rules
> governing values.

The problem with "validation rules" is that it really is
unbounded. When I see "validation rules" I think of lots of
domain-specific rule sets like the SDTM validator which examines a
cell in one table and runs a bevy of tests to see if has corresponding
entries in another cell, that those in tern have some other properties
with appropriate values, etc. It's hard to find a good description but
"structural" appears to be the most appropriate so far.

> >SHACL constraints are grouped into conjunctions called "shapes",
> which may also be referenced by constraints in other shapes.
> *Is "conjunction" the right word here? It doesn't match the
> grammatical use of this term. Union?

Struck " conjunctions called".

> >These constraints restrict the predicates of triples connecting
> nodes in the graph.
> *This really confuses me -- how do the constraints restrict the
> predicates? I mean, we do have min/max that can be applied to
> predicates, so it's the "these constraints" that doesn't work for me
> here.

These constraints restrict the triples connecting certain nodes in the
graph.  SHACL can restrict the number of triples with a particular
predicate and the permitted object datatype or object terms, require
that the subject or object match some shape or lexical and datatype

> >SHACL can restrict the number of these triples
> *I don't recall (but may not have read carefully) any discussion of
> restricting numbers of triples, unless you are referring to min/max?


> > and the permitted object datatype or object terms, require that
> the subject or object match some shape or lexical and datatype
> conditions.
> *these lexical and datatype conditions are what make the "structural
> constraints" above untrue.

Since datatypes are part of the RDF graph structure, I think that
"structural" is still better than any proposed alternative.

> Perhaps if we could develop a good definition of SHACL, other things
> could flow from it. I think these are the key areas that we need to
> define:
> - SHACL defines structures of RDF graphs in terms of focus nodes and
> member predicates, and values for objects

I suspect we don't want to talk about focus nodes in the abstract, but
I did add "triples connecting certain nodes". Does that seem like a
reasonable compromise between gentleness, brevity and accuracy?

> - SHACL definitions can be used as constraints for validation of RDF graphs
> - SHACL provides a closed-world semantics over RDF graphs
> - ?? more?
> Next, I think the document needs to define a focus shape[1], and the
> remainder of the constraints need to be described in relation to a
> focus shape. For example:
> >3.1 Property Constraint eval
> A property constraint has a predicate which identifies the triple's
> predicate and may have a minimum cardinality and maximum
> cardinality, to indicate how many triples with that predicate are
> expected.
> *"to indicate how many triples... are expected..." -> within that
> focus shape?

Hmm, the shape is composed (eventually) of property
constraints. Really we're trying to say that triples on the focus node
are matched against algebraic combinations of property constraints in
a shape. Does that seem like an improvement?

> kc
> [1] We seem to have an idea of where a focus shape starts, but not
> where it ends. This may relate to Peter's questions about recursion,
> but I'm not sure.
> -- 
> Karen Coyle
> kcoyle@kcoyle.net http://kcoyle.net
> m: 1-510-435-8234
> skype: kcoylenet/+1-510-984-3600


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Received on Sunday, 15 March 2015 14:59:08 UTC

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