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

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.

 >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?


 >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.


 >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.

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
- 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?

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

Received on Sunday, 15 March 2015 14:09:31 UTC