# Re: ISSUE-22 Recursion - Status of Core SHACL Semantics draft

```Le 13/11/2015 19:33, Peter F. Patel-Schneider a écrit :
> On 11/11/2015 07:50 PM, Arthur Ryman wrote:
> [...]
>> Iovka said that the draft is no longer being maintained. Her latest
>> version of the semantics of ShEx is given in [1]. I pointed out that I
>> had proposed a different approach to positive recursion. [2]
>>
>> We agreed to look at each others articles and decide how to proceed.
>>
>> [1] http://arxiv.org/abs/1510.05555
>> [2] http://arxiv.org/abs/1505.04972
> [...]
>
> I am continuing to look at [1].
>
>
>
> It appears to me that ShEx there is *not* built on top of RDF, as the notion
> of a graph there allows triples whose predicate is the inverse of a property.
>   This permits a different set of graphs than does RDF because the three graphs
> below are different
>
> 1/
> :a :p :c .
> :c ^:p :a .
>
> 2/
> :a :p :c .
>
> 3/
> :c ^:p :a .
>
> I am puzzled as to why this change was introduced.
This is not a change, just an abstraction on RDF graphs. Because 2/ and
3/ and not abstraction of any correct RDF graph, they will never be
considered.

>
> As well, there appears to be a single designated value for all blank nodes, so
> that it is not possible to distinguish between
>
> 4/
> :a :p _:b1 .
> :a :p _:b2 .
> _:b1 :p :c .
> _:b2 :p :d .
>
> and
>
> 5/
> :a :p _:b1 .
> :a :p _:b2 .
> _:b1 :p :c .
> _:b1 :p :d .
>
In the abstraction, nodes still have unique identifiers, and two blank
nodes are not the same.  The 4/ and 5/ graphs above will be distinct,
because in 4/ there will be two different nodes for _b1 and _b2, that
are n_b1 and n_b2, respectively.  The fact that val(n_b1) = val(n_b2) =
_b is not a problem, because in ShEx there is nothing to distinguish
between the local ids of two blan knodes anyway.

--
Iovka Boneva
Associate professor (MdC) Université de Lille
http://www.cristal.univ-lille.fr/~boneva/
+33 6 95 75 70 25
```

Received on Monday, 16 November 2015 08:37:09 UTC