Re: EXISTS : ways forward

good morning;

> On 2016-09-22, at 18:19, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> 
> On 09/22/2016 05:07 AM, james anderson wrote:
>> good afternoon;
>> 
>>> On 2016-09-22, at 13:46, Andy Seaborne <andy@apache.org
>>> <mailto:andy@apache.org>> wrote:
>>> 
>>> […]
>>> 
>>> There is a 3rd way which is to truly have bindings of variables as an
>>> initial set.  Restrict the range of values at the point a variable in bound.
>>> i.e. in the BGP and any AS usage (noting that BIND(... AS ?VAR) and ?VAR in
>>> a earlier/deeper BGP is already illegal in SPARQL generally but not if ?VAR
>>> is not in-scope at the point of BIND).
>> 
>> this is the (kind of) semantics which is entailed by the simple goal, to have
>> an exists definition which aims to be consistent with the remainder of the
>> language.
> 
> I don't understand what you mean by "consistent with the remainder of the
> language”.

that the definition is in terms of values from the language data model, not from its concrete syntax and that the definition follows established scoping rules or defines addition rules consistent with those which exist.

> 
>> it is straight-forward to define and realize this goal, if the semantics is
>> sited at the correct level of interpretation - that is in the abstract
>> algebra, rather than as a demonstration, that some things can never work, if
>> attempted on the basis of the surface syntax.
> 
> The definition of EXISTS actually works in the SPARQL algebra, not in the
> surface syntax.

if the proposed, problematic definition were not in terms of the surface syntax, there would be no problem with blank nodes and one would not conceive of substituting for symbols which bind variables rather than those which reference them only.

best regards, from berlin,
---
james anderson | james@dydra.com | http://dydra.com