Re: ACTION-29 Recursion: Wrap Up

Could we simply say "handling of recursion is left to the discretion of 
the implementations" and close all related tickets? This way, we can 
basically outsource the problem to the open source and research 
communities and move on. The description logics community also made 
progress over time, after the OWL standards were published.

Engines that have larger coverage get bonus points, but implementations 
that just want to cover basic SHACL compliance don't have to worry about 
all the details. We simply wouldn't define test cases that use 
recursion. The ShEx people can support it if they like.


On 24/02/2016 12:24, Arthur Ryman wrote:
> I've had this action for a while but have not been able to make
> progress due to other commitments. I'd like to summarize my thoughts
> here and will then close the action.
> Recursion arises when the evaluation of a shape at a node depends
> directly or indirectly on itself. If we allow this situation to occur,
> then we are obligated to define what it means.
> I believe SHACL has well-founded semantics in the absence of
> recursion. That is our starting point. Any new semantics that allows
> recursion must agree with the current semantics in the absence of
> recursion.
> The SHACL specification currently prohibits recursion. However, there
> is some motivation for allowing limited forms of recursion. See [1].
> I believe we can assign a well-founded sematics to recursion that does
> not involve either negation or disjunction. I wrote a Z specification
> that defined the semantics of this type of recursion as it appeared in
> OSLC Resource Shapes. [2] I had hoped to apply that approach to the
> current SHACL spec, but have not found the time. I still believe this
> approach is feasible.
> [1]
> [2]
> -- Arthur

Received on Wednesday, 24 February 2016 06:34:14 UTC