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ACTION-29 Recursion: Wrap Up

From: Arthur Ryman <arthur.ryman@gmail.com>
Date: Tue, 23 Feb 2016 21:24:37 -0500
Message-ID: <CAApBiO=pTbyND=d2sFAq5-v3WSMGdeEoZEOvyvf4myUKOYviBg@mail.gmail.com>
To: "public-data-shapes-wg@w3.org" <public-data-shapes-wg@w3.org>
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] https://www.w3.org/2014/data-shapes/wiki/ISSUE-66:_SHACL_spec_ill-founded_due_to_non-convergence_on_data_loops

[2] http://arxiv.org/abs/1505.04972

-- Arthur
Received on Wednesday, 24 February 2016 02:25:06 UTC

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