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Re: formal objection on excluding many well-behaved shapes from SHACL

From: Irene Polikoff <irene@topquadrant.com>
Date: Fri, 5 May 2017 10:20:17 -0400
Cc: "public-rdf-shapes@w3.org" <public-rdf-shapes@w3.org>
Message-Id: <6E1431E2-4C25-47C3-87F0-4010908F66F2@topquadrant.com>
To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>
Peter,

In this formal objection you have repeated what you have already said in the formal objection about paths syntax.

Does this formal objection, therefore, supersede the paths formal objection as you combined them into one?

If the answer is ‘no’ - please rephrase this formal objection not to include the content from another formal objection.

Irene

> On May 5, 2017, at 10:12 AM, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> 
> This is a formal objection to the exclusion from SHACL of numerous shapes
> that have well-behaved intuitive meanings.  A number of these exclusions
> first appear in the 11 April 2017 version of SHACL.  The overall severity of
> these exclusions was noticed during an attempt to produce an implementation
> of SHACL Core.
> 
> There is no technical reason to exclude numerous shapes that have been
> excluded from SHACL and good reasons to permit them.  The exclusions make
> SHACL harder to write for users.  As SHACL implementations are free to
> behave however they want on shapes graphs that contain these excluded shapes
> the exclusions serve as an impediment to interoperability in SHACL.
> 
> In most cases the only change required to SHACL to no longer exclude these
> shapes and thereby improve the utility and the interoperability of SHACL is
> to just remove syntax rules that exclude them.  There is no need to change
> the semantics of SHACL at all to include these shapes.   The burden on
> implementators will be very low, and in many cases there will be no burden
> at all, as these excluded shapes act like similar non-excluded shapes.
> 
> 
> Some of these exclusions are as if a programming language made (x>4 && x>6)
> illegal, but left (x>4 && x>=6) legal.  These syntax restrictions make it
> hard for SHACL users to write legal SHACL shapes graphs.  If there is a need
> to warn users about these shapes, it is better to do so using a lint-like
> tool that is not tied to the syntax of SHACL.
> 
> Actually, the situation is even worse in SHACL than it would be if a
> programming language had these sorts of exclusions.  Completely conforming
> SHACL implementations are free to do anything at all on most invalid syntax
> so a SHACL implementation can signal an error or return any set of
> validation results on shapes that contain the analog of (x>4 && x>6), even
> though the intuitive behaviour of these shapes is completely clear.
> 
> 
> Some of the excluded shapes are degenerate in that they will produce
> validation results for all focus nodes or all focus nodes that have a value
> for a particular property.  For example, the excluded shape
>  ex:false a sh:PropertyShape ;
>    sh:path ex:p ;
>    sh:datatype xsd:integer ;
>    sh:datatype xsd:int .
> will produce a validation result for every focus node that has a value for
> ex:p.   However, many shapes that are degenerate in this sense are not
> excluded.
> 
> Others of the excluded shapes are not degenerate in this way but instead
> have constraints that are redundant.  For example, in the excluded shape
>  ex:redundant a sh:PropertyShape ;
>    sh:path ex:p ;
>    sh:minInclusive 5 ;
>    sh:minInclusive 9 .
> one of the constraints is redundant.  However, many similar shapes that also
> have redundant constraints are not excluded.
> 
> Some of the excluded shapes don't even have any redundant constraints.  For
> example in the excluded shape
>  ex:dateTime a sh:PropertyShape ;
>    sh:path ex:p ;
>    sh:minInclusive "2002-10-10T12:00:00-05:00"^^xsd:dateTime ;
>    sh:minInclusive "2002-10-10T12:00:00"^^xsd:dateTime .
> neither of the constraints are redundant.  To exclude all dateTime values
> before "2002-10-10T12:00:00-05:00"^^xsd:dateTime and also before
> "2002-10-10T12:00:00"^^xsd:dateTime requires two uses of sh:minInclusive.
> 
> Some others of the excluded shapes are somewhat degenerate for standard RDF
> graphs but not for generalized RDF graphs, where literals can be subject of
> triples.  For example, in standard RDF the excluded shape
>  ex:generalized a sh:NodeShape ;
>     sh:lessThan ex:p .
> produces a validation report for nodes that have values for ex:p and can be
> better stated for standard RDF graphs as a property constraint with path
> sh:p and sh:maxCount 0.  In generalized RDF this shape is not degenerate as
> it also produces a validation report for some literals.  However, many
> shapes that are degenerate in this way for standard RDF graphs but not
> degenerate for generalized RDF graphs are not excluded.
> 
> Yet others of the excluded shapes are not excluded for any discernible
> reason at all.  For example, the excluded shape
>  ex:comments a sh:PropertyShape ;
>    sh:path [ rdfs:comment "inverse of ex:p" ;
>    	      sh:inversePath ex:p ] ;
>    sh:class ex:C .
> does not have any problems whatsoever.  Its meaning is clear.  It does not
> accept or reject all nodes.  It does not have any redundant pieces. It is
> excluded for no discernable reason.
> 
> It is easy for users to accidentally write these excluded shapes.  Automatic
> generation of SHACL shapes is made harder by these exclusions.  There are
> extra costs when implementing SHACL syntax checking because of these
> exclusions.
> 
> Interoperability is particularly harmed by these exclusions.  Fully
> conforming SHACL implementations may implement ex:false as producing a
> validation report for all nodes.  They may instead implement ex:false as
> never producing a validation report, on the argument that multiple
> sh:datatype constraints should just be removed.  They may implement ex:false
> as not producing a validation report for literals with datatype xsd:integer,
> or xsd:int, because the implementation assumes that only one sh:datatype can
> be present.  They may even implement ex:false as sometimes not producing a
> validation report for literals with datatype xsd:integer, and sometimes not
> producing a validation report for literals with datatype xsd:int.
> 
> 
> These exclusions should be removed from SHACL by removing the syntax rules
> 
>  datatype-maxCount, nodeKind-maxCount, minCount-scope, minCount-maxCount,
>  maxCount-scope, maxCount-maxCount, minExclusive-maxCount,
>  minInclusive-maxCount, maxExclusive-maxCount, maxInclusive-maxCount,
>  minLength-maxCount, maxLength-maxCount, languageIn-maxCount,
>  uniqueLang-scope, lessThan-scope, lessThanOrEquals-scope,
>  qualifiedValueShape-scope, and in-maxCount
> 
> and changing the syntax rules
> 
>  path-metarule, path-non-recursive, path-predicate, path-sequence,
>  path-alternative, path-inverse, path-zero-or-more, path-one-or-more, and
>  path-zero-or-one
> 
> to
> 
> path-non-recursive A node p is not a well-formed SHACL property path if
> 		   p is a blank node and any of the following rules require,
> 		   directly or indirectly, determining whether p is a
> 		   well-formed SHACL property path.
> 
> path-metarule     A node is a well-formed SHACL property
> 		  path if it satisfies exactly one of the following
> 		  rules and if it is a blank node it does not have a value
> 		  for more than one of rdf:first or rdf:rest,
> 		  sh:alternativePath, sh:inversePath, sh:zeroOrMorePath,
> 		  sh:oneOrMorePath, and sh:zeroOrOnePath.
> 
> path-predicate    A predicate path is any IRI.
> 
> path-sequence 	  A sequence path is a blank node that is a SHACL list with
> 		  at least two members and each member of the list is a
> 		  well-formed SHACL property path.
> 
> path-alternative  An alternative path is a blank node that has exactly one
> 		  value for sh:alternativePath and that value is a SHACL
> 		  list with at least two members and each member of the list
> 		  is a well-formed SHACL property path.
> 
> path-inverse      An inverse path is a blank node that has exactly one value
> 		  for sh:inversePath and that value is a well-formed
> 		  SHACL property path.
> 
> path-zero-or-more A zero-or-more path is a blank node that has exactly one
> 		  value for sh:zeroOrMorePath and that value is a
> 		  well-formed SHACL property path.
> 
> path-one-or-more  A one-or-more path is a blank node that has exactly one
> 		  value for sh:oneOrMorePath and that value is a
> 		  well-formed SHACL property path.
> 
> path-zero-or-one A zero-or-one path is a blank node that has exactly one
> 		 value for sh:zeroOrOnePath and that value is a
> 		 well-formed SHACL property path.
> 
> The change to path syntax not only permits paths that should not be excluded
> but also excludes paths that should not be included, such as
>  [ rdf:first ex:p ; rdf:rest ( ex:q ) ; sh:inversePath ex:p ]
> 
> These changes to the syntax of SHACL results in a SHACL that is easier to
> write, easier to generate, easier to implement, and more interoperable.
> 
Received on Friday, 5 May 2017 14:20:53 UTC

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