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Re: On various syntax issues

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Wed, 18 May 2016 09:39:56 -0700
To: Holger Knublauch <holger@topquadrant.com>, "public-data-shapes-wg@w3.org" <public-data-shapes-wg@w3.org>
Message-ID: <c96456e3-e71c-1b7c-f874-b07cec64f845@gmail.com>
	New Design for SHACL Syntax

This is a description of what Holger might have meant in his recent message
on SHACL syntax.  Sections that embody significant changes are @marked like
this@.  There are several remaining issues:
* Should sh:and, sh:or, and sh:not allow any kind of constraint that is
allowed where they are?
:: This makes the syntax somewhat more difficult to describe.
* Can arbitrary constraints have filters?
:: There does not appear to be any reason to disallow this.
* What is the exact relationship between the linking properties (sh:property,
...) and the constraint types (sh:PropertyConstraint, ...)?
:: When can the rdf:type links for constraints be missing?

Constraints are nodes in the shape graph.  @There are three kinds of
constraints in the core of SHACL: shapes, property constraints, and inverse
property constraints, which have SHACL type sh:shape, sh:propertyConstraint,
and sh:inversePropertyConstraint, respectively.@  No node in a shape graph
can have more than one of these among its types.

In SHACL, nodes are validated against constraints, resulting in validation
reports containing validation results.  (The details of validation results
are not described here.)  The node being validated is called the focus node.
If the validation of a node against a constraint produces no validation
results that have a severity level of violation then that node is said to
validate against the constraint.

Property constraints and inverse property constraints have exactly one value
for sh:predicate.  Property constraints work on the values of their
predicate for a focus node; inverse property constraints work on the inverse
values of their predicate for a focus node; @shapes work on the focus node
itself@.  The set of nodes that are being worked on are called the value
nodes.  Constraints also have a severity which can be informational,
warning, or violation.

@As well as being a kind of constraint, shapes also are used to initiate
validation, through scopes.@  Each node that is in any scope of a shape is
validated against that shape.  @Shapes also have filters.  Only nodes that
validate against all filters of a shape can produce validation results.@

Constraints are made up of components.  When a node is validated against a
constraint it is validated against each of its components.  The processing
of each component is described here.  If the condition described is not
satisfied, then one or more validation results are produced with the
severity level of the constraint.

Components that @are [should be]@ allowed for all kinds of constraints:

* sh:in ( n1 ... nm )
:: Each value node is one of the ni.
* sh:class c
:: Each value node has c as one of its SHACL types.
* sh:classIn ( c1 ... cn )
:: Each value node has one of the ci as one of its SHACL types.
* sh:datatype dt
:: Each value node has a datatype of dt.
* sh:datatypeIn ( dt1 ... dtn )
:: Each value node has a datatype that is one of the dti.
* sh:minLength non-negative integer
:: The SPARQL STR representation of each value node is at least this long.
* sh:maxLength non-negative integer
:: The SPARQL STR representation of each value node is at most this long.
* sh:minExclusive l
:: Each value node is greater than l, using SPARQL > test.
* sh:minInclusive l
:: Each value node is greater than or equal to l, using SPARQL >= test.
* sh:maxExclusive l
:: Each value node is less than l, using SPARQL < test.
* sh:maxInclusive l
:: Each value node is less than or equal to l, using SPARQL <= test.
* sh:nodeKind sh:BlankNode or sh:IRI or sh:Literal or BlankNodeOrIRI or
BlankNodeOrLiteral or IRIOrLiteral
:: Each value node is a blank node, an IRI, a literal, a blank node or IRI, a
blank node or literal, or an IRI or literal, respectively
* sh:pattern regex  or  sh:pattern regex ; sh:flags
:: The SPARQL STR representation each value node matches regex using SPARQL
REGEX, with flags if present.
* sh:property pc
:: Each value node validates against pc, a property constraint.
* sh:inverseProperty ipc
:: Each value node validates against ipc, an inverse property constraint.
* sh:and ( shape ... shape )
:: Each value node validates against each shape.
* sh:or ( shape ... shape )
:: Each value node validates against some shape.
* sh:not shape
:: Each value node does not validate against the shape.

Components that are only allowed for property and inverse property constraints:

* sh:hasValue n
:: Some value node is the same as n.
* sh:uniqueLang true
:: There are no repeated language tags among the value nodes.
* sh:minCount int
:: There are at least int value nodes.
* sh:maxCount int
:: There are at most int value nodes.
* sh:equals p
:: The set of value nodes is the same as the set of values of p for the focus
node.
* sh:disjoint p
:: The set of value nodes is the disjoint from set of values of p for the
focus node.
* sh:lessThan p
:: ...
* sh:lessThanOrEqual p
:: ...

Components that are allowed only for shapes:

* @sh:closed true ; sh:ignoredProperties ( p1 ... pn )@
:: Each value node has no values for any property not in the list or that is
not the property of a sh:property component of the shape.
Received on Wednesday, 18 May 2016 16:40:30 UTC

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