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Dropped ASK constraints https://lists.w3.org/Archives/Public/public-data-shapes-wg/2015Mar/0272.html
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@HolgerKnublauch
HolgerKnublauch committed Mar 19, 2015
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Expand Up @@ -123,6 +123,24 @@ <h2>Introduction</h2>
SHACL includes macro facilities that encapsulate reusable building blocks based on these executable languages into <span class="term">templates</span> and <span class="term">functions</span>.
These advanced topics are covered from section 7 onwards.
</p>
<section id="introduction-outline">
<h3>Document Outline</h3>
<p>
Unless otherwise noted in the section heading, all sections and appendices in this document are normative.
<span class="todo">TODO: We still need to mark non-normative sections.</span>
</p>
<p>
The remaining sub-sections of the Introduction provide an overview of the complete language and introduces relevant <a href="#introduction-overview">terminology</a>.
</p>
<p>
The sections 2 - 6 cover the <span class="term">SHACL Core Profile</span> and may be read independently from the later sections.
<span class="todo">We could add details on every section here, but I am not sure this is really needed.</span>
</p>
<p>
The sections 7 onwards are about the full SHACL language and its formal definition.
The appendix includes SPARQL-related implementation details.
</p>
</section>
<section id="introduction-overview">
<h3>Overview and Terminology</h3>
<p>
Expand Down Expand Up @@ -678,6 +696,9 @@ <h4>sh:allowedValues</h4>

ex:AllowedValuesExampleValidResource
ex:property ex:Value2 .</pre>
<p class="todo">
TODO: Some examples should also include the validation results.
</p>
<p class="issue" title="sh:allowedValue">
It is not yet decided whether the language should also support <code>sh:allowedValue</code>
that would point directly to the enumerated values instead of having an intermediate sh:Set.
Expand Down Expand Up @@ -1632,12 +1653,12 @@ <h2>SPARQL-based Execution (sh:sparql)</h2>
<section id="sparql-constraints">
<h3>SPARQL-based Constraints</h3>
<p>
The SPARQL queries attached to a <span class="term">constraint</span> via <code>sh:sparql</code> must be of the query form <code>ASK</code>, <code>CONSTRUCT</code> or <code>SELECT</code>.
The SPARQL queries attached to a <span class="term">constraint</span> via <code>sh:sparql</code> must be of the query form <code>CONSTRUCT</code> or <code>SELECT</code>.
<a href="#sparql-constraints-construct"><code>CONSTRUCT</code></a> is the most flexible option, so the following section starts with describing how these are executed.
The subsequent sections describe how <a href="#sparql-constraints-ask"><code>ASK</code></a> and <a href="#sparql-constraints-select"><code>SELECT</code></a> queries can be transformed into equivalent <code>CONSTRUCT</code> forms.
The subsequent section describes how <a href="#sparql-constraints-select"><code>SELECT</code></a> queries can be transformed into equivalent <code>CONSTRUCT</code> forms.
</p>
<p class="todo">
There is ongoing <a href="https://lists.w3.org/Archives/Public/public-data-shapes-wg/2015Feb/0359.html">discussion</a> about whether all options below shall be supported, and whether an approach based on variable names only would be better.
There is ongoing <a href="https://lists.w3.org/Archives/Public/public-data-shapes-wg/2015Feb/0359.html">discussion</a> about whether the CONSTRUCT option below shall be supported, and whether an approach based on variable names only would be better.
</p>
<section id="sparql-constraints-construct">
<h4>CONSTRUCT-based Constraints</h4>
Expand Down Expand Up @@ -1672,55 +1693,6 @@ <h4>CONSTRUCT-based Constraints</h4>
instance as subject and the surrounding <code>sh:Constraint</code> as its object.
</p>
</section>
<section id="sparql-constraints-ask">
<h4>ASK-based Constraints</h4>
<p class="todo">
Issue: It is quite possible that ASK queries will be removed from the language.
For many use cases, a corresponding SELECT query is easy to write.
Furthermore, ASK often leads to confusion because they need to return false to signal correctness.
</p>
<p>
SPARQL queries of the <code>ASK</code> form that produce <code>true</code> signal a constraint violation.
As shown in the following example, the subject node holding the <code>sh:sparql</code> query may also have other properties that define properties of the constraint violation.
</p>
<pre class="example" title="Global constraint based on SPARQL ASK">
ex:ExampleGlobalAskConstraint
a sh:GlobalConstraint ;
sh:level sh:Warning ;
sh:message "Properties marked as deprecated should not be used" .
sh:sparql """
ASK {
?subject ?predicate ?object .
?predicate ex:deprecated true .
}
""" .</pre>
<p>
The <code>ASK</code> query above gets translated into the following <code>CONSTRUCT</code>:
</p>
<pre class="example">
CONSTRUCT {
_:violation a sh:Warning .
_:violation sh:message "Properties marked as deprecated should not be used" .
}
WHERE {
?subject ?predicate ?object .
?predicate ex:deprecated true .
}</pre>
<p>
In its most basic form, an <code>ASK</code> query is translated into a <code>CONSTRUCT</code> form with an identical <code>WHERE</code> clause.
The <span class="term">graph template</span> of the <code>CONSTRUCT</code> must create exactly one blank node.
The <code>rdf:type</code> of that blank node must be the value of the property <code>sh:level</code> at the surrounding subject node.
If no <code>sh:level</code> has been specified, <code>sh:Error</code> must be used as default type.
<span class="todo">A proposed alternative to sh:level is <code>sh:severity</code>.</span>
</p>
<p>
If the surrounding subject node defines a <code>sh:message</code> then the constructed blank node needs to have a corresponding triple.
Similarly, the values of <code>sh:root</code>, <code>sh:path</code> and <code>sh:value</code> must be inserted (they cannot be blank nodes).
</p>
<p class="todo">
TODO: Maybe allow special variable names such as <code>?root</code>, <code>?path</code> etc to be projected similar to how this happens with SELECT queries.
</p>
</section>
<section id="sparql-constraints-select">
<h4>SELECT-based Constraints</h4>
<p>
Expand Down Expand Up @@ -1785,7 +1757,8 @@ <h4>SELECT-based Constraints</h4>
</tr>
</table>
<p>
Similar to <a href="#sparql-constraints-ask"><code>ASK</code></a> queries, corresponding values attached to the surrounding subject node must be used in the absence of the result variables above.
If the surrounding subject node defines a <code>sh:message</code> then the constructed blank nodes needs to have corresponding triples, unless the SELECT has corresponding result variables.
Similarly, the values of <code>sh:root</code>, <code>sh:path</code> and <code>sh:value</code> must be inserted (they cannot be blank nodes).
</p>
<p>
In local constraints the variable <code>?this</code> must be projected into <code>?root</code> unless the <code>SELECT</code> clause already projects to <code>?root</code>.
Expand Down

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