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<html>
<head>
<meta charset="utf-8"/>
<meta name="viewport" content="width=device-width"/>
<title>Shape Expressions ShExPath Language</title>
<!-- link rel="stylesheet" href="css/wgio.min.css"/ ask Iovka -->
<link rel="stylesheet" href="local.css"/>
<script src='https://www.w3.org/Tools/respec/respec-w3c-common'
async="async" class='remove'></script>
<script src="https://www.w3.org/2013/ShEx/demobits/jquery.min.js" type="text/javascript"></script>
<script class='remove'>
var respecConfig = {
specStatus: "base",
shortName: "shexpath",
editors: [
{ name: "Eric Prud'hommeaux",
url: "http://www.w3.org/Team/Eric",
company: "Janeiro Digital, Micelio" }
],
edDraftURI: "https://shexspec.github.io/spec/ShExPath",
// wg: "Shape Expressions ",
// wgURI: "https://shex.io/",
// wgPublicList: "public-shex-dev",
// wgPatentURI: "http://www.w3.org/2004/01/pp-impl/424242/status",
};
</script>
<style type="text/css">
em { font-weight: bold; }
.error { background-color: #fdd; }
</style>
</head>
<body>
<section id="abstract">
<h2>Abstract</h2>
<p>
ShEx is a schema language for RDF graphs. It provides structural constraint on graph structures and lexical forms of literals.
ShExPath addresses elements in a ShEx schema.
This can be used to anchor validation results, identify regions of RDF graphs, or tie external annotations to elements in a schema.
</p>
</section>
<section id="scope">
<h2>Scope</h2>
<p>
ShExPath identifies <a href="">Shape Expressions</a> and <a href="">Triple Expressions</a>.
It does not, on its own, identify portions of <a href="">Node Constraint</a>.
</p>
</section>
<section>
<h2>Document Conventions</h2>
<p>
This specification describes the structure of ShEx schemas in terms of <a href="http://shex.io/shex-semantics/index.html#shexj">ShExJ</a>.
Elements in the ShExJ are addressed in a javascript notation, e.g. <code>S.shapes[N]</code>.
</p>
</section>
<section id="data-model">
<h2>Data Model</h2>
<p>
A ShExPath is a Unicode string which defines a traversal of a Shape Expressions schema.
</p>
<p>
An <dfn>item</dfn> is one of these elements from a Shape Expressions schema:
</p>
<ul>
<li>some shape expression:<ul>
<li>a ShapeAnd expression</li>
<li>a ShapeOr expression</li>
<li>a ShapeNot expression</li>
<li>a NodeConstraint</li>
<li>a Shape</li>
</ul>some triple expression:<ul>
<li>an EachOf triple expression</li>
<li>a OneOf triple expression</li>
<li>a Triple Constraint</li>
</ul></li>
</ul>
<p>
A <dfn>value</dfn> is a sequence of <a>items</a>.
</p>
<p class="issue">This specification uses XPath's notion of <a href="https://www.w3.org/TR/2014/REC-xpath-30-20140408/#dt-item">item</a>, <a href="https://www.w3.org/TR/2014/REC-xpath-30-20140408/#dt-value">value</a>, <a href="https://www.w3.org/TR/2014/REC-xpath-30-20140408/#dt-step">step</a> and <a href="https://www.w3.org/TR/2014/REC-xpath-30-20140408/#dt-singleton">singleton</a> to leverage shared understanding. Would other terms be more useful?</p>
<p class="issue">is a value a set?</p>
<p>
A <dfn>singleton</dfn> is a sequence of exactly one <a>item</a>.
</p>
<p class="issue">Do we some way to say a singleton is expected?
This would be useful at end to say ShExPath should specify a single item, but also useful earlier in path for debugging.</p>
<p>
A <dfn>context label</dfn> identifies the expected element type in the ShExJ where each <a>item</a> should be.
The context labels are: <code>ShapeAnd</code> | <code>ShapeOr</code> | <code>ShapeNot</code> | <code>EachOf</code> | <code>OneOf</code> | <code>NodeConstraint</code> | <code>TripleConstraint</code>.
</p>
<p>
A <dfn>index</dfn> may be an integer <code>i</code>, RDF node (URL or blank node label) <code>N</code> with an optional integer <code>Ni</code>.
Integer indexes are 1-based accessors to array elements in a schema.
Out of range indexes are ignored (they do not contribute any new <a>items</a> to a result <a>value</a>).
The function <dfn>evaluateIndex</dfn> maps a <a>value</a> to a new <a>value</a>:
</p>
<ul>
<li>For each <a>item</a> <code>I</code> in <a>value</a>:
<ul>
<li>If I.type = "Schema",
<ul>
<li>If i, add I.shapes[i-1] to the new value.</li>
<li>Otherwise, if there exists a shape expression <code>E</code> in I.shapes with E.id = N and there is no <code>Ni</code> or <code>Ni</code> = 1, add <code>E</code> to the new value</li>
</ul></li>
<li>If I.type = "ShapeOr" or I.type = "ShapeAnd",
<ul>
<li>If i, add I.shapeExprs[i-1] to the new value.</li>
</ul></li>
<li>If I.type = "ShapeNot",
<ul>
<li>If i and i = 1, add I.shapeExpr to the new value.</li>
</ul></li>
<li>If I.type = "NodeConstraint", ignore the index.</li>
<li>If I.type = "Shape",
<ul>
<li>if I has an expression, add the results of <a>evaluateIndex</a>(I.expression) to new value.</li>
</ul></li>
<li>If I.type = "EachOf" or I.type = "OneOf"
<ul>
<li>If i, add I.expressions[i] to the new value.</li>
<li>Otherwise,
<ul>
<li>Let <code>TCs</code> be the (possibly empty) set of triple constraints the shape expression I.expressions with a predicate of N.</li>
<li>If Ni, add TCs[Ni] to new value.</li>
<li>Otherwise, add TCs to new value</li>
</ul></li>
</ul></li>
<li>If I.type = "TripleConstraint" <span class="issue">can we get here?</span>
<ul>
<li>If i and i = 1, add I to new value.</li>
<li>Otherwise, if N = I.predicate and there is no <code>Ni</code> or <code>Ni</code> = 1, add <code>I</code> to the new value</li>
</ul></li>
</ul></li>
</ul>
<p class="issue">
ShExPath indexes are 1-based, following XPath's precedent. Change to 0-based?
</p>
<p>
A ShExPath is divided into <dfn>steps</dfn> (see StepExpr [@@ add link when grammar is markup] in the <a href="grammar">grammar</a>).
The initial step may be the character <code>/</code>.
All following steps are separated by <code>/</code> and include an optional <a>context label</a> followed by a required <a>index</a>.
</p>
<p>
<dfn>evaluateShExPath</dfn> takes as arguments a ShExPath <code>P</code>, a schema <code>S</code> and an initial <a>value</a> <code>V</code>.
It iteratively calls <a>evaluateStep</a> with each <a>step</a> and either the initial value or the results of the last invocation of <a>evaluateStep</a>.
</p>
<p>
<dfn>evaluateStep</dfn>(P, S, V) is a function that takes as arguments a ShExPath <code>P</code>, schema <code>S</code> and a value <code>V</code> and produces a new <a>value</a>.
The operators are evaluated as follows:
.</p>
<table>
<thead>
<th><a>step</a></th> <th>action</th>
</thead>
<tbody>
<tr>
<td><code>/</code></td> <td><code>value</code> = the list of shapes in the schema</td>
</tr>
<tr>
<td><a>context label</a></td> <td>the items in <code>value</code> are tested for alignment with the <a>context label</a>.
It is a fatal error if the <code>item</code> is not the same as the <a>context label</a>.
This does not change the <code>value</code>.
<p class="issue">is an error or just a filter?</p>
</td>
</tr>
<tr>
<td><code><a>index</a></code></td> <td><code>value</code> = <a>evaluateIndex</a>(index, <code>value</code>)</td>
</tr>
<tr>
<td><i>empty string</i></td> <td>the evaluation is terminated, the result is <code>value</code>.</td>
</tr>
</tbody>
</table>
<div class="issue">
<p>
Should we advance through ShapeAnd automagically? This requires more aggressive searching but maybe that's worth it.
This would be useful when shape expr is a ShapeAnd of node constraint and a shape:
</p>
<pre>
<#UserShape> IRI /User\?id=[0-9]+/ {
foaf:mbox IRI
}
</pre>
Compare <code>@<#UserShape>/2/foaf:mbox</code> with <code>@<#UserShape>/foaf:mbox</code>.
</div>
</section>
<section id="issueExample">
<h2>Issue Example</h2>
<p>
This schema excerpt describes an issue that might appear in an issue tracking system.
</p>
<pre class="example">
<#IssueShape> {
:name STRING MinLength 4;
:category ["bug" "feature request"];
:postedBy @<#UserShape>;
:processing {
:reproduced [true false];
:priority xsd:integer
}?
}
<#UserShape> IRI /User\?id=[0-9]+/ {
( foaf:name xsd:string
| foaf:givenName +;
foaf:familyName
);
foaf:mbox IRI
}
</pre>
</section>
<section id="simple-access">
<h2>Simple Access</h2>
<p>
Elements can be addressed either by label or index.
For shape expressions, the label is the name of the shape expression.
Shape expression labels or indexes are prefixed by "<code>@</code>".
For triple constraints, the label is the name of the predicate for that triple constraints.
Elements of triple expressions may be selected by index within the triple expression.
</p>
<table class="example">
<thead>
<th style="min-width: 20em;">ShExPath</th> <th>value</th>
</thead>
<tbody>
<tr><td>/@<#IssueShape>/:category</td><td>the <code>:category</code> constraint in the <code>#IssueShape</code> shape, addressed by the <em>name</em> of the shape expression followed by the <em>name</em> of the <code>:category</code> property.</td></tr>
<tr><td>/@<#IssueShape>/2</td><td>the <code>:category</code> constraint in the <code>#IssueShape</code> shape, addressed by the <em>name</em> of the shape expression followed by the <em>index</em> of the <code>:category</code> property.</td></tr>
<tr><td>/@1/2</td><td>the <code>:category</code> constraint in the <code>#IssueShape</code> shape, addressed by the <em>index</em> of the shape expression followed by the <em>index</em> of the <code>:category</code> property.</td></tr>
</tbody>
</table>
<section id="context-tests">
<h3>Nested Expressions</h3>
<p>
A traversal through a schema, for instance, the results of a validation, can be expressed as a ShExPath.
Such a ShExPath can include shape references in value expressions.
These are can be appended to the triple expression path with a <code>/</code> separator.
</p>
<table class="example">
<thead>
<th style="min-width: 20em;">ShExPath</th> <th>value</th>
</thead>
<tbody>
<tr><td>/@<#IssueShape>/:postedBy/@<#UserShape>/foaf:mbox</td><td>the <code>:postedBy</code> constraint in the <code>#IssueShape</code> shape, which then references the <code>:category</code> property in the <code>#UserShape</code>.</td></tr>
<tr><td>/@1/3/@2/2</td><td>the same path, but with ordinals.</td></tr>
</tbody>
</table>
</section>
</section>
<section id="context-tests">
<h2>Context Tests</h2>
<p>
For added clarity and confidence, the ShExJ type of all shape expressions and triple expressions addressable can be tested.
If the expression type is specified and does not match the corresponding type in the schema, the path is invalid.
</p>
<section id="shape-expressions">
<h3>Shape Expressions</h3>
<p>
The axes <code>ShapeAnd</code>, <code>ShapeOr</code>, <code>ShapeNot</code>, <code>EachOf</code>, <code>OneOf</code>, <code>NodeConstraint</code> and <code>TripleConstraint</code> may be used to specify the expected expression type.
</p>
<table class="example">
<thead>
<th style="min-width: 20em;">ShExPath</th> <th>value</th>
</thead>
<tbody>
<tr><td>/@<#UserShape>/shapeAnd 2/foaf:mbox</td><td>the <code>foaf:mbox</code> constraint in <code>#UserShape</code>'s shape.
Note that <code>IRI /User\?id=[0-9]+/ {...}</code> compiles to a <code>ShapeAnd</code> with the first component a <code>NodeConstraint</code> and the second being a shape..
</td></tr>
</tbody>
</table>
</section>
<section id="triple-expressions">
<h3>Triple Expressions</h3>
<p>
The axes <code>EachOf</code>, <code>OneOf</code> and <code>TripleConstraint</code> may be used to specify the expected expression type.
</p>
<table class="example">
<thead>
<th style="min-width: 20em;">ShExPath</th> <th>value</th>
</thead>
<tbody>
<tr><td>/1/ShapeAnd 2/EachOf 2</td><td>the <code>:category</code> constraint in the <code>#IssueShape</code> shape, explicitly labeling the <em>index</em> axes.</td></tr>
<tr><td>/<#UserShape>/2/EachOf 1/OneOf 2</td><td>the <code>EachOf</code> containing <code>foaf:givenName</code> and <code>foaf:familyName</code> in the <code>#UserShape</code> shape.</td></tr>
<tr><td>/<#UserShape>/2/EachOf 1/EachOf 2</td><td class="error">invalid path for the given schema.</td></tr>
</tbody>
</table>
</section>
</section>
<section id="relative-shexpaths">
<h2>Relative ShExPaths</h2>
<p>
Invocation of <a>evaluateShExPath</a> includes a <a>value</a>.
ShExPaths starting with a <code>/</code> force the context to be the schema.
If the <a>value</a> is a schema, indexes access entries in the ShExJ <code>.shapes</code> property.
The application may provide a different <a>value</a>.
A <dfn>context path</dfn> is a ShExPath which, when evaluated against the schema, produces an equivalent <a>value</a>.
For instance validation results MAY be reported using relative ShExPaths.
</p>
<table class="example">
<thead>
<th style="min-width: 2em;">Context</th> <th style="min-width: 7em;">ShExPath</th> <th>value</th>
</thead>
<tbody>
<tr><td>/@<#IssueShape></td><td>:category</td><td>the <code>:category</code> constraint in the <code>#IssueShape</code> shape.</td ></tr>
</tbody>
</table>
<p>
Relative ShExPaths can be concatonated to their context with <code>/</code> separator.
</p>
</section>
<section id="disambiguation">
<h2>Disambiguation</h2>
<p>
If more than one triple constraint has the same predicate, they can be indexed by the order they would be encountered in a depth-first search.
If the index is omitted, it is assumed to be 1.
</p>
<pre class="example">
<BPObs> {
:component { code: "systolic"; value: xsd:double };
:component { code: "diastolic"; value: xsd:double };
:component { code: "posture"; value: @<Postures> }?;
}
</pre>
<table class="example">
<thead>
<th style="min-width: 20em;">ShExPath</th> <th>value</th>
</thead>
<tbody>
<tr><td>/<BPObs>/:component 3</td><td>the third <code>:component</code> triple expression (the one expecting a code of "posture").</td></tr>
<tr><td>/<BPObs>/:component</td><td>the first <code>:component</code> triple expression (the one expecting a code of "systolic").</td></tr>
</tbody>
</table>
</section>
<section id="grammar">
<h2>Grammar</h2>
<p>
[<a href="https://services.w3.org/yacker/uploads/ShExPath?lang=perl">Try it in yacker</a>], e.g. this <a href="https://services.w3.org/yacker/uploads/ShExPath?lang=perl&text=%2F%40%3Chttp%3A%2F%2Fa.example%2FShape1%3E%2FShapeAnd+1%2F1%2Fx%3Ay1+2%2FShapeOr+2%2F%40sch%3AShape2&action=validate+text">nested validation result test</a>.
</p>
<pre>
ShExPathExpr ::= AbsolutePathExpr | RelativePathExpr
AbsolutePathExpr ::= "/" RelativePathExpr
RelativePathExpr ::= StepExpr ("/" StepExpr)*
<dfn>StepExpr</dfn> ::= ContextTest? ExprIndex | ContextTest
ContextTest ::= ShapeExprContext | TripleExprContext
ShapeExprContext ::= "ShapeAnd" | "ShapeOr" | "ShapeNot"
| "NodeConstraint" | "Shape"
TripleExprContext ::= "EachOf" | "OneOf" | "TripleConstraint"
ExprIndex ::= ShapeExprIndex | TripleExprIndex
ShapeExprIndex ::= "@" (INTEGER | ShapeExprLabel)
ShapeExprLabel ::= iri | BLANK_NODE_LABEL
TripleExprIndex ::= INTEGER | TripleExprLabel
TripleExprLabel ::= (iri | BLANK_NODE_LABEL) INTEGER?
[136s] iri ::= IRIREF | prefixedName
[137s] prefixedName ::= PNAME_LN | PNAME_NS
@terminals
[18t] IRIREF ::= '<' ([^#x00-#x20<>\"{}|^`\\] | UCHAR)* '>'
[140s] PNAME_NS ::= PN_PREFIX? ':'
[141s] PNAME_LN ::= PNAME_NS PN_LOCAL
[142s] BLANK_NODE_LABEL ::= '_:' (PN_CHARS_U | [0-9]) ((PN_CHARS | '.')* PN_CHARS)?
[19t] INTEGER ::= [+-]? [0-9]+
[26t] UCHAR ::= '\\u' HEX HEX HEX HEX
| '\\U' HEX HEX HEX HEX HEX HEX HEX HEX
[160s] ECHAR ::= '\\' [tbnrf\\\"\']
[164s] PN_CHARS_BASE ::= [A-Z] | [a-z]
| [#x00C0-#x00D6] | [#x00D8-#x00F6] | [#x00F8-#x02FF]
| [#x0370-#x037D] | [#x037F-#x1FFF]
| [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF]
| [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD]
| [#x10000-#xEFFFF]
[165s] PN_CHARS_U ::= PN_CHARS_BASE | '_'
[167s] PN_CHARS ::= PN_CHARS_U | '-' | [0-9]
| [#x00B7] | [#x0300-#x036F] | [#x203F-#x2040]
[168s] PN_PREFIX ::= PN_CHARS_BASE ((PN_CHARS | '.')* PN_CHARS)?
[169s] PN_LOCAL ::= (PN_CHARS_U | ':' | [0-9] | PLX)
((PN_CHARS | '.' | ':' | PLX)*
(PN_CHARS | ':' | PLX))?
[170s] PLX ::= PERCENT | PN_LOCAL_ESC
[171s] PERCENT ::= '%' HEX HEX
[172s] HEX ::= [0-9] | [A-F] | [a-f]
[173s] PN_LOCAL_ESC ::= '\\' ('_' | '~' | '.' | '-' | '!' | '$' | '&' | "'"
| '(' | ')' | '*' | '+' | ',' | ';' | '=' | '/' | '?'
| '#' | '@' | '%')
</pre>
<p class="issue">
This grammar does not ensure that an absolute path start with a ShapeExprLabel and ShapeExprContext, which would require duplication of the <code>RelativePathExpr</code> and <code>StepExpr</code> productions.
Nor does it ensure a striping between a shape expression labels, triple expression labels, and any nested shape expression labels in triple expression value expressions.
</p>
</section>
</body>
</html>