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198 changes: 198 additions & 0 deletions spec/index.html
Original file line number Diff line number Diff line change
@@ -10467,7 +10467,205 @@ <h3>Evaluation Semantics</h3>
<a href="#defn_eval" class="evalFct">eval</a>( |D|(|G|), Slice(|L|, |start|, |length|) ) = <a href="#defn_algSlice" class="algFct">Slice</a>( <a href="#defn_eval" class="evalFct">eval</a>(|D|(|G|), |L|), |start|, |length| )
</p>
</div>

<!-- ValuesInsertion -->
<section>
<h3>Values Insertion and `EXISTS`</h3>
Comment on lines +10471 to +10473
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Values Insertion reads very oddly to me. Can this be changed (throughout) to Value Insertion?

Suggested change
<!-- ValuesInsertion -->
<section>
<h3>Values Insertion and `EXISTS`</h3>
<!-- ValueInsertion -->
<section>
<h3>Value Insertion and `EXISTS`</h3>

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It is inserting multiple values.

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Do you use an "eggs beater" or an "egg beater"?

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In US English, if not global English, the common phrasing for such things uses the singular — "rule book" (not "rules book"), "tool box" (not "tools box"), "book shelf" (not "books shelf").

Also, is this only and always "inserting multiple values", or can it be "inserting a single value"?

<div class="ednote">
<p>The following subsections contain draft material for a revised
"`exists`" operation.
</p>
<p>
Background: <a href="https://github.com/w3c/sparql-dev/blob/main/SEP/SEP-0007/sep-0007.md">SPARQL CG SEP-0007</a>
</p>
</div>
<section>
<h4>Syntax Restriction</h4>
<p><i>Additional [[[#sparqlGrammar]]] note:</i></p>
<blockquote>
Any variable that is assigned to in the graph pattern of `EXISTS`/`NOT EXISTS` must not be in-scope.
This applies to `BIND`, variables introduced by `AS` in a `SELECT` clause, variables in a `VALUES`
clause, and variables introduced by `AS` in `GROUP BY`.
</blockquote>
<p>
Extend the "in-scope" rules to include the possible
variables that are in-scope for the current row:
</p>
<table style="border-collapse: collapse; border-color: #000000; border-spacing:5px; border-width: 1px">
<tbody>
<tr>
<th>Syntax Form</th>
<th>In-scope variables</th>
</tr>
<tr>
<td>`EXISTS` and `NOT EXISTS` filters </td>
<td><code>v</code> is in-scope if it is in-scope for the pattern to which the `FILTER` is applied.
</td>
</tr>
</tbody>
</table>
<div class="note">
<p>
This restriction means that <a href="#defn_valuesinsertion">values inserted</a>
do not conflict with values assigned to variables within the pattern.
</p>
<p>
This operation is performed as part of query parsing.
</p>
</div>
</section>
<section>
<h4>Remapping</h4>
<p>
Remapping ensures that a variable name used inside a project expression,
but which is not part of the results of the evaluation of the project expression,
does not coincide with a variable mentioned anywhere else in the
algebra expression of a query.
</p>
<p>
Renaming these variables does not change the results of evaluating
the project expresssion.
</p>
<div class="defn">
<b>Definition: <span id="defn_projmap">Projection Expression Variable Remapping</span></b>
<p>
For a projection algebra operation #sparqlProjection `Project(A, PV)` acting on algreg express `A` and with set of variables `PV`, define
a partial mapping `F` from
`<a href="#sparqlQueryVariables">V</a>`,
the set of all variables, to `V` where:
</p>
<pre>F(v) = v1 if v is in PV, where v1 is a fresh variable
F(v) = v if v is not in PV</pre>
Comment on lines +10533 to +10538
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Considering the second line of this definition, it seems to me that F is actually not partial.

If I am wrong with this assumption, can you explain which variables are mapped by F and which ones are not.

<p>
Define the Projection Expression Variable Remapping `ProjectMap(P, PV)`
</p>
<pre>ProjectMap(Project(A, PV)) = Project(A1, PV)
where A1 is the result of applying F
to every variable mentioned in A.
</pre>
<p>
The Projection Expression Variable Remapping yields an algrebra expression that
evaluates to the same results as the Project argument. No variable of `ProjectMap(Project(A, PV))`
that is not in `PV` is mentioned anywhere else in the algebra expression for the query.
Comment on lines +10547 to +10549
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I don't think that's true based on the current version of this definition. By the current definition, the projection variables are renamed from A to A1, but not in PV. As a consequence, the evaluation of A1 will not even produce bindings for the projection variables.

Is it possible that you have a mistake in the earlier part of this definition? In particular, I have the feeling that the first line of the definition of F should have been as follows.

F(v) = v1 if v is mentioned in A but is not in PV , where v1 is a fresh variable

</p>
</div>
<p>This process is applied throughout the graph pattern of <code>EXISTS</code>:</p>
<div class="defn">
<b>Definition: <span id="defn_varrename">Variable Remapping</span></b>
<p>
For any algebra expression `X`, define the Variable Remapping `PrjMap(X)`
of algebra expression `X`:
</p>
<pre>PrjMap(X) = replace all project operations Project(P, PV)
with ProjectMap(P, PV) for each projection in X.</pre>
Comment on lines +10559 to +10560
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Is the ProjectMap(P, PV) here actually meant to be ProjectMap( Project(P, PV) ) as introduced in the previous definition? If not, how is ProjectMap(P, PV) defined?

</div>
<p>
The outcome of `PrjMap` is independent of the order of replacement
(e.g. bottom-up or top-down).
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W3C documents generally conform to US English, which uses a comma following e.g., i.e., and similar. (UK English often leaves this comma out.)

Suggested change
(e.g. bottom-up or top-down).
(e.g., bottom-up or top-down).

Edited to add: I prefer the comma, but I more prefer consistency, at least at the document level, if not at the document-suite level (i.e., all the RDF 1.2 & SPARQL 1.2 docs). If the group decides that some document(s) should be UK English, then that all the US vs UK variations within such chosen document(s) should be made UK.

Replacements may happen several times, depending on recursive order
but each time a replacement is made, the variable not used anywhere else.
</p>

<div class="note">
<p>
A variable inside a project expression that is not in the variables projected
is not affected by the values insertion operation because it is renamed apart.
</p>
<p>
This operation is as part of the <a href="#sparqlQuery">translation to the SPARQL
algebra</a>.
</p>
</div>
</section>
<section>
<h4>Values Insertion</h4>
<p>
Alternative 1: rewrite the algebra during
<a href="#sparqlQuery">translation to the SPARQL algebra</a>
to include a function that evaluates to the current row.
</p>

<div class="defn">
<div>
<b>Definition: <span id="defn_correlate">Values In Scope</span></b>
<p>
Define the function `BindingInScope()`.
Evaluation of `BindingInScope()` results in a table of one row,
being the current binding of the enclosing filter.
Comment on lines +10592 to +10594
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EXIST can be used not only within FILTER.

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Where else?

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Bind, select expression, order by…

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</p>
</div>
</div>

<div class="defn">
<div>
<b>Definition: Access to the current binding</b>
<p>
During <a href="#sparqlQuery">translation to the SPARQL algebra</a>
</p>
<pre>
Replace each occurence of `Y` in X where `Y` is one of
<a href="#sparqlTranslateBasicGraphPatterns">Basic Graph Pattern</a>,
<a href="#sparqlTranslatePathExpressions">Property Path Expression</a>,
<a href="#sparqlTranslateGraphPatterns">`Graph(Var, pattern)`</a>,
<a href="#https://www.w3.org/TR/sparql12-query/#sparqlTranslateGraphPatterns">Inline Data</a>
with `join(Y, BindingInScope())`.</pre>
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  1. What is X?
  2. Instead of Property Path Expression, do you actually mean Property Path Pattern?

</div>
<div class="note">
c.f. section <a href="#sparqlTranslateGraphPatterns">Translate Graph Patterns</a>
where an empty basic graph pattern start any
<a href="#rGroupGraphPattern">GroupGraphPattern</a>.
It happens before the <a href="#sparqlSimplification">simplification step</a>.
</div>
</div>

<div class="example">
<p>
Examples
</p>
</div>

<p>
Alternative 2: rewrite the algebra during execution. This corresponds to the
<a href="https://github.com/w3c/sparql-dev/blob/main/SEP/SEP-0007/sep-0007.md">original SEP-0007 proposal</a>.
</p>
<div class="defn">
<div>
<b>Definition: <span id="defn_valuesinsertion">Values Insertion</span></b>
<p>
Define the Values Insertion function `ValuesInsert(X, μ)`
</p>
<pre>Let Table(μ) = { μ } and multiplicity( μ | Table(μ) = { μ } ) = 1

Replace each occurence of `Y` in X where `Y` is one of
<a href="#sparqlTranslateBasicGraphPatterns">Basic Graph Pattern</a>,
<a href="#sparqlTranslatePathExpressions">Property Path Expression</a>,
<a href="#sparqlTranslateGraphPatterns">`Graph(Var, pattern)`</a>,
<a href="#https://www.w3.org/TR/sparql12-query/#sparqlTranslateGraphPatterns">Inline Data</a>
with `join(Y, Table(μ))`.</pre>

</div>
</div>
</section>

<section>
<h4>Evaluation of EXISTS</h4>
<div class="defn">
<b>Definition: <span id="x-defn_evalExists">Evaluation of Exists</span></b>
<p>
Let `μ` be the current solution mapping for a filter, and `X` a graph pattern,
define the Evaluation of Exists `exists(X)`
</p>
<pre>exists(X) = true
if eval( D(G), ValuesInScope(PrjMap(X)), μ)
is a non-empty solution sequence.
Comment on lines +10659 to +10660
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What is ValuesInScope( .. )? Where is this defined?

No matter how it is defined, there is another problem here. The formula uses the eval function with three arguments: i) D(G), ii) the output of ValuesInScope( .. ), whatever that might be, and iii) some solution mapping μ. Yet, eval is not defined for such arguments; it is defined for two arguments: i) D(G) and ii) an algebraic expression.

exists(X) = false otherwise</pre>
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exists(X) = false otherwise</pre>
Otherwise, exists(X) = false.</pre>

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The style here is that for function definition, with each case being aligned. It is not running text.

</div>
</section>
</section>
<!-- ValuesInsertion -->

</section>

<section id="sparqlBGPExtend">
<h3>Extending SPARQL Basic Graph Matching</h3>
<p>The overall SPARQL design can be used for queries which assume a more elaborate form of
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