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Generalize syntax of arrow expressions
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@michaelhkay
michaelhkay committed Feb 5, 2025
1 parent 4940b62 commit 4dbd9ac
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22 changes: 2 additions & 20 deletions specifications/grammar-40/xpath-grammar.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -1533,14 +1533,8 @@ VersionDecl ::= "xquery" (("encoding" StringLiteral) | ("version" StringLiteral

<g:production name="ArrowTarget" if="xpath40 xquery40 xslt40-patterns">
<g:choice>
<g:sequence>
<g:ref name="ArrowStaticFunction"/>
<g:ref name="ArgumentList"/>
</g:sequence>
<g:sequence>
<g:ref name="ArrowDynamicFunction"/>
<g:ref name="PositionalArgumentList"/>
</g:sequence>
<g:ref name="FunctionCall"/>
<g:ref name="DynamicFunctionCall"/>
</g:choice>
</g:production>

Expand Down Expand Up @@ -1913,18 +1907,6 @@ VersionDecl ::= "xquery" (("encoding" StringLiteral) | ("version" StringLiteral
<g:string process-value="yes">*</g:string>
</g:production>

<g:production name="ArrowStaticFunction" if="xpath40 xquery40 xslt40-patterns">
<g:ref name="EQName"/>
</g:production>

<g:production name="ArrowDynamicFunction" if="xpath40 xquery40 xslt40-patterns">
<g:choice>
<g:ref name="VarRef"/>
<g:ref name="InlineFunctionExpr"/>
<g:ref name="ParenthesizedExpr"/>
</g:choice>
</g:production>

<!-- ] end PathExpr -->

<!-- [ start PrimaryExpr -->
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102 changes: 66 additions & 36 deletions specifications/xquery-40/src/expressions.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -22115,6 +22115,12 @@ return string-join($chopped, '; ')
<div2 id="id-arrow-operator">
<head>Arrow Expressions</head>

<changes>
<change issue="1716">An arrow operator may now be followed by any dynamic function
call; the dynamic function call no longer needs to start with a variable reference
or a parenthesized expression.</change>
</changes>

<p>Arrow expressions apply a function to a value, using the value of the
left-hand expression as the first argument to the function.</p>

Expand Down Expand Up @@ -22162,38 +22168,44 @@ return string-join($chopped, '; ')
=!> upper-case()
=> string-join(" ")]]></eg>

<p diff="add" at="A">returns <code>"THE. CAT. SAT. ON. THE. MAT."</code>. The first arrow
<p>returns <code>"THE. CAT. SAT. ON. THE. MAT."</code>. The first arrow
could be written either as <code>=></code> or <code>=!></code> because the operand is a singleton; the next two
arrows have to be <code>=!></code> because the function is applied to each item in the tokenized
sequence individually; the final arrow must be <code>=></code> because the <code>string-join</code>
function applies to the sequence as a whole.</p>

<note diff="add" at="A"><p>It may be useful to think of this as a map/reduce pipeline. The functions
<note><p>It may be useful to think of this as a map/reduce pipeline. The functions
introduced by <code>=!></code> are mapping operations; the function introduced by <code>=></code>
is a reduce operation.</p></note>

<p diff="add" at="A">The following example introduces an inline function to the pipeline:</p>
<p>The following example introduces an inline function to the pipeline:</p>
<eg role="parse-test" diff="add" at="A"
><![CDATA[(1 to 5) =!> xs:double() =!> math:sqrt() =!> fn($a) { $a + 1 }() => sum()]]></eg>

<p diff="add" at="A">This is equivalent to <code>sum((1 to 5) ! (math:sqrt(xs:double(.)) + 1))</code>.</p>
<p>This is equivalent to <code>sum((1 to 5) ! (math:sqrt(xs:double(.)) + 1))</code>.</p>

<p diff="add" at="A">The same effect can be achieved using a <termref def="dt-focus-function"/>:</p>
<p>The same effect can be achieved using a <termref def="dt-focus-function"/>:</p>

<eg role="parse-test" diff="add" at="A"
<eg role="parse-test"
><![CDATA[(1 to 5) =!> xs:double() =!> math:sqrt() =!> fn { . + 1 }() => sum()]]></eg>

<p>It could also be expressed using the mapping operator <code>!</code>:</p>

<eg role="parse-test"
><![CDATA[(1 to 5) ! xs:double(.) ! math:sqrt(.) ! (. + 1) => sum()]]></eg>

<p>Where the value of an expression is a map containing functions, simulating the behavior
of objects in object-oriented languages, then the <term>lookup arrow operator</term> <code>=?></code>
can be used to retrive a function from the map and to invoke the function with the map as its
can be used to retrieve a function from the map and to invoke the function with the map as its
first argument. For example, if <code>my:rectangle</code> returns a map with entries <code>width</code>,
<code>height</code>, <code>expand</code>, and <code>area</code>, then it becomes possible to
write:</p>

<eg role="parse-test" diff="add" at="A"
><![CDATA[my:rectangle(3, 5) =?> expand(2) =?> area()]]></eg>

<note diff="add" at="A"><p>The <code>ArgumentList</code> may include <code>PlaceHolders</code>,
<note diff="add" at="A"><p>The <code>ArgumentList</code> of the function call
may include <code>PlaceHolders</code>,
though this is not especially useful. For example, the expression <code>"$" => concat(?)</code> is equivalent
to <code>concat("$", ?)</code>: its value is a function that prepends a supplied string with
a <code>$</code> symbol.</p></note>
Expand All @@ -22215,6 +22227,13 @@ return string-join($chopped, '; ')
operation.</p>
</note>

<note><p>The expression <code>U => $V(X)(Y)</code> satisfies the grammar, but its meaning
might not be obvious. It is interpreted as being equivalent to <code>$V(X)(U, Y)</code>.
The same applies to the mapping arrow operator: <code>U =!> $V(X)(Y)</code> is
interpreted as <code>for $u in $V return $V(X)($u, Y)</code>. This follows from
the way that the syntax of a <nt def="DynamicFunctionCall">DynamicFunctionCall</nt>
is defined.</p></note>




Expand All @@ -22229,18 +22248,28 @@ return string-join($chopped, '; ')


<ulist>
<item><p>Given a <nt def="UnaryExpr">UnaryExpr</nt>
<code>U</code>, an <nt def="ArrowStaticFunction">ArrowStaticFunction</nt>
<code>F</code>, and an <nt def="ArgumentList">ArgumentList</nt>
<code>(A, B, C...)</code>, the expression <code>U =&gt; F(A, B, C...)</code> is equivalent to the
expression <code>F(U, A, B, C...)</code>.</p></item>
<item><p>Given a <nt def="UnaryExpr">UnaryExpr</nt>
<code>U</code>, an <nt def="ArrowDynamicFunction">ArrowDynamicFunction</nt>
<code>F</code>, and an <nt def="PositionalArgumentList">PositionalArgumentList</nt>
<code>(A, B, C...)</code>, the expression <code>U =&gt; F(A, B, C...)</code> is equivalent to the
expression <code>F(U, A, B, C...)</code>.</p></item>
<item>
<p>If the arrow is followed by a static <nt def="FunctionCall">FunctionCall</nt>:</p>
<p>Given a <nt def="UnaryExpr">UnaryExpr</nt>
<var>U</var> and a <nt def="FunctionCall">FunctionCall</nt>
<code><var>F</var>(<var>A</var>, <var>B</var>, <var>C</var>...)</code>,
the expression <code><var>U</var> => <var>F</var>(<var>A</var>, <var>B</var>, <var>C</var>...)</code>
is equivalent to the expression
<code><var>F</var>(<var>U</var>, <var>A</var>, <var>B</var>, <var>C</var>...)</code>.</p></item>
<item>
<p>If the arrow is followed by a <nt def="DynamicFunctionCall">DynamicFunctionCall</nt>:</p>
<p>Given a <nt def="UnaryExpr">UnaryExpr</nt>
<var>U</var>, and a <nt def="DynamicFunctionCall">DynamicFunctionCall</nt>
<code><var>E</var>(<var>A</var>, <var>B</var>, <var>C</var>...)</code>, the expression
<code><var>U</var> => <var>E</var>(<var>A</var>, <var>B</var>, <var>C</var>...)</code> is equivalent to the
expression <code><var>E</var>(<var>U</var>, <var>A</var>, <var>B</var>, <var>C</var>...)</code>.</p></item>
</ulist>

<note><p>Although the syntax of an arrow expression makes use of the grammatical productions
<nt def="FunctionCall">FunctionCall</nt> and <nt def="DynamicFunctionCall">DynamicFunctionCall</nt>,
these are not evaluated in the same way as a function call that appears as a
free-standing expression.</p></note>

</div3>


Expand All @@ -22255,29 +22284,30 @@ return string-join($chopped, '; ')
</changes>


<p diff="add" at="A"><termdef term="mapping arrow operator" id="dt-mapping-arrow-operator">
<p><termdef term="mapping arrow operator" id="dt-mapping-arrow-operator">
The <term>mapping arrow operator</term> <code>=!></code> applies a function to each
item in a sequence.</termdef> It is defined as follows:</p>

<ulist diff="add" at="A">
<ulist>

<item>
<p>If the arrow is followed by an <nt def="ArrowStaticFunction">ArrowStaticFunction</nt>:</p>
<p>Given a <nt def="UnaryExpr">UnaryExpr</nt>
<code>U</code>, an <nt def="ArrowStaticFunction">ArrowStaticFunction</nt>
<code>F</code>, and an <nt def="ArgumentList">ArgumentList</nt>
<code>(A, B, C...)</code>, the expression <code>U =!&gt; F(A, B, C...)</code> is equivalent to the
expression <code>(for $u in U return F($u, A, B, C...))</code>.</p>

</item>
<p>If the arrow is followed by a static <nt def="FunctionCall">FunctionCall</nt>:</p>
<p>Given a <nt def="UnaryExpr">UnaryExpr</nt>
<var>U</var> and a <nt def="FunctionCall">FunctionCall</nt>
<code><var>F</var>(<var>A</var>, <var>B</var>, <var>C</var>...)</code>,
the expression <code><var>U</var> =!> <var>F</var>(<var>A</var>, <var>B</var>, <var>C</var>...)</code>
is equivalent to the expression
<code>for $u in <var>U</var> return
<var>F</var>(<var>$u</var>, <var>A</var>, <var>B</var>, <var>C</var>...)</code>.</p></item>
<item>
<p>If the arrow is followed by an <nt def="ArrowDynamicFunction">ArrowDynamicFunction</nt>:</p>
<p>Given a <nt def="UnaryExpr">UnaryExpr</nt>
<code>U</code>, an <nt def="ArrowDynamicFunction">ArrowDynamicFunction</nt>
<code>F</code>, and an <nt def="PositionalArgumentList">PositionalArgumentList</nt>
<code>(A, B, C...)</code>, the expression <code>U =!&gt; F(A, B, C...)</code> is equivalent to the
expression <code>(for $u in U return F($u, A, B, C...))</code>.</p>
</item>
<p>If the arrow is followed by a <nt def="DynamicFunctionCall">DynamicFunctionCall</nt>:</p>
<p>Given a <nt def="UnaryExpr">UnaryExpr</nt>
<var>U</var>, and a <nt def="DynamicFunctionCall">DynamicFunctionCall</nt>
<code><var>E</var>(<var>A</var>, <var>B</var>, <var>C</var>...)</code>, the expression
<code><var>U</var> => <var>E</var>(<var>A</var>, <var>B</var>, <var>C</var>...)</code> is equivalent to the
expression <code>for $u in U return <var>E</var>(<var>$u</var>, <var>A</var>, <var>B</var>, <var>C</var>...)</code>.</p></item>



</ulist>

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