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Document PHP 7.4 arrow functions #71

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164 changes: 164 additions & 0 deletions language/functions.xml
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Expand Up @@ -1737,6 +1737,170 @@ Warning: Cannot bind an instance to a static closure in %s on line %d
</sect2>

</sect1>
<sect1 xml:id="functions.arrow">
<title>Arrow Functions</title>

<simpara>
Arrow functions were introduced in PHP 7.4 as a more concise syntax for
<link linkend="functions.anonymous">anonymous functions</link>.
</simpara>
<simpara>
Both anonymous functions and arrow functions are implemented using the
<link linkend="class.closure"><classname>Closure</classname></link> class.
</simpara>

<simpara>
Arrow functions have the basic form
<code>fn (argument_list) =&gt; expr</code>.
</simpara>

<simpara>
Arrow functions support the same features as
<link linkend="functions.anonymous">anonymous functions</link>,
except that using variables from the parent scope is always automatic.
</simpara>

<simpara>
When a variable used in the expression is defined in the parent scope
it will be implicitly captured by-value.
In the following example, the functions <varname>$fn1</varname> and
<varname>$fn2</varname> behave the same way.
</simpara>

<para>
<example>
<title>Arrow functions capture variables by value automatically</title>
<programlisting role="php">
<![CDATA[
<?php

$y = 1;

$fn1 = fn($x) => $x + $y;
// equivalent to using $y by value:
$fn2 = function ($x) use ($y) {
return $x + $y;
};

var_export($fn1(3));
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
4
]]>
</screen>
</example>
</para>
<simpara>
This also works if the arrow functions are nested:
</simpara>
<para>
<example>
<title>Arrow functions capture variables by value automatically, even when nested</title>
<programlisting role="php">
<![CDATA[
<?php

$z = 1;
$fn = fn($x) => fn($y) => $x * $y + $z;
// Outputs 51
var_export($fn(5)(10));
?>
]]>
</programlisting>
</example>
</para>
<simpara>
Similarly to anonymous functions,
the arrow function syntax allows arbitrary function signatures,
including parameter and return types, default values, variadics,
as well as by-reference passing and returning.
All of the following are valid examples of arrow functions:
</simpara>
<para>
<example>
<title>Examples of arrow functions</title>
<programlisting role="php">
<![CDATA[
<?php

fn(array $x) => $x;
static fn(): int => $x;
fn($x = 42) => $x;
fn(&$x) => $x;
fn&($x) => $x;
fn($x, ...$rest) => $rest;

?>
]]>
</programlisting>
</example>
</para>
<simpara>
Arrow functions use by-value variable binding.
This is roughly equivalent to performing a <code>use($x)</code> for every
variable <varname>$x</varname> used inside the arrow function.
A by-value binding means that it is not possible to modify any values
from the outer scope.
<link linkend="functions.anonymous">Anonymous functions</link>
can be used instead for by-ref bindings.
</simpara>
<para>
<example>
<title>Values from the outer scope cannot be modified by arrow functions</title>
<programlisting role="php">
<![CDATA[
<?php

$x = 1;
$fn = fn() => $x++; // Has no effect
$fn();
var_export($x); // Outputs 1

?>
]]>
</programlisting>
</example>
</para>

<sect2 role="changelog">
&reftitle.changelog;
<para>
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry>&Version;</entry>
<entry>&Description;</entry>
</row>
</thead>
<tbody>
<row>
<entry>7.4.0</entry>
<entry>
Arrow functions became available.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect2>

<sect2 role="notes">
&reftitle.notes;
<note>
<simpara>
It is possible to use <function>func_num_args</function>,
<function>func_get_arg</function>, and <function>func_get_args</function>
from within an arrow function.
</simpara>
</note>
</sect2>
</sect1>

</chapter>

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