phpday 2021 digital edition

箭头函数

箭头函数是 PHP 7.4 的新语法,是一种更简洁的 匿名函数 写法。

匿名函数和箭头函数都是 Closure 类的实现。

箭头函数的基本语法为 fn (argument_list) => expr

箭头函数支持与 匿名函数 相同的功能,只是其父作用域的变量总是自动的。

当表达式中使用的变量是在父作用域中定义的,它将被隐式地按值捕获。在下面的例子中,函数 $fn1$fn2 的行为是一样的。

示例 #1 箭头函数自动捕捉变量的值

<?php

$y 
1;

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

var_export($fn1(3));
?>

以上例程会输出:

4

在箭头函数嵌套的情况下同样有效。

示例 #2 箭头函数自动捕捉变量的值,即使在嵌套的情况下

<?php

$z 
1;
$fn fn($x) => fn($y) => $x $y $z;
// 输出 51
var_export($fn(5)(10));
?>

和匿名函数一样,箭头函数语法同样允许标准的函数声明,包括参数和返回类型、缺省值、变量,以及通过引用传递和返回。以下都是箭头函数的有效例子。

示例 #3 合法的箭头函数例子

<?php

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

?>

箭头函数会自动绑定上下文变量,这相当于对箭头函数内部使用的每一个变量 $x 执行了一个 use($x)。这意味着不可能修改外部作用域的任何值,若要实现对值的修改,可以使用 匿名函数 来替代。

示例 #4 来自外部范围的值不能在箭头函数内修改

<?php

$x 
1;
$fn fn() => $x++; // 不会影响 x 的值
$fn();
var_export($x);  // 输出 1

?>

更新日志

版本 说明
7.4.0 新增箭头函数语法。

注释

注意: 可以对箭头函数使用 func_num_args(), func_get_arg(), 和 func_get_args() 函数。

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User Contributed Notes 5 notes

up
10
Koushil Mankali
8 months ago
In example 4  (Values from the outer scope cannot be modified by arrow functions)

<?php

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

?>

Here we can use reference variable in fn(&$x) and pass the value from function call $fn($x) so that we will get the output as expected with out using Anonymous functions.

Example:

<?php

$x
= 1;
$fn = fn(&$x) => $x++;
$fn($x);
var_export($x);

?>

Output : 2 (as expected)

But here it will not take values from parent scope automatically but we have to pass them explicitly.
up
4
InvisibleSmiley
3 months ago
Unlike anonymous functions, arrow functions cannot have a void return type declaration.

May seem obvious, but if you thought you could make use of the benefits of arrow functions (using variables from the parent scope) to simplify a function or method call, keep in mind that this is only possible if you do NOT tell PHP that the arrow function does indeed return void.
up
5
dexen dot devries at gmail dot com
8 months ago
Beware compact() not being able to access (import) variables from external scope (known in versions: 7.4.0, 7.4.8) (bug: https://bugs.php.net/bug.php?id=78970).

A workaround is available - use the variable directly; this will cause it to be imported into the arrow function's namespace and make it available to the compact() too.

<?php
$aa
= 111;
$accessing_variable_works = fn($bb) => [ $aa, $bb ];
$compact_is_broken = fn($bb) => compact('aa', 'bb');
$compact_can_work_with_workaround = fn($bb) => compact('aa', 'bb') + ['workaround' => $aa];
var_dump($accessing_variable_works(333));
var_dump($compact_is_broken(555));
var_dump($compact_can_work_with_workaround(777));
?>

result:
array(2) {
  [0]=>
  int(111)
  [1]=>
  int(333)
}
PHP Notice:  compact(): Undefined variable: aa in /home/m/vlt/guitar/tlb/s/public_html/index.php on line 9
array(1) {
  ["bb"]=>
  int(555)
}
array(3) {
  ["aa"]=>
  int(111)
  ["bb"]=>
  int(777)
  ["workaround"]=>
  int(111)
}
up
1
itsunclexo at gmail dot com
18 days ago
As you already know, variable bindings occur in arrow functions by "by-value".  That means, an arrow function returns a copy of the value of the variable used in it from the outer scope.

Now let us see an example of how a arrow function returns a reference instead of a copy of a value.

<?php

$x
= 0;

$fn = fn &(&$x) => $x// Returns a reference

$y = &$fn($x);  // Now $y represents the reference

var_dump($y);  // Outputs: 0

$y = 3// Changing value of $y affects $x

var_dump($x);  // Ouputs: 3

?>
up
-2
zhangchengming at kkguan dot com
4 months ago
<?php

$x
= 1;

(
fn() => print($x))(); // Outputs 1

(fn($x) => print($x))(2); // Outputs 2
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