在 php 7.4 中重写匿名函数 [英] Rewriting an anonymous function in php 7.4
问题描述
有如下匿名递归函数:
$f = function($n) use (&$f) {返回 ($n == 1) ?1 : $n * $f($n - 1);};回声 $f(5);//120
我尝试重写到 7.4 版本,但出现错误,请告诉我缺少什么?
$f = fn($n) =>($n == 1) ?1 : $n * $f($n - 1);回声 $f(5);
<块引用>
注意:未定义变量:f
致命错误:未捕获的错误:函数名必须是字符串
正如 Barmar 所说,你不能在作用域外使用 $f
,因为当隐式绑定发生时 $f
仍未定义.
没有什么可以阻止您稍后将其作为参数传递.
$f = fn($f, $n) =>$n == 1 ?1 : $n * $f($f, $n - 1);回声 $f($f, 5);//120
箭头函数的工作方式是,在定义期间,它们将使用外部作用域变量的按值绑定.
<块引用>如前所述,箭头函数使用按值变量绑定.这大致相当于对箭头函数中使用的每个变量 $x
执行 use($x)
.- https://wiki.php.net/rfc/arrow_functions_v2
闭包对变量 $f
的赋值发生在闭包定义之后,而变量 $f
在它之前是未定义的.
据我所知,在定义箭头函数时没有任何通过引用绑定的机制.
There is the following anonymous recursive function:
$f = function($n) use (&$f) {
return ($n == 1) ? 1 : $n * $f($n - 1);
};
echo $f(5); // 120
I try to rewrite to version 7.4, but there is an error, please tell me what I'm missing?
$f = fn($n) => ($n == 1) ? 1 : $n * $f($n - 1);
echo $f(5);
Notice: Undefined variable: f
Fatal error: Uncaught Error: Function name must be a string
Just like Barmar said, you can't use $f
from the outside scope, because when the implicit binding takes place $f
is still undefined.
There is nothing stopping you from passing it later as a parameter.
$f = fn($f, $n) => $n == 1 ? 1 : $n * $f($f, $n - 1);
echo $f($f, 5); // 120
The way arrow functions work, is that during definition time they will use by-value binding of the outer scope's variables.
As already mentioned, arrow functions use by-value variable binding. This is roughly equivalent to performing a
use($x)
for every variable$x
used inside the arrow function. - https://wiki.php.net/rfc/arrow_functions_v2
The assignment of the closure to the variable $f
happens after closure's definition and the variable $f
is undefined prior to it.
As far as I am aware, there isn't any mechanism to bind by reference while defining arrow functions.
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