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.