Since PHP 5.3 now has closures, all things that other languages with closures do should also be possible. One of them is having recursive closures. I.e. something like this:
$factorial = function($n) {
if ($n <= 1)
return 1;
else
return $n * call_user_func(__FUNCTION__, $n - 1);
};
which does not work. One of the ways to do it is to use Y combinator function, which allows, by application of dark magic and friendly spirits from other dimensions, to convert non-recursive code to recursive code. In PHP, Y combinator function would look like this:
function Y($F) {
$func = function ($f) { return $f($f); };
return $func(function ($f) use($F) {
return $F(function ($x) use($f) {
$ff = $f($f);
return $ff($x);
});
});
}
And then the factorial function would be:
$factorial = Y(function($fact) {
return function($n) use($fact) {
return ($n <= 1)?1:$n*$fact($n-1);
};
});
Which does work:
var_dump($factorial(6)); ==> int(720)
Of course, we could also cheat and go this way:
$factorial = function($n) use (&$factorial) {
if ($n <= 1)
return 1;
else
return $n * $factorial($n - 1);
};
Doing Y-combinator in PHP was attempted before (and here), but now I think it works better. It could be even nicer if PHP syntax allowed chaining function invocations – ($foo($bar))($baz) – but for now it doesn’t.
If you wonder, using such techniques does have legitimate applications, though I’m not sure if doing it in PHP this way is worth the trouble.
