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Date: Tue, 02 Mar 2010 00:08:18 +0200
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Subject: Re: [PHP-DEV] Closure/lambda semantic comparaison
From: ionut.g.stan@gmail.com ("Ionut G. Stan")

In my opinion this is consistent with the fact that PHP has no 
block-scoped variables. The situation is similar with the one in JavaScript.

In JavaScript the problem has been solved by Mozilla with so-called let 
definitions[1], which are block-scoped.

1: 
https://developer.mozilla.org/en/New_in_JavaScript_1.7#Block_scope_with_let

On 3/1/10 2:33 PM, mathieu.suen wrote:
> Hi,
>
> I am proposing a comparison between PHP closure with lisp one.
>
> First thing to compare is how scope are capture:
> Suppose I want to create a function that add.
> The common idiom to do it in lisp is:
>
> (defun adder (x) #'(lambda (y) (+ x y)))
>
> Then if I want the add3 method you can do:
>
> (setf (symbol-function 'add3) (adder 3))
> (add3 4) => 7
>
>
> Now let reproduce that in php:
>
> $adder = function ($x) {return function ($y) use($x) {return $y + $x;};};
> $add3 = $adder(3);
> echo $add3(4); => 7
>
> Every thing is fine but let change the adder into a counter :
> In lisp you could do:
>
> (let ((sum 0))
> (defun counter () (setf sum (1+ sum))))
> (counter) => 1
> (counter) => 2
> (counter) => 3 ...
>
> In other to have the same behavior in PHP you first need to transform
> the let into a lambda which make no difference in lisp:
>
> (funcall #'(lambda (sum) (defun counter () (setf sum (1+ sum)))) 0)
>
> Now we are ready to do it in PHP.
>
> $createCounter = function ($sum) { return function () use ($sum) {return
> ++$sum;};};
> $counter = $createCounter(0);
> $counter(); => 1
> $counter(); => 1
> $counter(); => 1
> ...
>
> So that's not the expected behavior.
> In oder to have the expected behavior you need to add & in front of $sum.
> But & is kind of evil since it mimics dynamic scoping:
>
> $array = array(1, 2, 3);
> foreach($array as $pos)
> {
> $adders[] = function ($x) use (&$pos) { return $pos+ $x;};
> }
>
> foreach($adders as $addIt)
> {
> echo $addIt(5);
> }
>
> Thanks for your attention
>
> -- Mathieu Suen
>

-- 
Ionut G. Stan
I'm under construction  |  http://blog.igstan.ro/