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Date: Mon, 01 Mar 2010 13:33:29 +0100
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Subject: Closure/lambda semantic comparaison
From: mathieu.suen@easyflirt.com ("mathieu.suen")

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