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Date: Mon, 1 Mar 2010 14:30:02 +0100
Message-ID: <e666c3891003010530g2af44b7cu894660377b0cca3c@mail.gmail.com>
To: "mathieu.suen" <mathieu.suen@easyflirt.com>
Cc: internals@lists.php.net
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Subject: Re: [PHP-DEV] Closure/lambda semantic comparaison
From: colder@php.net (Etienne Kneuss)

Hello,

On Mon, Mar 1, 2010 at 1:33 PM, mathieu.suen <mathieu.suen@easyflirt.com> w=
rote:
> 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) =3D> 7
>
>
> Now let reproduce that in php:
>
> $adder =3D function ($x) {return function ($y) use($x) =A0{return $y + $x=
;};};
> $add3 =3D $adder(3);
> echo $add3(4); =3D> 7
>
> Every thing is fine but let change the adder into a counter :
> In lisp you could do:
>
> (let ((sum 0))
> =A0 =A0 (defun counter () (setf sum (1+ sum))))
> (counter) =3D> 1
> (counter) =3D> 2
> (counter) =3D> 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.
>
> $createCounter =3D function ($sum) { return function () use ($sum) {retur=
n
> ++$sum;};};
> $counter =3D $createCounter(0);
> $counter(); =3D> 1
> $counter(); =3D> 1
> $counter(); =3D> 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:
>
> $array =3D array(1, 2, 3);
> foreach($array as $pos)
> {
> =A0 $adders[] =3D function ($x) use (&$pos) { return $pos+ $x;};
> }
>
> foreach($adders as $addIt)
> {
> =A0echo $addIt(5);
> }
>

use($var) will import by copy, so you won't have the behavior you
expect. If you want to keep a value in a function between calls, you
can use "static $sum =3D 0;".

> Thanks for your attention
>
> -- Mathieu Suen
>
> --
> PHP Internals - PHP Runtime Development Mailing List
> To unsubscribe, visit: http://www.php.net/unsub.php
>
>



--=20
Etienne Kneuss
http://www.colder.ch