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In-Reply-To: <b3fba6e2-8547-4de6-a09f-bd980b24a97a@www.fastmail.com>
Date: Wed, 30 Dec 2020 20:50:50 +0100
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To: Larry Garfield <larry@garfieldtech.com>
Cc: php internals <internals@lists.php.net>
Content-Type: multipart/alternative; boundary="000000000000c9cceb05b7b3d34b"
Subject: Re: [PHP-DEV] Analysis of property visibility, immutability, and
 cloning proposals
From: olleharstedt@gmail.com (=?UTF-8?Q?Olle_H=C3=A4rstedt?=)

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On Wed, 30 Dec 2020, 20:27 Larry Garfield, <larry@garfieldtech.com> wrote:

>
> > > That's a good summary of why immutability and with-er methods (or some
> > > equivalent) are more ergonomic.
> > >
> > > Another point to remember: Because of PHP's copy-on-write behavior,
> full on
> > > immutability doesn't actually waste that much memory.  It does use up
> some,
> > > but far less than you think.  (Again, based on the tests MWOP ran for
> PSR-7
> > > a ways back.)
> >
> > I thought copy-on-write was only for arrays, not objects?
> >
> > Olle
>
> Copy on write applies to all values; the caveat is that with objects, the
> value being copied is the handle that points to an object in memory, rather
> than the object itself.  That means passing an object by reference can do
> some seriously unexpected things, which is why you basically never do so.
>
> The point here is that if you have an object with 15 internal properties,
> it's memory usage is 15 zvals plus one zval for the object, plus one zval
> for the variable that points to it.  (I'm over-simplifying here.  A lot.)
> If you pass it to a function, only the one zval for the handle is
> duplicated, which is the same as for an integer.
>
> If you clone the object, you don't duplicate 15+1 zvals.  You duplicate
> just the one zval for the object itself, which reuses the existing 15
> internal property entries.  If in the new object you then update just the
> third one, PHP then duplicates just that one internal zval and modifies the
> new one.  So you still are using only 18 zvals, not 36 zvals.  (Engine
> people: Yes, I am *very* over-simplifying.  I know.)
>
> Basically, what in most languages would require manually implementing
> "immutable data structures" we get for free in PHP, which is seriously
> sweet.
>
> The net result is that a with-er chain like this:
>
> $foo2 = $foo->withBar('x')->withBaz('y')->withBeep('z');
>
> is way, way less expensive than it looks, both on memory and CPU.  It is
> more expensive than setters, but not by much.
>

Ok. You have a benchmark for this? I can make one otherwise, for the query
example.

It worries me a little that immutablility is pushed into the ecosystem as a
silver bullet. Main reason functional languages are using it is because
ownership is a newer concept, so it hasn't been adapted as much.

>

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