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In-Reply-To: <07f176a5c2ff0338cb67c9755bf37af6dcc2d465.camel@schlueters.de>
Date: Sat, 28 Mar 2020 17:44:28 +0100
Message-ID: <CAF+90c_w=igQi3skmiOKeJ+t=BjRTmM-7u=-SOpj_ri+gxZ08Q@mail.gmail.com>
To: =?UTF-8?Q?Johannes_Schl=C3=BCter?= <johannes@schlueters.de>
Cc: "Christoph M. Becker" <cmbecker69@gmx.de>, Arnold Daniels <arnold.adaniels.nl@gmail.com>, 
	PHP internals <internals@lists.php.net>, =?UTF-8?Q?Jan_B=C3=B6hmer?= <jan.h.boehmer@gmx.de>
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Subject: Re: [PHP-DEV] [VOTE] Userspace operator overloading
From: nikita.ppv@gmail.com (Nikita Popov)

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On Sat, Mar 28, 2020 at 5:28 PM Johannes Schl=C3=BCter <johannes@schlueters=
.de>
wrote:

> On Sat, 2020-03-28 at 17:05 +0100, Christoph M. Becker wrote:
> > On 28.03.2020 at 15:57, Johannes Schl=C3=BCter wrote:
> >
> > > On March 28, 2020 1:25:11 PM GMT+01:00, "Christoph M. Becker" <
> > > cmbecker69@gmx.de> wrote:
> > >
> > > > This "try left/right" approach is how operator overloading works
> > > > for
> > > > internal classes[1], and apparently, it works quite well, as long
> > > > as it
> > > > is not overused.
> > >
> > > The fact that it works in one or two cases as an implementation
> > > detail where the full implementation is controlled by a single
> > > group (internals) is no indication for it to work at large.
> >
> > Fair enough.  But maybe Python, where userland operator overloading
> > works similar to the proposal at hand, is? :)
>
> It doesn't:
>
>
> Python 3.6.9 (default, Nov  7 2019, 10:44:02)
> [GCC 8.3.0] on linux
> Type "help", "copyright", "credits" or "license" for more information.
> >>> class A:
> ...     def __add__(self, other):
> ...         print("add")
> ...
> >>> a =3D A()
> >>> a + 1
> add
> >>> 1 + a
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: unsupported operand type(s) for +: 'int' and 'A'
>

It does.

Because Python uses instance methods for operator overloading, it has to
use pairs of methods like __add__ and __radd__ to handle commuted variants.
If you want to implement a commutative add, you need to implement both
__add__ and __radd__. If __add__ is not implemented or returns
NotImplemented, then __radd__ will be called.

So, as Christoph has said, this is indeed the same system as what is being
proposed here, with the difference that we do not need two separate methods
per operator, but can use a static method instead.

Regards,
Nikita

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