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In-Reply-To: <CAF+90c8yhAfzTP7apPSbbrcSuqhiBv2Y_XAD=U5MLG+SoezFRA@mail.gmail.com>
Date: Fri, 25 Mar 2016 13:46:29 +0100
Cc: PHP internals <internals@lists.php.net>
Message-ID: <7C805132-E4B3-482A-8B4D-8E949C06639F@gmail.com>
References: <9399032E-5BAA-4ABF-81AD-13716790D8DB@gmail.com> <CAF+90c8yhAfzTP7apPSbbrcSuqhiBv2Y_XAD=U5MLG+SoezFRA@mail.gmail.com>
To: Nikita Popov <nikita.ppv@gmail.com>
Subject: Re: [PHP-DEV] Combine ??= and ?:= assignment operator rfc's.
From: mtkocak@gmail.com (Midori Kocak)

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h=
ttp://www.rubyinside.com/what-rubys-double-pipe-or-equals-really-does-5488=
.html =
<http://www.rubyinside.com/what-rubys-double-pipe-or-equals-really-does-54=
88.html>

> On 25 Mar 2016, at 13:42, Nikita Popov <nikita.ppv@gmail.com> wrote:
>=20
> On Fri, Mar 25, 2016 at 11:59 AM, Midori Kocak <mtkocak@gmail.com =
<mailto:mtkocak@gmail.com>> wrote:
> Hi Everyone,
>=20
> I think it's better idea to combine those two assignment operator =
RFC=E2=80=99s. So I am going to close the current one and open ??=3D =
with ?:=3D
> What do you think? And we have to find better names.
>=20
> Wishes,
> Midori Kocak
>=20
> I'd prefer to keep them separate, or at least keep their votes =
separate. The ??=3D operator vote is currently unanimous at 24:0, while =
the ?:=3D vote was closed at something like 9:2, so there clearly are =
differences of opinion regarding these two operators.
>=20
> I'll use this chance for some comments on the proposal. I can see the =
general usefulness of ??=3D, but right now the RFC is severely =
underspecified and I'm uncomfortable voting on it in it's current form =
as so much will depend on the final implementation. So, what do I mean =
by underspecified?
>=20
> The only statement the RFC essentially makes is that $a ??=3D $b will =
be the same as $a =3D $a ?? $b, for variable-expression $a and =
expression $b. This statement, while a good high-level illustration, =
does not explain the exact behavior of this operator.
>=20
> For example, consider the expression $a[print 'X'] ??=3D $b. A simple =
desugaring into $a[print 'X'] =3D $a[print 'X'] ?? $b will result in 'X' =
being printed twice. However, this is not how all other existing =
compound assignment operators behave: They will print X only once, as =
the LHS is only evaluated once. I assume that ??=3D would behave the =
same way.
>=20
> However, with ??=3D the problem becomes more complicated. Let us =
assume that $a is an ArrayAccess object and consider the expression =
$a[0] ??=3D $b. Let us further assume that $x =3D $a->offsetGet(0) is =
non-null. Will $a[0] ??=3D $b result in a call to $a->offsetSet(0, $x)? =
This is what would normally happen with a compound assignment operator =
and what would be implied by the desugaring $a[0] =3D $a[0] ?? $b. =
However this assignment is not really necessary, as we're just =
reassigning the same value. So, does the call happen or not? Is the =
proper desugaring maybe if (!isset($a[0])) $a[0] =3D $b?
>=20
> Let us now assume that $a is a recursive ArrayAccess object with =
by-reference offsetGet() and consider the expression $a[0][1] ??=3D =
expr. For a normal compound assignment operator, this would issue the =
call sequence
>=20
>     $b =3D expr;
>     $x =3D& $a->offsetGet(0);
>     $y =3D $x->offsetGet(1);
>     $y OP=3D $b;
>     $x->offsetSet(1, $y);
>=20
> Note that we only issue one offsetSet() at the end. We do not refetch =
$x via $a->offsetGet(0). How would the same work with the ??=3D =
operator? As the RHS is evaluated lazily, it is my opinion that only =
performing the offsetSet() call without refetching $x beforehand would =
violate PHP's indirection memory model. Additionally as ??=3D has to =
fetch offsets in BP_VAR_IS mode, we likely wouldn't be able to write =
them without refetching anymore.
>=20
> So, what would be the desugared call sequence for $a[0][1] ??=3D expr? =
Something like this?
>=20
>     if (!$a->offsetHas(0)) {
>         goto assign;
>     }
>     $x =3D $a->offsetGet(0);
>     if (x =3D=3D=3D null) {
>         goto assign;
>     }
>     if (!$x->offsetHas(0)) {
>         goto assign;
>     }
>     $y =3D $x->offsetGet(0);
>     if ($y =3D=3D=3D null) {
>         goto assign;
>     }
>     goto done;
> assign:
>     $b =3D expr;
>     $x =3D& $a->offsetGet(0);
>     $x->offsetSet(1, $b);
> done:
>=20
> That would be some first thoughts on the issue, though I'm sure there =
are more subtleties involved. I'd like to see the exact behavior of ??=3D =
(and ?:=3D) specified.
>=20
> I'm also pretty sure that writing a patch for this will not be =
entirely easy. The combination of execute-once LHS side-effects and lazy =
RHS execution does not translate well to PHP's VM constraints.
>=20
> Regards,
> Nikita


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