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Date: Fri, 28 May 2021 17:01:29 -0400
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To: Nikita Popov <nikita.ppv@gmail.com>
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Subject: Re: [PHP-DEV] A little syntactic sugar on array_* function calls?
From: mike@newclarity.net (Mike Schinkel)

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Hi Nikita,

Thank you for taking the time to explain in detail. =20

One more question below.

-Mike

> On May 28, 2021, at 10:31 AM, Nikita Popov <nikita.ppv@gmail.com> =
wrote:
>=20
> On Fri, May 28, 2021 at 3:11 AM Mike Schinkel <mike@newclarity.net =
<mailto:mike@newclarity.net>> wrote:
> > On May 26, 2021, at 7:44 PM, Hendra Gunawan =
<the.liquid.metal@gmail.com <mailto:the.liquid.metal@gmail.com>> wrote:
> >=20
> > Hello.
> >=20
> >>=20
> >> Yes, but Nikita wrote this note about technical limitations at the =
bottom of the repo README:
> >>=20
> >> Due to technical limitations, it is not possible to create mutable =
APIs for
> >> primitive types. Modifying $self within the methods is not possible =
(or
> >> rather, will have no effect, as you'd just be changing a copy).
> >>=20
> >=20
> > If it is solved, this is a great accomplishment for PHP. But I think
> > scalar object is not going anywhere in the near future. If you are =
not
> > convinced, please take a look
> > =
https://github.com/nikic/scalar_objects/issues/20#issuecomment-569520181 =
<https://github.com/nikic/scalar_objects/issues/20#issuecomment-569520181>=
.
>=20
> Nikita's comment actually causes me more questions, not fewer.
>=20
> Nikita says "We need to know that $a[$b][$c is an array in order to =
determine that the call should be performed by-reference. However, we =
already need to convert $a, $a[$b] and $a[$b][$c] into references before =
we know about that." =20
>=20
> How then are we able to do the following?:
>=20
> $a[$b][$c][] =3D 1;
>=20
> In this case, we're clearly performing a write operation on the array. =
If you want to know the technical details, the compiler will convert =
this into a sequence of FETCH_DIM_W ops followed by ASSIGN_DIM. The "W" =
bit here is for "write", which will perform all the necessary special =
handling, such as copy-on-write separation and auto-vivification.
>=20
> How also can we do this:
>=20
> byref($a[$b][$c]);
> function byref(&$x) {
>     $x[]=3D 2;
> }
>=20
> See https://3v4l.org/aPvTD <https://3v4l.org/aPvTD> =
<https://3v4l.org/aPvTD <https://3v4l.org/aPvTD>>
>=20
> This is a more complex case. In this case the compiler doesn't know in =
advance whether the argument is passed by value or by reference. What =
happens here is:
>=20
> 1. INIT_FCALL determines that we're calling byref().
> 2. CHECK_FUNC_ARG for the first arg determines that this argument is =
passed by-reference for this function.
> 3. FETCH_DIM_FUNC_ARG on the array will be perform either an =
FETCH_DIM_R or to FETCH_DIM_W operation, depending on what =
CHECK_FUNC_ARG determined.
>=20
> I assume that in both my examples $a[$b][$c] would be considered an =
"lvalue"[1] and can be a target of assignment triggered by either the =
assignment operator or calling the function and passing to a by-ref =
parameter. =20
>=20
> [1] =
https://en.wikipedia.org/wiki/Value_(computer_science)#Assignment:_l-value=
s_and_r-values =
<https://en.wikipedia.org/wiki/Value_(computer_science)#Assignment:_l-valu=
es_and_r-values>
>=20
> So is there a reason that -> on an array could not trigger the same?  =
Is Nikita saying that the performance of those calls performed =
by-reference would not matter because they are always being assigned, at =
least in the former case, but to do so with array expressions would be =
problematic? (Ignoring there is no code in the wild that currently uses =
the -> operator, or does that matter?)
>=20
> Note that the byref($a[$b][$c]) case only works because we know which =
function is being called at the time the argument is passed. If you have =
$a[$b][$c]->test() we need to pass $a[$b][$c] by reference (FETCH_DIM_W) =
or by value (FETCH_DIM_R) depending on whether $a[$b][$c]->test() =
accepts the argument by-value or by-reference. But we can only know that =
once we have already evaluated $a[$b][$c] and found out that it is =
indeed an array.
>=20
> The only way around this is to *always* perform a for-write fetch of =
$a[$b][$c], even though we don't know that the end result is going to be =
an array. However, doing so would pessimize the performance of code =
operating on objects. Consider $some_huge_shared_array[0]->foo(). If we =
fetch $some_huge_shared_array for write, we'll be required to perform a =
full duplication of the array in preparation for a possible future =
write. If it turns out that $some_huge_shared_array[0] is actually an =
object, or that $some_huge_shared_array[0] is an array and the performed =
operation is by-value, then we have performed this copy unnecessarily.
>=20
> I don't believe this is acceptable.
>=20
> I ask honestly to understand, and not as a rhetorical question.
>=20
> Additionally, if the case of updating an array variable is not a =
problem but updating an array expression is a problem then why not just =
limit the -> operator to only work on expressions for immutable methods =
and require variables for mutable methods?  I would think should be easy =
enough to throw an error for those specific "methods" that would be =
mutable, such as shift() and unshift() if $a[$b][$c]->shift('foo') were =
called?
>=20
> There are externalities associated even with the simple $x->foo() =
case, though they are less severe. They primarily involve reduced =
ability to analyze code in opcache.
>=20
> In either case, this limitation does not seem reasonable to me from a =
language design perspective. If $a->push($b) works, then =
$a[$k]->push($b) can reasonably be expected to work as well.
> =20
> Or maybe just completely limit using the -> operator on array =
variables. Don't work on any array expressions for consistency. There is =
already precedence in PHP for operators that work on variables and not =
on expressions:  ++, --, and &.
>=20
> IF we can get a thumbs up from Nikita that one of these would actually =
be possible then I think the next step should be to write up a list of =
proposed array methods that would be implemented to support the -> =
operator with arrays and put them in an RFC, and to flesh out any edge =
cases.=20
>=20
> The only correct way to resolve this issue is to not support mutable =
operations.

I don't think I agree that this is the only correct way, but I respect =
your position of authority on the matter.

> I don't think there's much need for mutable operations. sort() and =
shuffle() would be best implemented by returning a new array instead. =
array_push() is redundant with $array[]. array_shift() and =
array_unshift() should never be used.

Why do you say array_shift() and array_unshift() should never be used?  =
When I wrote the above questions the use-case I was thinking about most =
was $a->unshift($value) as I use array_unshift() more than most of the =
other array functions.

Do you mean that these if applied as "methods" to an array should not be =
use immutably =E2=80=94 meaning in-place is bad but returning an array =
value that has been shifted would be okay =E2=80=94 or do you have some =
other reason you believe that shifting an array is bad?  Note the reason =
I have used them in the past is when I need to pass an array to a =
function written by someone else that expects the array to be ordered.

Also, what about very large arrays?  I assume =E2=80=94 which could be a =
bad assumption =E2=80=94 that PHP internally can be more efficient about =
how it handles array_unshift() instead of just duplicating the large =
array so as to add an element at the beginning?

-Mike=

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