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Date: Wed, 23 Jul 2025 08:55:41 -0500
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Subject: Re: [PHP-DEV] [RFC] Partial Function Application v2
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From: larry@garfieldtech.com ("Larry Garfield")

On Wed, Jul 23, 2025, at 3:56 AM, Tim D=C3=BCsterhus wrote:
> Hi
>
> Am 2025-06-28 07:06, schrieb Larry Garfield:
>> https://wiki.php.net/rfc/partial_function_application_v2
>
> I've now given the RFC an in-depth read. I have the following remarks:
>
> 1.
>
>>  If the function is variadic, there are two additional rules:
>>=20
>> - Any positional placeholders that run into the variadic portion beco=
me=20
>> required.
>> - If any positional placeholders run into the variadic portion, all=20
>> prior remaining placeholders become required. However, those paramete=
rs=20
>> may not be called with named arguments, as there is no name to use.
>
> I do not understand what =E2=80=9Crunning into the variadic portion=E2=
=80=9D means. An=20
> example would probably be helpful. The first bullet point is probably=20
> intended to mean the following:
>
>      function foo($a, ...$b) { }
>      $pfa =3D foo(?, ?, ?, ?);
>
> Now `$pfa` has 4 required parameters, despite `foo()` only having 1?
>
> But I don't understand what the second bullet point is intended to say.

I have added the following example:

function foo(int $a =3D 5, int $b =3D 1, string ...$c) { }
$pfa =3D foo(?, ?, ?, ?);

// Equivalent to this:
// Note that $a and $b become required, because there must be at least 4=
 arguments.
$pfa =3D fn(int $a, int $b, string $c1, string $c2) =3D> foo($a, $b, $c1=
, $c2);


> 2.
>
> In the placeholder semantics section:
>
>>  While in theory that means a call like this would be legal:
>
>  From what I see this specific case is not part of the examples sectio=
n=20
> either. Including the desugaring of this extreme case would certainly=20
> help understanding.

I have added the following equivalent:

$c =3D fn(string $s, Point $p, int $m =3D 0) =3D> stuff(1, $s, 3.14, $m);

> 3.
>
> In the =E2=80=9CExamples=E2=80=9D section:
>
> The choice of parameters names makes it hard to understand the example=
s,=20
> particularly when named parameters are used to provide arguments out o=
f=20
> order. Including the =E2=80=9Cposition=E2=80=9D in the name of the par=
ameter would make=20
> it easier to follow the example, since it is not necessary to look up=20
> the signature of `stuff()` all the time. (it becomes manageable if you=20
> read it as a sentence "is FPM")

LOL.  That was completely unintentional. :-)

However, I have gone through and added numbers to the variable names to =
clarify their original ordering.

> 4.
>
> In the =E2=80=9CExamples=E2=80=9D section:
>
> Is the =E2=80=9Cthunk=E2=80=9D example accurate? It's the only example=
 where the=20
> resulting =E2=80=9CPFA=E2=80=9D includes a variadic picking up the rem=
aining arguments.

Hm.  I think you're right, with the Extraneous Args section's clarificat=
ion, the others should likely have a trailing ...$args as well.  I will =
clarify with Arnaud when he returns and update accordingly.

> 5.
>
> In the =E2=80=9CExamples=E2=80=9D section:
>
>> $c =3D stuff(?, p: $point, f: ?, s: ?, m: 4);
>
> It is unexpected to me that this definition will take `$s` before `$f`=20
> in the resulting Closure. The explanation says that the =E2=80=9Corder=
 does not=20
> matter=E2=80=9D, but it is certainly unexpected that the order *change=
s* during=20
> desugaring.
>
> In fact, the "(four(c: ?, d: 4, b: ?, a: 1))(2, 3);" example further=20
> down below seems to contradict this.

The order is determined by the original function.  The same is true for =
a normal function call.

function foo(int $a, int $b, int $c) {}

// All of these are equivalent.
foo(a: 1, b: 2, c: 3);
foo(b: 2, a: 1, c: 3);
foo(c: 3, b: 2, a: 1);
foo(1, 2, 3);

So the same is true of a PFA:

foo(a: ?, b: 2, c: ?);
foo(b: 2,  a: ?, c: ?);
foo(c: ?, b: 2,  a: ?);
foo(?, 2, ?);

All of those produce the same result.

> 6.
>
> In the =E2=80=9CExtraneous arguments=E2=80=9D section:
>
> I disagree with silently ignoring trailing arguments, because it *adds=
*=20
> to the existing inconsistency between userland and internal functions.=20
> In fact there was a recent-ish discussion (I would provide a link, but=20
> can't find it right now) about allowing userland functions to define=20
> that they want to be called with a strict arity.
>
> While it would add to the symbol soup, having an explicit =E2=80=9Cign=
ore the=20
> argument at this position=E2=80=9D indicator would be useful. For exam=
ple:=20
> `$firstNonZero =3D array_find($arr, intval(?, _));` (I understand this=20
> specific one doesn't work, because `_` is a valid constant).
>
> 7.
>
> It is not clear to me from the RFC, why the `...` placeholder to=20
> indicate =E2=80=9Call remaining arguments=E2=80=9D must come between p=
ositional and=20
> named parameters. It would make more sense to me for it to become last=
,=20
> because it would also make it clearer that named parameters take=20
> priority over `...`. Is there some technical limitation that caused th=
is=20
> choice to be made?

I can't find an answer to that in my notes, so I'll have to defer to Arn=
uad when he returns.  (Ilija tells me he's back Monday.)  I think so, bu=
t I'm not certain.

--Larry Garfield