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Date: Thu, 22 Jan 2026 17:08:21 +0100
To: Derick Rethans <derick@php.net>
Cc: php internals <internals@lists.php.net>, Arnaud Le Blanc
 <arnaud.lb@gmail.com>
Subject: Re: [PHP-DEV] [RFC] Partial Function Application for instance of
 non-static methods ("$this")
In-Reply-To: <253ee811-0c1d-7eab-4c36-38607401aaf6@php.net>
References: <64953ec741a4c6609519e1878ad37b54@bastelstu.be>
 <253ee811-0c1d-7eab-4c36-38607401aaf6@php.net>
Message-ID: <28461544eb11b41dce7c7563ad879e63@bastelstu.be>
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From: tim@bastelstu.be (=?UTF-8?Q?Tim_D=C3=BCsterhus?=)

Hi

Am 2026-01-22 16:33, schrieb Derick Rethans:
>> https://wiki.php.net/rfc/partial_function_application_this
> 
> 	We propose allowing a named parameter $this: ? to be combined with the
> 	“scope resolution operator” syntax of referring to a method.
> 
> I can't say I am entirely sold on the syntax here — starting with a $
> makes it look like a variable.

Yes, it certainly requires some getting used to it. It's the best we 
could come up with, but if there are any better suggestions, we're open 
to hearing those. I'd like to note the “Rejected Features” section, 
since it lists some non-workable alternatives we considered.

> 	Since the $this: ? parameter does not refer to a parameter in
> 	the underlying argument list and thus there is no ambiguity with
> 	regard to positional parameters it may be placed at any position
> 	of the parameter list
> 
> I think I would be keen on *requiring* this to be the **first** in the
> argument list.
> 
> Although there is no ambiguity, I also can't see why you would want to
> have it anywhere else.

Being able to “reorder” parameters to (better) fit a specific callback 
signature is an explicit feature of PFA that we also wanted to support 
for `$this`.

As an example, suppose objects could be used as an array key. 
`array_find()` first passes the value and then the key to the callback. 
In this case users might want to do the following:

     class SomeObject {
         public function __construct(private string $value) { }
         public function hasMatchingValue(string $value) { return $value 
=== $this->value; }
     }

     $array = [
         new SomeObject('foo') => 'bar',
         new SomeObject('baz') => 'baz',
     ];

     array_find(SomeObject::hasMatchingValue(value: ?, $this: ?), 
$array);

In this case `array_find()` is supposed to return `'baz'`, since this is 
where the values match.

> 	It may also not be used when partially applying an instance
> 	method with a known object.
> 
> 	$c = $dateTime->format($this: ?, "c"); // Fatal error: Invalid use of 
> $this placeholder
> 
> Can you explain (in the RFC) what you mean by "known object" — I don't
> think the language on what this means is clear enough.

I have rephrased that to “It may also not be used when partially 
applying an instance method using the -> operator, since the object is 
already specified:”

Best regards
Tim Düsterhus