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Date: Sat, 7 Feb 2026 17:51:28 +0100
User-Agent: Mozilla Thunderbird
Subject: Re: [PHP-DEV] [RFC] Partial Function Application for instance of
 non-static methods ("$this")
To: =?UTF-8?Q?Tim_D=C3=BCsterhus?= <tim@bastelstu.be>
Cc: php internals <internals@lists.php.net>,
 Arnaud Le Blanc <arnaud.lb@gmail.com>
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From: bobwei9@hotmail.com (Bob Weinand)

Hey Tim, I think you jumped to a conclusion without exploring the space 
of possibilities fully:

On 6.2.2026 16:12:05, Tim Düsterhus wrote:
>
> With regard to your comment on the PR and the “Open Issue” listed in 
> the RFC about the name for the `$this` parameter in the resulting 
> Closure we have two options to offer:
>
> 1. `ClassName::methodName(this: ?)` with the parameter being called 
> `$__this`.
>
> 2. `ClassName::methodName($this: ?)` with the parameter being called 
> `$this`.
>
> Notably `ClassName::methodName(this: ?)` with the parameter being 
> called `$this` is not an option, because it would result in 
> inconsistent behavior:
>
> `ClassName::methodName(this: ?)` semantically relies on `$this` never 
> being a valid parameter name, such that `this: ?` can unambiguously 
> refer to the “implicit `$this` value” for a method call. However when 
> the parameter in the generated Closure would be called `$this`, there 
> is some ambiguity for cases like:
>
>      $c = DateTime::format(this: ?, format: ?);
>      $c2 = $c(this: ?, format: 'c');
>
> Is `this: ?` in the definition of `$c2` referring to the `$this` 
> parameter of the generated Closure or is it an attempt to partially 
> apply the `Closure` object for the `Closure::__invoke()` method that 
> is referenced by `$c`?
>
> Similarly allowing `this: $object` with a concrete value is explicitly 
> disallowed, because of possible ambiguity with regard to inheritance:
>
>      class P { public function m() { echo "P"; } }
>      class C extends P { public function m() { echo "C"; } }
>
>      // is this calling P::m() or is this calling C::m()?
>      P::m(this: new C());
>
> This would however prevent calling a partially applied instance method 
> with named arguments:
>
>      $c = DateTime::format(this: ?, format: ?);
>      // Disallowed, because this: must be partially applied.
>      $c(this: new DateTime(), format: 'c');
>
> If the syntax to define the PFA was using `C::m($this: ?)`, `$this` in 
> the resulting Closure would just work like any other parameter.
> 1. `ClassName::methodName(this: ?)` with the parameter being called 
> `$__this`.
>
> 2. `ClassName::methodName($this: ?)` with the parameter being called 
> `$this`. 


Allowing P::m(this: $object) with a concrete value should behave 
identically to P::m(this: ?)($object), which in turn should behave 
identically to $object->m() if called outside of C (or its children), 
otherwise equivalently to `private function forwardM() { return P::m(); 
} $object->forwardM();` (i.e. a (grand)parent class method can always be 
called).

This approach is consistent with how method calling works in child classes.


More normatively for any class P and method m, P::m(this: ?) needs to 
store the class where the PFA closure is created as called_scope on the 
Closure when that class is instanceof P unless P::m is abstract, so that 
any call to the resulting closure is checked against that called_scope: 
If the $this object the Closure is ultimately called with is instanceof 
the called_scope of the Closure, the specific given method must be 
called (i.e. specifically P::m()), otherwise the object is merely 
checked for being instanceof P and the method m on the $this object is 
called directly.


Defining it this way preserves LSP guarantees, with maximum flexibility 
- making it for example perfectly possible to call 
array_map(ParentClass::someValue(this: ?), $objectsArray) without being 
surprised that it subtly actually calls the child method someValue on 
objects which are instanceof the containing class.


The neat benefit is that any $obj->method() call is now generalized (on 
the surface of the language semantics, obviously not implementation 
wise) as {get_class($obj)}::method(this: $obj), which makes the this-PFA 
a trivial extension of just having ? for the this parameter.
It also obsoletes any concerns about how the this parameter ends up 
called in the resulting PFA - because it's just a "normal" parameter 
then, from the perspective of the caller.

The only small caveat is Closure::__invoke(this: ?), which literally is 
just sort of an identity function and thus useless. To solve that, we 
should just decide to have an explicit this parameter on Closures take 
precendence. (Ultimately __invoke is more of an implementation detail of 
Closure, than anything else).

Also of note is that having a proper implicit $this parameter on methods 
must not have a position (as in positional parameters). Otherwise 
conflicts arise with e.g. the implicit $this forwarding in parent::m() 
syntax in class scope. (obviously, once you create a PFA P::m(this: ?, 
?) the first positional parameter becomes the $this parameter and the 
original first parameter is now the second one.)


To summarize: Please introduce a first-class implicit $this parameter on 
any non-static method call, and have PFA work naturally with it, without 
doing contortions in language semantics / introducing a very 
PFA-specific syntax.


Thanks,
Bob


P.s.: You should possibly add to future scope allowing 
object::someMethod(this: ?) or class::someMethod(this: ?) to allow 
proper duck typing without having to know the actual object behind. 
That's something I see people asking for, too.