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Date: Wed, 22 Apr 2020 17:30:10 +0100
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Subject: Re: [PHP-DEV] Typed callable properties
From: rowan.collins@gmail.com (Rowan Tommins)

On 22/04/2020 16:01, Michał Brzuchalski wrote:
> I agree, a callable brings as much information as a resource type - you
> know the type
> but are unable to use it without additional information.


While this is true, it's somewhat orthogonal to the question Benas 
raised in this thread - namely, how to handle values that are callable 
in one scope but not another.

That is, is the following code valid (error-free) or not?

delegate Reducer (?int $sum, int $item = 0): int;
class X {
     private static function myPrivateMethod(?int $sum, int $item = 0): 
int {
          return 0;
     }
     public static function runReducer(Reducer $r) {
          return $r(0, 0);
     }
}
echo X::bar(['X', 'myPrivateMethod']);


The equivalent code with a type constraint of "callable" currently runs 
fine, even though passing the same value to a different function would 
fail due to scoping. Making the delegate-constrained version throw an 
error for any private or protected method would just trade one 
inconsistency for another.

A more reasonable restriction, IMO, would be to say that delegates only 
match Closure instances (and possibly other objects with a public 
__invoke method), not the various string and array syntaxes that 
"callable" currently allows. The case above would then need to bind the 
private method into a closure that is callable everywhere, e.g. using 
Closure::fromCallable([self::class, 'myPrivateMethod']);

The obvious downside is that Closure::fromCallable is both verbose and a 
potential performance hit. So it seems we're back again to wanting a new 
syntax to succinctly and optimally generate such a closure. For 
instance, in the discussion of ::func, one suggestion was 
"$closure={self::myPrivateMethod};" and more recently it was mentioned 
that a variant of partial application is to not actually bind anything, 
and write "$closure=self::myPrivateMethod(?, ?);"


Regards,

-- 
Rowan Tommins (né Collins)
[IMSoP]