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Date: Mon, 13 Jun 2022 14:36:26 +0100
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Subject: Re: [PHP-DEV] [RFC] Short Closures 2, aka auto-capture take 3
From: rowan.collins@gmail.com (Rowan Tommins)

On 13/06/2022 13:57, Arnaud Le Blanc wrote:
> The proposal is to extend the Arrow Functions syntax so that it allows
> multiple statements.


That's one perspective. The other perspective is that the proposal is to 
extend closure syntax to support automatic capture.


> Currently the `use()` syntax co-exists with auto-capture, but we could change
> it so that an explicit `use()` list disables auto-capture instead:
>
> ```php
> fn () use ($a) { } // Would capture $a and disable auto-capture
> fn () use () { }   // Would capture nothing and disable auto-capture
> ```


That's an interesting idea. I was coming from the other direction, but 
it might make sense I guess.


By the way, the current RFC implies you could write this:

fn() use (&$myRef, $a) { $myRef = $a * $b; }

It's clear that $myRef is captured by reference, and $a by value; but 
what about $b? Is it local to the closure as it would be in a "long" 
closure, or implicitly captured by value as it would be with no "use" 
statement?

It might be best for such mixtures to raise an error.



> Unfortunately, Arrow Functions already auto-capture today, so requiring a
> `use(*)` to enable auto-capture would be a breaking change.


I'm not suggesting any change to arrow functions, just the ability to 
write "use(*)" (or "use(...)") in all the place you can write 
"use($foo)" today.

I don't think that introduces any problems, if you think of "fn" as an 
alternative spelling of "function", and "=>" as expanding to "use(*) { 
return"


>> I don't find the comparison to a foreach loop very convincing. Loops are
>> still only accessing variables while the function is running, not saving
>> them to be used at some indeterminate later time.
> Do you have an example where this would be a problem?


I didn't say anything was a problem; I just said that the comparison 
didn't make sense, because the scenarios are so different.


> Auto-capture in PHP is by-value. This makes this impossible. It also makes
> explicit declarations non-necessary and much less useful.

> Live-variable analysis is mentioned in as part of implementation details. It
> should not be necessary to understand these details to understand the behavior
> of auto-capture.


As noted in my other e-mail, by-value capture can still have side 
effects, so users may still want to ensure that their code is free of 
such side effects.

Currently, the only way to do so is to understand the "implementation 
details" of which variables will be captured, and perhaps add dummy 
statements like "$foo = null;" or "unset($foo);" to make sure of it.


Regards,

-- 
Rowan Tommins
[IMSoP]