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To: Larry Garfield <larry@garfieldtech.com>, Dan Ackroyd <Danack@basereality.com>
Cc: internals@lists.php.net
Date: Tue, 14 Jun 2022 12:29:09 +0200
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Subject: Re: [PHP-DEV] [RFC] Short Closures 2, aka auto-capture take 3
From: arnaud.lb@gmail.com (Arnaud Le Blanc)

Hi Dan,

On lundi 13 juin 2022 19:49:10 CEST Dan Ackroyd wrote:
> > Auto-capture in PHP is by-value. This makes this impossible. It also makes
> > explicit declarations non-necessary and much less useful.
> 
> Separating off some pedantism from the hopefully constructive comment,
> 
> I think some of the words in the RFC are a bit inaccurate:
> > A by-value capture means that it is not possible to modify any variables
> > from the outer scope:
> > 
> > Because variables are bound by-value, the confusing behaviors often
> > associated with closures do not exist.
> > 
> > Because variables are captured by-value, Short Closures can not have
> > unintended side effects.
> Those statements are true for scalar values. They are not true for objects:

This is shown in the "No unintended side-effects" section of the RFC.

I agree that the choice of words is inaccurate, as "modify any variable" could 
be interpreted not only as "bind a variable", but also as "mutate a value".

The section you have quoted is meant to show how by-value capture, which is 
the default capture mode in all PHP closures, is less error prone than by-
variable/by-reference capture, by a huge margin. Especially since variable 
bindings do not have side-effects unless a variable was explicitly captured 
by-reference. Do you agree with this ?

The "No unintended side-effects" section assumes that property assignments to 
captured variables are intended side-effects. In your example, the programmer 
intended to have a side effect because `$a` can only come from the declaring 
scope (the code would result in an error otherwise) :

> $a = new Foo('bar');
> $f = fn() {
>     $a->value = 'explicit scope is nice';
> };

Do you have an example where the intent would be less obvious ? With code you 
would actually write ?

Cheers,
--
Arnaud Le Blanc