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Date: Tue, 23 Jul 2024 16:26:59 +0000
To: "php internals" <internals@lists.php.net>
Subject: Re: [PHP-DEV] [RFC] [VOTE] Deprecations for PHP 8.4
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From: larry@garfieldtech.com ("Larry Garfield")

On Tue, Jul 23, 2024, at 4:00 PM, Matthew Weier O'Phinney wrote:

>> However, a few people indicated a desire to have an explicit wildcard _ anyway, even if it's redundant, as it's a more common and standard approach in other languages.  We've indicated that we are open to making that an optional secondary vote in the pattern matching RFC if there's enough interest (it would be trivial), though I haven't bothered to add it to the RFC text yet.
>> 
>> Having _ available could also be used in other "wildcard" or "ignore this" cases, like exploding into a list assignment or similar, though I don't believe that has been fully explored.
>
> Can you provide examples of what that usage would look like? And the 
> question I have really is, does this actually _require_ using "_", or 
> could another token be used for such matches?

Hypothetical pattern matching example:

$foo is ['a' => int, 'b' => $b, 'c' => mixed];

That would assert that there's 3 keys.  "a" may be any integer (but only an integer), "b" can be anything and will be captured to a variable, and "c" must be defined but we don't care what it is.

The suggestion is to basically alias _ to "mixed" for pattern purposes:

$foo is ['a' => int, 'b' => $b, 'c' => _];

As "there's a var here but I don't care what it is, ignore it" is a common meaning of _ in other languages.  But that would need to be disambiguated from a pattern saying "c must be an instance of the class _".  Technically any symbol/set of symbols could be used there (as it's just an alias to mixed, which has the exact same effect), but _ is a common choice in other languages.

In theory, that could be expanded in the future to something like (note: this hasn't been seriously discussed that I know of, I'm just spitballing randomly):

[$a, $b, _] = explode(':', 'foo:bar:baz');

To assign $a = "foo", $b  to "bar", and just ignore "baz".  Which might cause parser issues if _ is a legal class name, I'm not sure.  There's probably other "ignore this" cases we could come up with, but I haven't actually thought about it.

Again, whether any of the above is a compelling argument or not I leave as an exercise for the reader.

--Larry Garfield