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Subject: Re: [PHP-DEV] [RFC] [VOTE] Deprecations for PHP 8.4
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Date: Wed, 24 Jul 2024 05:47:54 -0400
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To: Larry Garfield <larry@garfieldtech.com>
From: mike@newclarity.net (Mike Schinkel)

> On Jul 23, 2024, at 12:26 PM, Larry Garfield <larry@garfieldtech.com> =
wrote:
>=20
> Hypothetical pattern matching example:
>=20
> $foo is ['a' =3D> int, 'b' =3D> $b, 'c' =3D> mixed];
>=20
> 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.
>=20
> The suggestion is to basically alias _ to "mixed" for pattern =
purposes:
>=20
> $foo is ['a' =3D> int, 'b' =3D> $b, 'c' =3D> _];
>=20
> 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.

I do not see this use-case as compelling.   =20

`mixed` is perfectly sufficient and using `_` for a data types just =
gives two ways to do the same. Not that multiple ways to do the same =
thing is necessarily wrong, but I think it needs a better justification =
than just to save characters.  Besides, it has the potential to confuse =
people as to its exact meaning whereas `mixed` does not.

OTOH, if you really want to say characters =E2=80=94 albeit not as many =
=E2=80=94 then choose `any`, which is certainly less likely to be =
confusing and has an analog in Go, TypeScript, and Python, at least.

Also, AFAIK, few (no?) other languages actually allow for using `_` for =
types, they only allow using them for variables.  I know that to be the =
case for Go, and if I understand the docs correctly it is also true for =
Rust, Zig, Haskell and Swift, with caveats for Rust. =20

- Rust allows underscore for type inference, e.g.:  let x: _ =3D "Hello, =
world!";
- Also for a Generics' type placeholder, e.g.:  let vec: Vec<_> =3D =
vec![1, 2, 3];
- But as for Rust pattern matching, the underscore is only used for =
values, not types.

For any other languages, I cannot say.

> 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):
>=20
> [$a, $b, _] =3D explode(':', 'foo:bar:baz');

This is actually where a "blank" variable represented by `_` actually =
makes a lot of sense.  It is also how Go and Zig use them and =
effectively also how Rust, Haskell, and Swift use them. =20

Unlike for types where we have `mixed`, there is no current globally =
consistent alternate to using a blank variable in PHP. The only option =
is to use an arbitrary name that other developers won't know the =
intention of unless the developer adds comments to the effect.

In summary, although I don't have strong feelings about deprecating =
classes named `_`, I do not think the arguments made for disallowing =
them actually have any analog in any other languages so I question if =
there is valid justification for the deprecation. #jmtcw #fwiw

-Mike=