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Subject: Re: [PHP-DEV] Local constants
In-Reply-To: <27c3909f-05f4-4256-a447-10e8d8760fff@app.fastmail.com>
Date: Wed, 4 Sep 2024 16:17:07 -0400
Cc: internals@lists.php.net
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Message-ID: <F9C739E2-2FA3-4CAF-9A9B-CE343F84B461@zort.net>
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 <27c3909f-05f4-4256-a447-10e8d8760fff@app.fastmail.com>
To: Rob Landers <rob@bottled.codes>
From: jbafford@zort.net (John Bafford)

On Sep 4, 2024, at 15:23, Rob Landers <rob@bottled.codes> wrote:
>=20
> On Tue, Sep 3, 2024, at 05:20, HwapX wrote:
>> Hello internals!
>>=20
>> I was wondering, has there been any discussion about supporting local =
constants (variables that cannot be reassigned, perhaps even function =
parameters)?
>=20
> Out of curiosity, what value would this bring to PHP? In my =
experience, modern php methods and functions tend to fit on a single =
screen, at most being a few hundred lines for complex logic.
>=20
> =E2=80=94 Rob

This is about semantic clarity. If you define a variable as a constant, =
then, not only do you know for certain that it cannot change, you are =
also stating "it is my intent to not change this variable after setting =
it", and so if someone later tries to do so, they (should) have to =
answer the question, "*why* do I need to make this variable change?".

If it's useful to be able to annotate class members as being "readonly", =
it is likewise useful to do that on a local scope.

With sufficient compiler support, *typed* constant variable declarations =
might also allow for this:

	let SomeType $foo; //or readonly, or writeonce, or whatever
=09
	if($something) {
		$foo =3D ...;
	} else if($somethingElse) {
		$foo =3D getSomeString(); //Error, type mismatch; =
possibly catchable at compile time, definitely with static analysis
	} else if($thirdCondition) {
		$foom =3D ...; //oops, typo
	} else {
		throw new Exception(...)
	}
=09
	doSomething($foo); //Compiler error: $foo not initialized on all =
call paths.
}

You might also tighten scoping with such variables:

	foreach(...) {
		let $foo =3D //local working variable; possibly even =
shadowing a variable in the parent scope
	}

	//$foo is undefined; you can't access the last-set value from =
the loop outside of the loop

Static analysis can catch all these errors, but it'd be nicer to be able =
to do it without requiring docblocks. The language providing tools for =
being more explicit about intent in code is never bad.

-John