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To: Bishop Bettini <bishop@php.net>
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Thread-Topic: [PHP-DEV] Re: Straw poll: Places to allow function calls in
 constant expressions
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Date: Sun, 23 Feb 2020 02:01:49 +0000
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Subject: Re: [PHP-DEV] Re: Straw poll: Places to allow function calls in
 constant expressions
From: tysonandre775@hotmail.com (tyson andre)

> The original motivation was, IIRC, to initialize class constants with an =
expression.=0A=
> Following KISS, I feel we should constrain our efforts to that scope, and=
 that's how I voted:=0A=
> "Yes" on class constants and static members, "No" on everything else.=0A=
=0A=
I'd also personally found the `static $x =3D null; if ($x =3D=3D=3D null) {=
 $x =3D some_fn(); }` pattern inconvenient and wanted that to be just `stat=
ic $x =3D some_fn();` to make the initial value `some_fn()` if possible. It=
 also seemed more readable (e.g. doesn't matter if some_fn() can be constan=
t).=0A=
=0A=
My personal use case for parameter defaults and global constants isn't as s=
trong (especially with define()),=0A=
but it's still something I'd want.=0A=
=0A=
- I'd be able to use `private const FOO_DEFAULT =3D array_keys(...); functi=
on foo($x =3D FOO_DEFAULT) {}` if constants did pass, so that's less off a =
concern, and ensures&makes clear that the expression is only evaluated once=
, so it might be a better place to start with.=0A=
=0A=
> As for implementation, we must manage the complexity.=0A=
> I'm a hard "No" on restricting use to an arbitrary, variable list of bles=
sed functions.=0A=
> It's a dev UX nightmare to include a bevy of array_* functions but no str=
* functions.=0A=
=0A=
If the RFC succeeded with a whitelist, I'd planned to create RFCs to add to=
 the whitelist.=0A=
Locale dependency of string functions was the reason for leaving it out ini=
tially, since that may also be a hard "no" for others.=0A=
=0A=
> Consequently, the only way to safely initialize class constants and stati=
c members is at run-time, and I can only think of one way to do it.=0A=
> Apologies if this has already been suggested: taking a cue from C#, a sta=
tic class constructor ([1]) =0A=
> would allow us to have expression-initialized constants and static member=
s.=0A=
> <?php=0A=
> class Config {=0A=
>     public const URL =3D null; // compile time initialization=0A=
>     protected static $mtime =3D null; // compile time initialization=0A=
>     private static function __constructStatic() {=0A=
>         $env =3D json_decode(file_get_contents('config.json'));=0A=
>         self::URL =3D $env->url;=0A=
>         self::$mtime =3D filemtime('config.json');=0A=
>    }=0A=
>    public function reload() {=0A=
>        if (self::$mtime < filemtime('config.json'))=0A=
>            self::__constructStatic();=0A=
>        }=0A=
>    }=0A=
> }=0A=
> =0A=
> echo Config::URL; // assert: runtime has already called _constructStatic=
=0A=
> $config =3D new Config; // assert: __constructStatic called only once by =
runtime=0A=
> $config->reload(); // instance may call its own static constructor=0A=
?>=0A=
=0A=
This seems to allow the same functionality my RFC would allow, in a slightl=
y different way.=0A=
I didn't go with that syntax because it'd move the values of the constants/=
properties away from the declaration.=0A=
Also, if `json_decode()` threw, then there would never be a way for a PHP i=
mplementation to initialize `self::$mtime` in that example, even though the=
y're unrelated in how they're initialized there.=0A=
Allowing calling `__constructStatic` multiple times (thus assigning to the =
**const** `self::URL` multiple times) would be a significant change to php =
internals, and out of the scope of this RFC=0A=
=0A=
https://wiki.php.net/rfc/calls_in_constant_expressions_poll ("Put the RFC(P=
oll) URL into all your replies.")=0A=
=0A=
- Tyson=