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To: Mike Schinkel <mike@newclarity.net>, PHP internals
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Thread-Topic: [PHP-DEV] Initializing constants once, with code?
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Date: Sun, 5 Jan 2020 22:04:34 +0000
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Subject: Re: [PHP-DEV] Initializing constants once, with code?
From: tysonandre775@hotmail.com (tyson andre)

I have a work in progress wrapper for allowing const-like syntax for global=
 constants `static const X =3D $dynamicExpression;`.=0A=
This can be seen in https://github.com/TysonAndre/php-src/pull/10=0A=
This is basically a wrapper around if (!defined(...)) { define(...); }=0A=
If anyone's planning to implement changes to the way constants can be decla=
red,=0A=
then the POC might be worth looking at (e.g. to see what parts of the code =
would need to be changed),=0A=
even if their design and approach is different.=0A=
=0A=
- The issues this solves has overlap and differences with what Mike Schinke=
l requests here.=0A=
  It's intended as a solution to limitations in different use cases that I'=
ve encountered/thought about (which this thread reminded me of).=0A=
=0A=
  Notably, this deliberately can't refer to other *dynamic* "static const" =
values that haven't been declared.=0A=
- This deliberately limits it to the top-level statements in files=0A=
- Class constants below the global 'static const' can then use the eagerly =
evaluated const.=0A=
=0A=
Reasons for me to implement the proof of concept this way:=0A=
- defined()/define() is already supported by the engine.=0A=
  This is a slight improvement due to shorter syntax,=0A=
  and being more certain the const is only set or used in one file.=0A=
- Eagerly initialized dynamic constants are easier to reason about in some =
ways (e.g. fetching the constant of an already fetched class is less likely=
 to cause pauses or exceptions.)=0A=
  This makes un-noticed errors in rare code paths less likely.=0A=
- Attempting to support function calls, property accesses, class constant a=
ccesses (e.g. inherited), and variables in class constants recursively and/=
or lazily=0A=
  would probably break a lot of code/assumptions in the php engine.=0A=
=0A=
  I might be mistaken about that, though - I'd be glad if it turned out to =
be easy to implement.=0A=
=0A=
Declaring 'static const' as a top level statement of a function/closure/met=
hod might also be possible, but would require more work.=0A=
=0A=
------=0A=
=0A=
It's also worth mentioning that some expressions that definitely have const=
ant values=0A=
can't be written (in a convenient way) because they use syntax that is prev=
ented right now, such as function calls, closures, etc.=0A=
=0A=
My preference would be to make it easier to use dynamic expressions, and re=
commend in coding guidelines that=0A=
certain types of expressions "SHOULD NOT" be used for constants that vary o=
n a particular host.=0A=
(network calls, time, etc)=0A=
=0A=
```=0A=
const DEFAULT_PAYLOAD =3D json_encode(['key' =3D> 'value']);=0A=
const ALLOWED_ELEMENT_TYPES =3D array_merge(self::FIRST_TYPES, self::SECOND=
_TYPES);=0A=
const ELEMENT_LENGTHS =3D array_map('strlen', self::ELEMENTS);=0A=
```=0A=
=0A=
=0A=
=0A=
> For derive-once global constants, you may be looking for define.=0A=
> That's a runtime operation and effectively boils down to a set-once globa=
l.=0A=
> It doesn't help with constants defined in a class,=0A=
> but for an already-global constant define already gives you a one-time se=
t operation,=0A=
> just not on-demand.=0A=
=0A=
This POC adds an alternate syntax for a set-once global.=0A=
It prevents accidentally reusing the same global name in multiple files (e.=
g. with copy and paste),=0A=
and lets people looking at it know the constant is only used in that file.=
=0A=
=0A=
>=A0Another use-case that just occurred to me is to support testing:=0A=
> =0A=
> class Api {=0A=
>   const URL: string{=0A=
>     return $_ENV['EXAMPLE_API_URL'] ?? 'https://api.example.com';=0A=
>   }=0A=
> }=0A=
=0A=
That example would be written with the POC as:=0A=
=0A=
static const API_URL =3D $_ENV['EXAMPLE_API_URL'] ?? 'https://api.example.c=
om';=0A=
=0A=
class Api {=0A=
  const URL =3D API_URL;=0A=
}=