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To: Rowan Tommins <rowan.collins@gmail.com>
Subject: Re: [PHP-DEV] Initializing constants once, with code?
From: mike@newclarity.net (Mike Schinkel)

> On Jan 5, 2020, at 3:17 PM, Rowan Tommins <rowan.collins@gmail.com> =
wrote:
>=20
> They seem a poor example of something that you would later want to =
"evolve" using the proposed new feature. An enum is, by definition, a =
set of values that covers all possibilities, often with completely =
arbitrary values.

Yet you called my list of URLs enums, and they are not at all arbitrary =
and absolutely want to change.

We can both agree that enums for 0, 1, 2 etc. are not in use-case I am =
discussing.

Maybe you misspoke when you called my list of URLs enums?  Or I misread?

> I'm struggling to imagine how this would work. I've seen plenty of =
code that needs to know "the current environment", but "the name of the =
current development environment" seems an odd thing to have a constant =
for.

I take it you have never used Pantheon hosting then?  You create a named =
environment to match a Git branch.=20

"dev" is not "local" in this content.  "dev" is the place developers =
push code to test remotely.  Then they push to "test" for QA to =
evaluate.  At least for this client.

That said, my client wants to move to many "dev" and "test" =
environments, one for each JIRA epic, so we would like to be able to =
initialize the API urls based on the current subdomain.=20

> Well, it would be more "scannable" if every method in the class was =
written on one line - and indeed you can do so if you want - but that's =
not inherently better.

What is "better" in this context is one's opinion.  You might not find =
it beneficial, but I would.=20

> If a class has a bunch of different functions and constants in, I'll =
generally use an IDE or editor plugin to navigate them, and rely on them =
being well-named, rather than minimising the whitespace between them.

I use and have used PhpStorm extensively, since prior to 1.0.   But =
PhpStorm does not make it easier to do code reviews on Github, or even =
read code in PhpStorm.  Yes, I can navigate them, but to still adds =
cognitive load.=20

Honestly, we are debating preferences here.  Can we focus on objective =
things?

> "Constants are defined at compile-time" is a status quo in the same =
sense as "variables can be assigned to" is a status quo - you could =
certainly design a language differently, but it's not just something =
that happened by accident, it's a design decision.

It was a design decision in that maybe it did not occur to them that it =
would be useful; IOW a design by omission.

Design decisions can be changed.  PHP 5 changed the design decisions of =
PHP objects and added the __set() method. Before then it could have =
easily been argued that variable accessed a memory location, but never =
called code. But then that design decision was changed. =20

So there is no reason I've heard thus far that this one could not be =
changed too.

> An interesting exercise is to ask if there are other languages that =
take a different decision - it's not decisive, either way, but it can be =
suggestive.
> "Can constants be evolved into something that is initialised =
on-demand?" is not something I'm aware of examples of elsewhere. That =
could be because nobody's thought of it before, or just that I haven't =
come across it, but it could be because there's a better solution to the =
underlying requirement.

Excellent point. =20

Ruby has the .replace method for constants and can thus assign at =
runtime:
=3D=3D=3D=3D=3D=3D=3D=3D=3D
https://stackoverflow.com/a/6712438/102699

Javascript can assign constants at runtime:
=3D=3D=3D=3D=3D=3D=3D=3D=3D
fn =3D function() { return "World"; };
const who =3D fn();
alert( "Hello " + who );

C++ has const member functions (not exactly the same, but similar):
=3D=3D=3D=3D=3D=3D=3D=3D=3D
=
https://www.linuxtopia.org/online_books/programming_books/thinking_in_c++/=
Chapter08_014.html

Python adds a final qualifier to turn variables into constants. Thus is =
supports initializing constants:
=3D=3D=3D=3D=3D=3D=3D=3D=3D
https://stackoverflow.com/a/47821212/102699
http://code.activestate.com/recipes/65207-constants-in-python/

Java creates class constants with `public static final` which can be =
initialized with code:
=3D=3D=3D=3D=3D=3D=3D=3D=3D
=
https://www.javaworld.com/article/2076481/use-constant-types-for-safer-and=
-cleaner-code.html

I think that is a fairly conclusive list of highly-used languages, all =
of which allow constants to be initialized with code. This list would =
imply any assertion that constants must be handled at compile time is =
not applicable for many other languages so I would argue to not a =
universal principle.

Also it would imply that allowing constants to be initialized via =
runtime code would not disrupt the time-space continuum, either.

> I think the syntax being the same when you *reference* the value =
bothers me more than how it is declared. If I see ClassName::USER_ID, I =
expect the value to be the same each time the code runs, and to be able =
to find the value directly in the source.

Again, I ask if that your unease has more to do with status quo bias vs. =
something fundamentally wrong with the concept?

> It's a limitation in the very basic sense that it limits what you can =
do. You can't write this, for instance:
>=20
> class API {
>     private $config;
>     public function __construct($config) {
>         $this->config =3D $config;
>     }
>=20
>     const URL {
>          return $this->config->getValue('API_URL');
>     }
> }

But you could do this:

class Api {
   const URL:string {
         return Config::get()->api_url;
    }
}

class Config {
   public $api_url;
   private static $_current;
   function __construct($args =3D array()){
      $this->api_url =3D $args['api_url'];
   }
   static function get():Config{
      if ( ! isset( self::$_current ) ) {
         trigger_error('must call Config::set() before calling =
Config::get()');
         die();
      }
      return self::$_current;
   }
   static function set( Config $config ){
      self::$_current =3D $config;
   }
}

Config::set( new Config(['api_url' =3D> 'https://api.example.com']) );

//...
$api_url =3D Api::URL;

Any way, why again must perfect be the enemy of the good?

> Laravel's "facades" are global variables hidden behind static methods. =
So you write "DB::query($sql)" as a short-hand for =
"DB::$currentInstance->query($sql)". Most of the time, you just treat it =
as static, but if you want to write tests, for instance, you can inject =
a mock as the current instance.

Ah.=20

> An interesting thing is that you can evolve a class that really is =
static and replace it with one of these facades. Unfortunately, the same =
wouldn't be true with initialise-once constants, because once the =
constant had been read, you couldn't swap it out in another test.

Well, I am suggested one-time only because I assumed that one-time only =
_is_ the only thing that is fundamental to `const.`  But I can see uses =
for being able to redefine them =E2=80=94 when running unit tests =E2=80=94=
 but I was not planning to ask for that.

> Many people would say the kind of global state you're talking about is =
an anti-pattern.

Well, Larry did, which is reality is an appeal-to-authority argument and =
not an argument on the merits. =20

But I used "anti-pattern" because I did not think I was trying to =
convince anyone against adding a pre-processor, so it was just easier to =
write!

> I'm just looking for solutions to your use case, which as I understand =
it is values which change rarely based on configuration.

I honestly do appreciate it.  You are generally very good about that =
kind of thing.

-Mike=