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Date: Wed, 29 Mar 2023 10:47:18 +0200
Message-ID: <CADyq6sKjFWDf1ZqHE4F_ddSCi-=b8uRsxYzxW5xB8YrqZKoODw@mail.gmail.com>
To: =?UTF-8?Q?Rokas_=C5=A0leinius?= <raveren@gmail.com>
Cc: internals@lists.php.net
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Subject: Re: [PHP-DEV] [IDEA] allow extending enum
From: ocramius@gmail.com (Marco Pivetta)

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The point of an ENUM is precisely to restrict options within a certain
range.

Assume you declared `ErrorCode` with `SOMETHING_BROKE` and `PEBKAC` as
possible enumeration values.

If you can (somewhere, anywhere in the code) check `instanceof ErrorCode`,
then you know it can only be `SOMETHING_BROKE` and `PEBKAC`, and you can
write logic around it accordingly.

This restriction/assumption is what makes ENUMs powerful: expanding
pre-existing enums would eliminate this very core design principle, since
you may now no longer have an `ErrorCode`, but some child class designed
somewhere that may not even be under your control.

Greets,

Marco Pivetta

https://mastodon.social/@ocramius

https://ocramius.github.io/


On Wed, 29 Mar 2023 at 10:31, Rokas =C5=A0leinius <raveren@gmail.com> wrote=
:

> Enums were a very useful addition to PHP, however one aspect of them is
> neither
> explicitly documented - or seemingly even talked about.
>
> Enums were implemented as final so they cannot be extended nor can extend
> anything else.
>
> From a user perspective it's surprising - and actually limiting.
>
> USAGE EXAMPLE:
> I am making an error management system: each error presented to the user
> must have a unique code visible.
>
> ```php
> class SystemError
> {
>     public function __construct(
>         private string $errorText,
>         private ErrorCode $code
>     ) {
>     }
>
>     public function __toString():
>     {
>         return $this->errorText . ' ' . $this->code->toString();
>     }
> }
> // ...
>
> enum ErrorCode
> {
>     case Code_1;
>     case Code_2;
>
>     public function toString(): string
>     {
>         return 'Error code:' . substr($this->name, strlen('Code_'));
>     }
> }
> ```
>
> Now I want to modify it to support different modules with different
> namespaces for
> errors, e.g. an ApiError, simple enough:
>
> ```php
> enum BaseErrorCode
> {
>     // ...
> }
>
> enum ErrorCode extends BaseErrorCode
> {
>     case Code_1;
>     case Code_2;
>
>     // ...
> }
>
> enum ApiErrorCode extends BaseErrorCode {
>     // ...
>     function toString(): string
>     {
>         return 'Error code:API-' . substr($this->name, strlen('Code_'));
>     }
> }
> ```
>
> This results in a syntax error.
>
> PROPOSAL:
>
> Enums should be able to extend other enums.
>
> For a complete wishlist, add:
>  * abstract enums;
>  * enums allowed to implement interfaces;
>
> However since I have no experience in PHP source code, I can only
> provide the test suite for a possible PR this might have :(
>
> Do you think this is likely to get implemented?
>
> --
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>
>

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