On Wed, 29 Mar 2023 at 09:31, Rokas Šleinius <[email protected]> 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.
>
This is by design.
Enumerations are in type theory parlance sum types.
Objects in PHP, in general, are what are called product types.
> From a user perspective it's surprising - and actually limiting.
>
The point of enums is to be 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.
>
You are approaching the design of this in a fundamentally wrong way.
Enums, as sum types, are meant to enclose a *finite* number of states.
If you want to expand those finite number of states, the solution is to use
a union type.
Let's say you have those 3 enums:
enum GenericErrors {
case ErrorType1;
case ErrorType2;
}
enum FileErrors {
case FileErrorType1;
case FileErrorType2;
case FileErrorType3;
}
enum NetworkErrors {
case NetworkErrorType1;
case NetworkErrorType2;
case NetworkErrorType3;
}
And you have a function that can fail with either a generic error or a file
error then you can be explicit by having:
function foo(): T|GenericErrors|FileErrors { /* Do something or Fail */ }
This signature provides you with great type safety and can be checked via
static analysis that *all* error cases are handled.
Moreover, you know you don't need to handle NetworkErrors.
Extending an Enum *loses* you this type safety, because now ff you say
something returns a GenericErrors well who knows if you cover all cases
because someone might have ad-hock added a new one.
PHP's type system is not perfect yet, but it has become powerful enough
that you can do things like what I just described.
Ideally we would have generics, and you could use a Result/Either Monad
(fancy word for "wrapper") to have a return type of Result<T,
GenericErrors|FileErrors>
Use the type system properly instead of trying to shove everything into an
"Inheritance" view of things.
Union types are not "bad", they are another way of creating a sum type.
(You could, with a lot of effort, create "proper" enumerations since PHP
8.0 by using union types and singleton classes.)
PROPOSAL:
>
> Enums should be able to extend other enums.
>
> For a complete wishlist, add:
> * abstract enums;
>
* enums allowed to implement interfaces;
>
Enums can already implement interfaces: https://3v4l.org/tKtQ0
> 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?
>
I dear hope so not.
Breaking fundamental type theoretical assumptions is terrible.
Best regards,
George P. Banyard
