Fwd: Re[2]: PHP 8.1 enum const expressions problem

Hello internals! I apologize if such a discussion has already taken
place, but I didn't find anything like it.
 
When working with enums, I ran into problems that are currently not
resolved. In some cases, enumerations require self references and
constant arithmetic expressions:
 
enum Example: int
{
    case A = 0b0001;
    case B = 0b0010;
    case C = 0b0100;
 
    public const EXAMPLE_MASK = self::A | self::B; // << Invalid expression
    public const EXAMPLE_MASK2 = self::A->value | self::B->value; // << Same
}
 
Similar examples can be taken in the existing PHP code, for example,
for attributes (Attribute::TARGET_ALL) which is a binary mask for
existing «targets». Thus, if targets for attributes were implemented
through enumerations, and not through constants, then it would be
impossible to implement such functionality.
 
In addition, enumeration values are not part of constant expressions,
so implementation through "Example::A->value" is also not available.
 
Can you please tell me, maybe there have already been some discussions
of this problem? And if not, then maybe we should solve this problem
before PHP 8.1 release? Since it seems to me that the lack of such
functionality is quite critical.
What you're describing is enum sets. That is, a value that is itself a union of two enum values. Those are not supported at this time. It's actually a bit tricky to do, since internally enum cases are not a bit value but an actual full PHP object. So self::A | self::B would be trying to bitwise-OR two objects, which is of course meaningless.

The second doesn't work because technically self::A is an object, not a constant, so it cannot be used in a constant expression. The only dynamicness added in 8.1 was for "new" expressions in initializers. The objects don't exist at compile time so they cannot be reduced down to a constant value pre-runtime.

I agree the current situation is suboptimal. I don't think anyone is opposed to enum sets, but they're a non-trivial amount of work to do and we haven't figured out how to do them yet. There's no simple way to address it as as bug, so any larger effort will have to wait for 8.2.

--Larry Garfield

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Yes, I saw this short description in your RFC ( https://wiki.php.net/rfc/enumerations ) about «enum sets». However, I do not quite understand why we can not now add a cast to scalars (string and int) and math expressions, which would solve this problem? This behavior has already been implemented for GMP objects.

$mask = gmp_init(0b0001) | gmp_init(0b0010); // object(GMP) { num = 3; }
echo $mask; // 3
 
$mask = $mask + 1; // object(GMP) { num = 4; }
$mask instanceof \GMP; // true
 
I mean, for such cases, we can create a new "virtual enum case" containing a new value instead special «EnumSetCase».
 
enum Some: int
{
    case A = 0b0001;
    case B = 0b0010;
}
 
var_dump(Some::A | Some::B); // enum(Some::@anonymous) { value = 3; }
 
I don’t think that it is necessary to consider the «enum sets» as a separate case, cause addition is also a fairly popular case:
 
case LAST = self::B + 1;
 
Like any other mathematical operations.
 
 

Kirill Nesmeyanov