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Date: Fri, 05 Sep 2025 21:01:04 +0100
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Subject: Re: [PHP-DEV] enum flag backed enum
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")

On 4 September 2025 15:50:08 BST, Rob Landers <rob@bottled=2Ecodes> wrote:
>I think this is a fair observation and a fair question; but I think it is=
 important not to have "magic"=2E The power-of-two rule is to make it possi=
ble to work back how $enum->value =3D=3D=3D 15 (0x1111) even if you are com=
pletely new to the language=2E If you just use some magical cardinal order,=
 it is impossible to reserve ranges, handle communications with external sy=
stems, etc=2E


A set does not need elements to have a defined order, only a defined ident=
ity; that is available on any enum, even one with no backing at all=2E That=
 pure set could be serialised in various ways, based on the available seria=
lisations of its elements (comma-separated list, integer bitmask, binary st=
ring bitmask, etc)=2E That's the strongly typed model=2E

The weakly typed model is to keep the values permanently in their serialis=
ed form, and manipulate that directly=2E That is, you have a set of integer=
s for the flags, and construct a new integer for the set of flags=2E That h=
as the advantage of being simple and efficient, at the cost of safety and e=
asy tool affordance=2E

What you're suggesting sounds like somewhere between the two: the individu=
al flags are of a specific type, rather than raw integers, but the set itse=
lf is just an integer composed of their "backing values"=2E The big downsid=
e I see is that you can't natively label a parameter or return value as bei=
ng a "set of flags from this enum"=2E You could probably make a docblock ty=
pe annotation work with an external static analyser, but in that case, you =
might as well use that tool to enforce the powers of 2 on your enum or Plai=
n Old Constants=2E

Indeed, enforcing "this integer must be a power of 2" is not really anythi=
ng to do with enums, it would be useful on *any* type declaration=2E


Personally, I find the concept of enum cases having a single backing value=
 unnecessarily limiting, and would much prefer Java-style case properties=
=2E

enum FooFlag: object {
    case None(0, '');
    case ThrowOnError(0b1, 'T');
    // etc=2E
    case PrettyPrint(0b1000, 'P');
    // etc=2E

    public function __construct(public int $flagForBinaryApi, public strin=
g $flagForTextApi){}
}

FooFlag::ThrowOnError->flagForBinaryApi; // 0b1
FooFlag:: PrettyPrint->flagForTextApi; // 'P'


Ideally we would declare that $flagForBinaryApi must be a power of 2, and =
that $flagForTextApi must be a single character, using some general-purpose=
 feature of the type system=2E=20

The only thing that might be enum-specific is a way to say whether the val=
ues of a property must be unique (something we don't currently have with si=
ngle-value backed enums)=2E


Then the ideal would be an EnumSet customised to work with any properties =
or methods it wanted:

$foo =3D FooFlag::ThrowOnError | FooFlag::PrettyPrint;
$foo->serialiseForBinaryApi(); // 0b1001
$foo->serialiseForTextApi(); // 'TP'


Rowan Tommins
[IMSoP]