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Date: Sat, 06 Sep 2025 12:48:19 +0100
To: internals@lists.php.net
Subject: Re: [PHP-DEV] enum flag backed enum
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")

On 6 September 2025 01:47:35 BST, Rob Landers <rob@bottled=2Ecodes> wrote:
>
>Interesting=2E=2E=2E but we don=E2=80=99t really have a way to say that a=
 parameter or return value must satisfy *any* constraints except that it be=
 of a certain type=2E Hmmm, I guess we have "true" and "false" as values th=
at can be returned/accepted by functions =E2=80=94 but I think they=E2=80=
=99re the only ones=2E


Declaring an enum already declares a type=2E I wasn't thinking about const=
raining on lists of values, just saying that something should be a "set of =
T" where T is the enum type=2E


Relying on integer ops, the result ends up as an int:

enum FooOption: flag { =2E=2E=2E }

class Foo {
=2E=2E=2E
function setOption(FooOption $opt): void { =2E=2E=2E }
function getCurrentOptions(): int { =2E=2E=2E }
}


With general-purpose generics, you can make that strongly typed:

enum FooOption { =2E=2E=2E }

class Foo {
=2E=2E=2E
function setOption(FooOption $opt): void { =2E=2E=2E }
function getCurrentOptions(): Set<FooOption> { =2E=2E=2E }
}



A specific EnumBitSet<T> could implement the integer serialisation:

class EnumBitSet<T> extends Set<T> {
    function asBitMask(): int { =2E=2E=2E }
}

enum FooOption: int { =2E=2E=2E }

class Foo {
=2E=2E=2E
function setOption(FooOption $opt): void { =2E=2E=2E }
function getCurrentOptions(): EnumBitSet<FooOption> { =2E=2E=2E }
}

$foo =3D new Foo;
=2E=2E=2E
$foo->getCurrentOptions()->asBitMask();


With the recently proposed declare-time generics, you'd just need an extra=
 declaration alongside your enum:

class EnumBitSet<T> extends Set<T> {
    function asBitMask(): int { =2E=2E=2E }
}

enum FooOption: int { =2E=2E=2E }
class FooOptionSet extends EnumSet<FooOption> {
    // Can add additional methods here, but don't need to
}

class Foo {
=2E=2E=2E
function setOption(FooOption $opt): void { =2E=2E=2E }
function getCurrentOptions(): FooOptionSet { =2E=2E=2E }
}

$foo =3D new Foo;
=2E=2E=2E
$foo->getCurrentOptions()->asBitMask();


All the operator overloads or methods for things like intersect, union, co=
ntains, etc could be on the top-level Set<T>, since they don't intrinsicall=
y have anything to do with enums=2E That's the joy of our enum implementati=
on being object-based: any algorithm that works on object identity automati=
cally works on enum cases=2E



Rowan Tommins
[IMSoP]