Re: [PHP-DEV] enum flag backed enum

[Rowan Tommins [IMSoP]]

On 6 September 2025 01:47:35 BST, Rob Landers <rob@bottled.codes> wrote:
>
>Interesting... but we don’t really have a way to say that a parameter or 
>return value must satisfy *any* constraints except that it be of a certain 
>type. Hmmm, I guess we have "true" and "false" as values that can be 
>returned/accepted by functions — but I think they’re the only ones.


Declaring an enum already declares a type. I wasn't thinking about constraining 
on lists of values, just saying that something should be a "set of T" where T 
is the enum type.


Relying on integer ops, the result ends up as an int:

enum FooOption: flag { ... }

class Foo {
...
function setOption(FooOption $opt): void { ... }
function getCurrentOptions(): int { ... }
}


With general-purpose generics, you can make that strongly typed:

enum FooOption { ... }

class Foo {
...
function setOption(FooOption $opt): void { ... }
function getCurrentOptions(): Set<FooOption> { ... }
}



A specific EnumBitSet<T> could implement the integer serialisation:

class EnumBitSet<T> extends Set<T> {
    function asBitMask(): int { ... }
}

enum FooOption: int { ... }

class Foo {
...
function setOption(FooOption $opt): void { ... }
function getCurrentOptions(): EnumBitSet<FooOption> { ... }
}

$foo = new Foo;
...
$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 { ... }
}

enum FooOption: int { ... }
class FooOptionSet extends EnumSet<FooOption> {
    // Can add additional methods here, but don't need to
}

class Foo {
...
function setOption(FooOption $opt): void { ... }
function getCurrentOptions(): FooOptionSet { ... }
}

$foo = new Foo;
...
$foo->getCurrentOptions()->asBitMask();


All the operator overloads or methods for things like intersect, union, 
contains, etc could be on the top-level Set<T>, since they don't intrinsically 
have anything to do with enums. That's the joy of our enum implementation being 
object-based: any algorithm that works on object identity automatically works 
on enum cases.



Rowan Tommins
[IMSoP]