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Date: Sat, 05 Dec 2020 20:21:13 +0100
To: Rowan Tommins <rowan.collins@gmail.com>,
	<internals@lists.php.net>
Message-ID: <9EA6E41D-B41F-4DF7-9A11-225DE74CED14@mabe.berlin>
Thread-Topic: [PHP-DEV] [RFC] Enumerations
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Subject: Re: [PHP-DEV] [RFC] Enumerations
From: marc@mabe.berlin (Marc Bennewitz)



=EF=BB=BFAm 05.12.20, 16:21 schrieb "Rowan Tommins" <rowan.collins@gmail.com>:

    (Apologies if this posts twice, I may have accidentally hit send before=
=20
    I'd finished writing it)


    On 05/12/2020 14:21, Marc Bennewitz wrote:
    > * Do cases have a stable ordinal number / position and is that access=
ible?
    >    This would be very interesting on implementing an optimized EnumSe=
t using bit-array / bit-set internally

    If you want to associate a stable value with each case, you can use a=20
    "primitive-equivalent enum" and assign each case an integer manually. A=
s=20
    the RFC says:

     > There are no auto-generated primitive equivalents (e.g., sequential=20
    integers).


    I think that's a good design decision, because the language itself can'=
t=20
    actually guarantee a stable ordinal number if the enum's author makes=20
    changes; e.g. given:

    enum Colour { case RED; case GREEN; case BLUE; }

    somebody might decide the cases should be in alphabetical order:

    enum Colour { case BLUE; case GREEN; case RED; }

    It seems unnecessarily confusing to have this be a breaking change=20
    because code somewhere relied on the ordering. It's much clearer if the=
=20
    author has to declare an intentional value, and can then maintain it=20
    when making cosmetic changes to their own code:

    enum Colour { case BLUE=3D3; case GREEN=3D2; case RED=3D1; }


Sorry for the confusion - I mean stable within the same process - not over =
different processes / systems.


    > * I often use metadata in enumerations and so I would be very=20
    > interested to allow constants.


    Could you give an example what you mean? Metadata on individual cases i=
s=20
    supported by methods, which map more cleanly to things like interfaces,=
=20
    and the notion of each case as a singleton object, not a static class.


    That said, I have a related question: can enum cases be used as the=20
    *value* of constants? e.g.:

    class OldMaid {
         public const SUIT =3D Suit::Spades;
         public const VALUE =3D CardValue::Queen;
    }


    If so, this leads to an interesting use case for constants on the enum=20
    itself (not the cases) to define aliases:

    enum Suit {
       case Hearts;
       case Diamonds;
       case Clubs;
       case Spades;
       const Tiles =3D self::Diamonds;
       const Clovers =3D self::Clubs;
       const Pikes =3D self::Spades;
    }

    // The constants and cases can be used interchangeably, since constants=
=20
    and cases are de-referenced with the same syntax
    assert(Suit::Spades =3D=3D=3D Suit::Pikes);


I mean on mapping something to something else defined as a single assoc arr=
ay constant.
Something like:

enum Role {
  case User,
  case Admin,
  ...
}

enum Action {
  case Order_Edit,
  case Order_Read,
 =20
  private const BY_ROLE =3D [
    Role::User =3D> [self::Order_Read],
    Role::Admin =3D> [self::Order_Read, self::Order_Edit],
  ];

  public function isAllowed(User $user) {
    return in_array($this, self::BY_ROLE[$user->role]);
  }
}



    Regards,

    --=20
    Rowan Tommins (n=C3=A9 Collins)
    [IMSoP]

    --=20
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