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Date: Fri, 18 Sep 2015 10:42:44 -0400
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To: Levi Morrison <levim@php.net>
Cc: John Bafford <jbafford@zort.net>, Bob Weinand <bobwei9@hotmail.com>, 
	Rowan Collins <rowan.collins@gmail.com>, internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Let's discuss enums!
From: ircmaxell@gmail.com (Anthony Ferrara)

Levi et al,

On Fri, Sep 18, 2015 at 10:11 AM, Levi Morrison <levim@php.net> wrote:
> On Thu, Sep 17, 2015 at 5:52 PM, John Bafford <jbafford@zort.net> wrote:
>> On Sep 17, 2015, at 19:16, Bob Weinand <bobwei9@hotmail.com> wrote:
>>>
>>>> Am 18.09.2015 um 01:06 schrieb Rowan Collins <rowan.collins@gmail.com>=
:
>>>>
>>>> This has come up in passing a few times recently, but I'm not sure the=
re's ever been a dedicated discussion of it: would it be useful for PHP to =
have a built-in Enumeration type, and if so, how should it look?
>>>
>>> I like enums in general, but I'd like to note that there's already a RF=
C in draft by Levi:
>>>
>>> https://wiki.php.net/rfc/enum <https://wiki.php.net/rfc/enum>
>>>
>>> As far as I know, the RFC is already fairly final and just lacks an imp=
lementation.
>>>
>>> So, I'd consider bikeshedding an actual RFC first.
>>
>>
>> If we=E2=80=99re bikeshedding, one feature I would really like to see, w=
ith typehinting, is warnings if all cases of an enum aren=E2=80=99t handled=
 in a switch. So, for example, given our example Weekdays enum, if I wrote =
this code:
>>
>> switch(Weekday $someWeekday) {
>>         case Weekday::MONDAY: break;
>>         case Weekday::TUESDAY: break;
>> }
>>
>> By providing the typehint, I=E2=80=99m indicating that I want to get a w=
arning/error that the switch does not cover all enum values. This would be =
very handy if an enum value is added after initial development and someone =
misses a switch statement in cleanup.
>>
>> The typehint would also allow generating a warning if someone did someth=
ing like
>>
>> switch(Weekday $someWeekday) {
>>         //case =E2=80=A6 all the weekdays: break;
>>         case =E2=80=98I am not a Weekday=E2=80=99: echo =E2=80=98Generat=
e a fatal error here because string is not a Weekday.=E2=80=99;
>> }
>
> I've looked into this but not in the same syntax you've provided. I
> was exploring a `match` construct that has different semantics:
>
>     match ($foo) {
>         case Weekday::Monday: {
>             echo "Monday";
>         } // it is not possible to fall-through with match/case
>
>         case Weekday::Tuesday:  echo Monday; // single expression
> doesn't need braces
>     } // if it hits the end without a match a runtime error will be emitt=
ed
>
> I think this fits into the dynamic behavior of PHP a little better
> than forcing a case for each enum value at compile-time, even if it
> was possible (I suspect it is not).

This is basically pattern matching. And while I'd LOVE to see support
for it in PHP, I think it's a bit off-topic here. Perhaps a new thread
could be opened for it (or ideally a RFC)...?

As far as enums, I wonder if they would be necessary if we supported
algebraic types, since you could define an "enum" as a virtual type:

const MONDAY =3D 0;
const TUESDAY =3D 1;
const WEDNESDAY =3D 2;
const THURSDAY =3D 3;
const FRIDAY =3D 4;
use MONDAY | TUESDAY | WEDNESDAY | THURSDAY | FRIDAY as WEEKDAY;

function foo(WEEKDAY $day) {
    // must be an integer 0-4, or castable to 0-4 unless strict_types is on=
.
}

Just a thought...

Anthony