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Date: Fri, 18 Sep 2015 08:11:21 -0600
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To: John Bafford <jbafford@zort.net>
Cc: Bob Weinand <bobwei9@hotmail.com>, Rowan Collins <rowan.collins@gmail.com>, 
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Subject: Re: [PHP-DEV] Let's discuss enums!
From: levim@php.net (Levi Morrison)

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 ther=
e's ever been a dedicated discussion of it: would it be useful for PHP to h=
ave 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 RFC=
 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 impl=
ementation.
>>
>> So, I'd consider bikeshedding an actual RFC first.
>
>
> If we=E2=80=99re bikeshedding, one feature I would really like to see, wi=
th 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 t=
his 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 wa=
rning/error that the switch does not cover all enum values. This would be v=
ery handy if an enum value is added after initial development and someone m=
isses a switch statement in cleanup.
>
> The typehint would also allow generating a warning if someone did somethi=
ng 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=98Generate=
 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 emitted

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).