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Date: Tue, 14 Feb 2017 14:20:02 -0700
Message-ID: <CAFMT4NroSFbiLnHqEJE9UXwsTrutB3OSo2sysn0j=zFRHyD2-w@mail.gmail.com>
To: Dan Ackroyd <danack@basereality.com>
Cc: "internals@lists.php.net" <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC][DISCUSSION] Object type hint, now with added variance
From: levim@php.net (Levi Morrison)

On Tue, Feb 14, 2017 at 2:14 PM, Levi Morrison <levim@php.net> wrote:
> On Tue, Feb 14, 2017 at 11:55 AM, Dan Ackroyd <danack@basereality.com> wr=
ote:
>> Hi Levi,
>>
>> On 24 November 2016 at 08:58, Dan Ackroyd <danack@basereality.com> wrote=
:
>>> Hi Levi,
>>>
>>> On 23 November 2016 at 01:25, Levi Morrison <levim@php.net> wrote:
>>>
>>>> For instance enumerations (or enums) are one possible type I can see u=
s
>>>> adding that may not be objects.
>>>
>>> I very much look forward to the RFC for enums, and have for some time.
>>> When are you thinking of submitting it for discussion and voting?
>>
>> I can understand your concerns. At the same time, I hope you can
>> understand that people won't want to put off introducing features that
>> they want in the meantime.
>>
>> I think if you were able to give a timeline for when an Enum RFC could
>> be introduced, before the PHP 7.2 release, we could talk about whether
>> it would be right to hold off on this RFC for now.
>>
>> Otherwise I suggest Micha=C5=82 updates the voting options, and we open =
the
>> voting in a week or so.
>>
>> cheers
>> Dan
>
> Let me clarify that I'm not against this object variance; there is
> nothing intrinsic about object variance that prevents enums (or
> typedefs/type aliases). Rather it's *how* it is achieving that
> variance that is an issue. This is something that should ship along
> with full covariance/contravariance and not be a special case which
> may unintended side-effects later (such as preventing enums without
> breaking BC). Does that make sense?

Here's a concrete compatibility concern. Let's assume we have the
following code:

    interface Factory {
        function make(...$args): object;
    }

    class FooFactory {
        function make(...$args): Foo;
    }

Currently the engine would not have to know anything about Foo to know
if it is a sub-type of object. It would not need to require it to even
be defined.

However, if at some point in the future we add non-object types it may
now error or autoload.

The way to avoid the compatibility concern is to implement this
feature the same way we wold implement full covariance, which would
similarly need to know about the types involved.

Does that make more sense as to why the assumption that unknown types
are object types is an issue?