[swift-evolution] [Pitch] Remove type inference for associated types

Dave Abrahams dabrahams at apple.com
Mon Jun 27 17:20:43 CDT 2016

on Mon Jun 27 2016, Austin Zheng <austinzheng-AT-gmail.com> wrote:

> Thanks for your response, Dave.
> There is a bit of a dilemma here: wait until the generics and type system
> features have stabilized at the risk of making major source-breaking
> changes after 3.0, or make changes now without clarity about the future of
> the generics system.
> Given that this topic showed up both in Chris's list of open design topics
> and in the generics manifesto, I assume that someone on the core team
> wanted a discussion about it before Swift 3 closes. I would be interested
> in knowing if that's true.

I don't know anything more than you do, I'm afraid.

> Best,
> Austin
> On Mon, Jun 27, 2016 at 12:56 PM, Dave Abrahams via swift-evolution <
> swift-evolution at swift.org> wrote:
>> on Sat Jun 25 2016, Austin Zheng <swift-evolution at swift.org> wrote:
>> >> On Jun 25, 2016, at 6:23 AM, Matthew Johnson <matthew at anandabits.com>
>> wrote:
>> >>
>> >> Hi Austin,
>> >>
>> >> I’m sorry to say, but this proposal makes me really sad.  I consider
>> >> associated type inference one of the more elegant aspects of Swift.
>> >> It would be very unfortunate to lose it.
>> >
>> > There are lots of "elegant" things that Swift could do, but has chosen
>> > not to do for pragmatic reasons (e.g. generalized implicit
>> > conversions, type inference that crosses statement boundaries). Given
>> > how terrible the development experience can be right now in the worst
>> > case, I would happily trade off some measure of convenience for better
>> > tooling.
>> Well, the type checker's inference engine has *always* been kinda
>> unreliable, and the experience is made much worse by the lack of
>> recursive protocol requirements and the inability to express other
>> constraints that would better guide inference, and by the “underscored
>> protocols” such as _Indexable that are required to work around those
>> limitations.  IMO it's premature to remove this feature before the
>> inference engine is made sane, the generics features are added, and the
>> library is correspondingly cleaned up, because we don't really know what
>> the user experience would be.
>> Finally, I am very concerned that there are protocols such as Collection,
>> with many inferrable associated types, and that conforming to these
>> protocols could become *much* uglier.
>> --
>> Dave
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