[swift-evolution] [Review] SE 0192 - Non-Exhaustive Enums

Xiaodi Wu xiaodi.wu at gmail.com
Tue Jan 2 12:36:28 CST 2018

On Tue, Jan 2, 2018 at 12:11 PM, Jason Merchant via swift-evolution <
swift-evolution at swift.org> wrote:

> I think this whole thing has been unnecessarily convoluted. As a result,
> the majority of the replies are rabbit holes.
> In my opinion, the true root of the concept in question is as follows:
> *A list of something is desired:*
> 1 - Pancake
> 2 - Waffle
> 3 - Juice
> *Developer wishes to be able to:*
> *A)* Add new things to the list of choices in the future as they come up
> with new ideas
> *B)* Sometimes select one of the choices to be chosen as the normal
> choice if no choice is made by the user
> A and B are *separate desires*. In some circumstances a developer may
> want to add a new choice and make it the normal choice when there was no
> normal choice was clarified before.

I don't think this is an accurate summary of the problem being tackled
here. Rather, we are how to enable the vendor of a nonexhaustive enum to
add new cases without breaking binaries compiled against previous versions.
There is little here to do with what a "default" should be. Indeed, it is
an explicit design decision of Swift not to support types having an
implicit default value.

> ____________________
> *Part 2:*
> After this simple desire is clear, there should be two discussions:
> *A)* In a text only coding language, what would we like the syntax to
> look like? (Without regard to past-bias. What should it really be, forget
> what mistaken design choices were made in Swift in the past)
> *B)* How do we approach making this happen behind the scenes?
> *Bonus:* Given that some of us have changed our approach to programming
> significantly beyond text based coding, and into more dynamic mediums of
> programming in other niches, and even here and there in Xcode - I would
> recommend considering how the IDE would show a modern version of this
> concept. I feel too often that Swift design syntax has a *lack of
> awareness between the distinctions of what the IDE should do, as opposed to
> what the syntax of the language should be*, and what should be handled
> behind the scenes by automated tooling.
> _____________________
> *My opinion*, in answering the above questions is in preference to a
> simple easy to read and write syntax, something like the following:
> choices Breakfast {
>     Pancake, *Waffle*, Juice
> }
> If a "default" choice is desired, it is obvious to me that I would select
> the choice from the IDE, and it would be visually indicated that it was the
> default.
> When changes occur, whether new choices are added, old ones are removed or
> changed, or a default is added, changed, or removed - a behind the scenes
> automated tool analyzes the changes and presents migration options through
> the IDE.
> _____________________
> Sincerely,
> Jason
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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