[swift-evolution] [Pitch] Introducing role keywords to reduce hard-to-find bugs

Paul Cantrell cantrell at pobox.com
Wed Jun 14 23:41:07 CDT 2017

> On Jun 14, 2017, at 4:51 PM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
>> On 14 Jun 2017, at 22:37, Vladimir.S via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> On 14.06.2017 21:23, Haravikk via swift-evolution wrote:
>>>> On 14 Jun 2017, at 19:08, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>> wrote:
>>>> On Wed, Jun 14, 2017 at 1:01 PM, David Hart via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>> wrote:
>>>>    Sorry, initially sent off-list:
>>>>    I think this proposal is a great idea. But I would vote for the alternative of
>>>>    only having default and implicitly deducing extend when default is not specified:
>>>> This wouldn't work with the fundamental design decision that these are optional keywords, which IMO is absolutely key.
>>> Hmm, I'm inclined to agree with David that only the default keyword really seems like it's necessary, and that extend can be implied.
> I think a good plan would be to make default required in a later Swift version (Swift 5) for example, and only warn for now.

I like only having “default,” and I like David’s plan for achieving that.

• • •

It seems that this proposal could help mitigate the problems described in the classic “Ghost of Swift Bugs Future” (https://nomothetis.svbtle.com/the-ghost-of-swift-bugs-future <https://nomothetis.svbtle.com/the-ghost-of-swift-bugs-future>). There’s a nice new convention (I think?) of:

`extend` / no modifier in extension → static dispatch
`default`  in extension → dynamic dispatch

(Does this always hold in an extensions?)

Knowing whether the extension’s author •intended• static or dynamic dispatch could also allow the compiler to give better warnings: about shadowing at declaration sites, and also also potentially at call sites where multiple extension methods could match depending on the compile-time type of the receiver.

Might it be worth describing some of these warnings out in the proposal? I realize warnings don’t require full swift-evo treatment, but it would be nice to at least sketch out in the “Impact on Existing Code” section what the warnings might look like.



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20170614/8b7e535b/attachment.html>

More information about the swift-evolution mailing list