[swift-dev] Value-type bound protocols?

[David Zarzycki]

> On Sep 13, 2017, at 15:23, Matthew Johnson via swift-dev <swift-dev at swift.org> wrote:
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> Sent from my iPhone
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>> On Sep 13, 2017, at 11:56 AM, David Zarzycki via swift-dev <swift-dev at swift.org> wrote:
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>>> On Sep 13, 2017, at 13:53, David Sweeris <davesweeris at mac.com> wrote:
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>>>> On Sep 13, 2017, at 09:54, David Zarzycki via swift-dev <swift-dev at swift.org> wrote:
>>>> 
>>>> Hello,
>>>> 
>>>> As a part of a research project that I’m working on, I’ve started bumping into the need for value-type bound protocols (as opposed to the existing class bound protocols). Is this something that would be worth proposing formally? Or should I just keep the patch I have on my research branch?
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>>> I think it'd be worth a proposal, especially if can talk about why you needed it.
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>> While I look forward to talking about my research, I’m not ready to do in the near future.
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>> That being said, value-type bound protocols seem independently useful and that is why I emailed the list.
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>> I think the use case for this is generic algorithms. Why? Because it can be hard to impossible to write *robust* generic code when you don’t know whether an abstract type copies by value or by reference during assignment/initialization. With class-bound protocols, you can guarantee reference semantics, but there is no analogous feature for ensuring value semantics. I have a small (~150 line) patch that fixes this.
> 
> Value types and value semantics are not the same.  Most people who have asked for this capability actually want a constraint for value semantics, not value types.  Is that what you're asking for as well?

The patch that I’m ready to put forth is only a value-type bound. In other words only structs and enums would be able to conform to a value-type bound protocol. Enforcing value semantics is arguably a separable language goal.

Dave