[swift-dev] Value-type bound protocols?

[Slava Pestov]

> On Sep 13, 2017, at 12:23 PM, Matthew Johnson via swift-dev <swift-dev at swift.org> wrote:
> 
> 
> 
> Sent from my iPhone
> 
>> On Sep 13, 2017, at 11:56 AM, David Zarzycki via swift-dev <swift-dev at swift.org> wrote:
>> 
>> 
>> 
>>> On Sep 13, 2017, at 13:53, David Sweeris <davesweeris at mac.com> wrote:
>>> 
>>> 
>>>> 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?
>>> 
>>> I think it'd be worth a proposal, especially if can talk about why you needed it.
>> 
>> While I look forward to talking about my research, I’m not ready to do in the near future.
>> 
>> That being said, value-type bound protocols seem independently useful and that is why I emailed the list.
>> 
>> 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?

Right. For instance, if I have a struct containing a field of reference type, is this struct a ‘value type’?

Slava

> 
>> 
>> Dave
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