> On Sep 18, 2017, at 17:54, Ben Cohen via swift-dev <swift-dev@swift.org>
> wrote:
>
>
>
>> On Sep 13, 2017, at 1:06 PM, David Zarzycki via swift-dev
>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote:
>>
>>
>>
>>> On Sep 13, 2017, at 15:23, Matthew Johnson via swift-dev
>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote:
>>>
>>>
>>>
>>> Sent from my iPhone
>>>
>>>> On Sep 13, 2017, at 11:56 AM, David Zarzycki via swift-dev
>>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote:
>>>>
>>>>
>>>>
>>>>> On Sep 13, 2017, at 13:53, David Sweeris <daveswee...@mac.com
>>>>> <mailto:daveswee...@mac.com>> wrote:
>>>>>
>>>>>
>>>>>> On Sep 13, 2017, at 09:54, David Zarzycki via swift-dev
>>>>>> <swift-dev@swift.org <mailto:swift-dev@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?
>>
>> 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.
>>
>
> But knowing something is a value type isn’t particularly useful, given it
> doesn’t guarantee value semantics. It could even do more harm than good, by
> being confusable with enforcing value semantics.
>
> Can you go into the use cases you have where you would use the knowledge that
> a type is a value type?
Hi Ben,
As a part of a much larger goal, I’m experimenting with enforced value
*semantics* and I found that value-type bound protocols are a wholly separable
and independently useful prerequisite. Here is a contrived but representative
example:
protocol ValueThingy : !class { // From the patch sent to the list
mutating func increment()
}
func incrementByCopy<T : ValueThingy>(_ arg : T) -> T {
var copy = arg
copy.increment()
return copy
}
Without value-type bound protocols, generic code cannot ensure that required
copies are actually happening. This is independently useful and good.
Dave_______________________________________________
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