[swift-dev] Value-type bound protocols?

[David Zarzycki]

> On Sep 18, 2017, at 17:54, Ben Cohen via swift-dev <swift-dev at swift.org> wrote:
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> 
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>> On Sep 13, 2017, at 1:06 PM, David Zarzycki via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
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>> 
>> 
>>> On Sep 13, 2017, at 15:23, Matthew Johnson via swift-dev <swift-dev at swift.org <mailto: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 <mailto:swift-dev at swift.org>> wrote:
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>>>> 
>>>>> On Sep 13, 2017, at 13:53, David Sweeris <davesweeris at mac.com <mailto:davesweeris at mac.com>> wrote:
>>>>> 
>>>>> 
>>>>>> On Sep 13, 2017, at 09:54, David Zarzycki via swift-dev <swift-dev at swift.org <mailto: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?
>> 
>> 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.
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> 
> 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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