[swift-evolution] [Pitch] Non-class type requirements on protocols (eg : struct, : enum)

[Karl Wagner]

Copying is a much more nuanced issue than just reference-or-value though. I would also support some good copying protocols in the standard library, but that's additive.
 
   
 
 
 

 
 
 
 

 
 
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> On Oct 21, 2016 at 5:11 pm,  <Mike Kasianowicz (mailto:mike at ap14.com)>  wrote:
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> Just from an outside perspective, the class restriction seems to be there as a kludge for technical reasons... but that's neither here nor there. 
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> It is not so much to enforce a lack of identity - in the struct case, it would be to enforce copy-by-value semantics.    I think the strongest argument I've got is, say, a serialization or caching framework where you want to enforce that something is entirely writeable via memory pointer or copyable.    A value-type restriction would get us mostly there, albeit there would still be ways to break the contract.    However, as noted in my previous email, I see a lot of possibilities for enums too - in that case the protocol somewhat acts as 'base type' without adding the complexity of a base type.
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> I listed some of my examples in my previous email - I could elaborate if it helps.
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> On Fri, Oct 21, 2016 at 9:51 AM, Karl Wagner  <razielim at gmail.com (mailto:razielim at gmail.com)>  wrote:
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> > IIRC, the reason we have "class" there is for the optimiser, so it can optimise for the protocol being satisfied by a reference-counted type. Classes are semantically unique from values because they have identity, which is also something a protocol might want to codify.
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> > There may be some optimisation gains by requiring all conformers to be values, but   I struggle to think of why you might want to codify that a conformer should not have identity.
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> > Personally I don't really like this asymmetry in the language either, and would support changes to make these two elements more explicit. For example, a magic "hasIdentity" protocol which is automatically satisfied only by classes, and moving the optimisation guides to usage site (e.g. when declaring a variable of type MyProto, I could declare it of type AnyClass<MyProto>  or AnyValue<MyProto>  instead, to annotate this specific instance as being refcountable or not, without making such optimisation hints part of the MyProto definition)
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> > - Karl
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> > > On Oct 21, 2016 at 8:39 am,  <Mike Kasianowicz via swift-evolution (mailto:swift-evolution at swift.org)>  wrote:
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> > > Currently protocols can have the class constraint: 
> > > protocol MyProtocol : class {}
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> > > It would be (a) intuitive and (b) useful to allow such things as:
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> > > protocol Model : struct {} or protocol Event : enum {}
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> > > These types of restrictions can help prevent accidental anti-patterns or misuse of APIs.
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> > > Seems simple and non-controversial... right?
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> > > [Note: I'd like to see even more heavy-handed protocol restrictions in the future.    For example, a protocol describing an enum with a common case, or a struct with no reference members. Great stuff for defensively coding APIs.]
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> > >   _______________________________________________ swift-evolution mailing list  swift-evolution at swift.org (mailto:swift-evolution at swift.org)   https://lists.swift.org/mailman/listinfo/swift-evolution
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