[swift-evolution] Static Dispatch Pitfalls

[Matthew Johnson]

> On May 22, 2016, at 4:22 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Sun, May 22, 2016 at 3:38 PM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> > The proposal is well thought out and makes a valiant attempt at handling all of the issues necessary.  But I don't support it for a number of reasons.  I think it highlights how awkward it would be to try to address shadowing on a case-by-case basis, which isn't necessarily obvious until you explore what a solution might look like.
> 
> It does, but I'm just not sure what else you can do about it. If there's a warning, you need a way to silence it. If you ignore some cases (like creating a conflict by importing two modules), you'll miss some of the subtlest and hardest-to-fix bugs.
> 
> Honestly, I'm tempted to say "you just can't ever shadow a final protocol method" and be done with it. If that prevents certain conformances or stops certain imports, so be it. You can always work around that with wrapper types or other techniques.
> 
> You know, I think this might be cleverest solution. It adds a small limit to the language, but it doesn't unduly penalize retroactive modeling. If you control either the protocol or the conforming type, you can change the name of one of the methods so it doesn't shadow/get shadowed by the other.

If you control the conforming type this isn’t too big an issue as long as the protocol was well designed.  However, if the protocol was poorly designed it could be an issue.  Maybe a method that can be more efficiently implemented by some types was not made a requirement, but an extension method (with a slower implementation) takes the obvious name.  Maybe you would be willing to live with the slower implementation when your type is accessed via the protocol, because at least it can still be used via the protocol, but you don’t want to burden callers who use the concrete type with the slow implementation.  What do you do then?  

If you control the protocol but want to retroactively model types you do not control this assumes you are willing to design your protocol around those types.  What if one of those types happens to implement a method that should not be a requirement of the protocol for one reason or another, but will be implemented as an extension method.  What do you do then?

And of course there are cases where you do not control either.   Some people write code with a lot of 3rd party dependencies these days (not my style, but pretty common).  This is not a trivial concern.

>  
> 
> > (And btw, 'final' in this proposal is not exactly, because when combined with @incoherent the methods are not actually 'final' - there is a necessary escape hatch).
> 
> There is no particular reason you couldn't allow similar annotated shadowing of `final` methods on classes; they would have basically the same semantics as you get here, where if a piece of code knows it's working with the subclass you get subclass semantics, but otherwise you get superclass ones. I do not claim this is a good idea. :^)
> 
> > Second, we should require annotation of methods in protocol extensions that are not default implementation of requirements.  Maybe 'shadowable' or 'staticdispatch'?  These are awkward, but so is the behavior and they describe it better than anything else I've seen so far (maybe we can do better though).
> 
> I don't think `shadowable` makes sense here; that doesn't acknowledge a limitation, which is what we're trying to do here.
> 
> I continue to wish we hadn't taken `static` for statically-dispatched type methods. But I lost that argument years before swift-evolution became a thing.
> 
> > I don't like 'nondynamic' both because it is not aligned with the meaning of 'dynamic' and also because it only says what the behavior *is not* rather than what the behavior *is*.
> 
> I do understand what you mean here. Unfortunately, the word "virtual" in a keyword makes me break out in hives, and I'm not sure what else we might base it on.
> 
> This is why I selected `final` in my proposal. `final` is desperately close to the actual semantic here, far closer than anything else in the language.
> 
> How about `nonoverridable`? That said, I agree with earlier comments that training-wheel annotations probably aren't the way to go. Maybe, as you suggest above, just don't allow shadowing at all.

Unfortunately, ‘nonoverridable’ doesn’t really make sense because you don’t ‘override’ protocol requirements.

>  
> 
> --
> Brent Royal-Gordon
> Architechies
> 
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