[swift-evolution] Static Dispatch Pitfalls

[Xiaodi Wu]

On Sun, May 22, 2016 at 11:20 PM, Matthew Johnson <matthew at anandabits.com>
wrote:

>
> 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> 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 a method that really ought to be a protocol requirement isn't a
requirement and you don't control the protocol, well you're pretty much out
of luck even today. Any conforming type accessed via the protocol will use
the less efficient extension method and nothing about Brent's proposal
would make that worse or better.

Shadowing of the slow extension method doesn't remove the burden. It may
make calling your fast implementation look nicer, but a less informed user
of your type would unwittingly call the slower implementation if they
access your type via the protocol. You could instead:
* come up with another name for your fast implementation; maybe the
"obvious" name for the method is "frobnicate"--then name your method
"quicklyFrobnicate";
* or, decide you don't want to conform your type to a poorly designed
protocol after all, instead retroactively modeling your type and other
types of interest with a better designed protocol of your making.


> 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?
>

I'm not sure I quite understand when this arises. Surely, by construction,
if you wish to retroactively model types, you are willing to design your
protocol around them. What else could it mean to retroactively model
existing types? Can you give a concrete example where during retroactively
modeling you simply have no choice but to name an extension method using a
name that it is shadowed by a conforming type?


> 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.
>

You are saying that it would be possible for a protocol extension in one
dependency to conflict with a conforming type in another? This issue can be
avoided if enforcement of non-shadowing by the compiler is such that when
neither conforming type nor protocol extension is under your control
everything continues to work as-is.

>
>> > (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.
>

You don't override protocol requirements, but you do override their default
implementations, whereas you cannot 'override' statically dispatched
extension methods. Within a protocol extension, there are methods that are
overridable by conforming types (i.e. default implementations of protocol
requirements) and methods that are not (i.e. statically dispatched
non-requirements).


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