[swift-evolution] Renaming for Protocol Conformance

Xiaodi Wu xiaodi.wu at gmail.com
Tue Aug 23 10:35:27 CDT 2016

On Tue, Aug 23, 2016 at 3:02 AM, Jonathan Hull <jhull at gbis.com> wrote:

> On Aug 22, 2016, at 11:32 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> On Mon, Aug 22, 2016 at 11:59 PM, Jonathan Hull via swift-evolution <
> swift-evolution at swift.org> wrote:
>> Hi everyone,
>> We talked about this before when we were discussing mixins, and there
>> seemed to be generally positive feelings towards it as a feature for the
>> future.
> It's been some time now since the original discussion, so perhaps you
> could refresh our collective memory (or at least, mine): although it
> *seems* like this feature might be useful, I can't recall a concrete use
> case where I've felt like I needed this feature--do you have some examples?
> Ideally, the biggest use is that it helps to (partially) solve the diamond
> problem (and similar issues) by forcing/allowing disambiguation when there
> are multiple protocols being conformed to.  This will become more of an
> issue if we allow protocols or extensions to add storage.  Your proposed
> syntax actually does a better job of it than mine because mine was always
> shown as attached to some sort of implementation, whereas yours could
> potentially allow access to a default implementation under a new name.
> Other than that, it generally allows us to bypass/mitigate conflicts
> between protocols.  In the current version, you are unable to conform to
> both protocols (either because it won’t compile or because you can’t
> satisfy the semantics of both protocols) without designing the protocols
> together to avoid conflicts.  (I have definitely had to go back and
> rename/refactor properties on a protocol for this reason… which I couldn’t
> have done if I didn’t control both protocols).

I understand something of the difficulty of confronting the diamond
problem. As I wrote above, I'm inclined to believe that this proposed
feature would help solve a real issue. However, the point I'm trying to
make is that, on reflection, I have never actually been hampered by the
lack of this feature, and so I'd like to continue the discussion to get a
fuller sense of just how impactful this proposal would be, both positive
and negative.

It's true, of course, that if you control at least one of two protocols
(you don't need to control both protocols), it is trivially easy to cause
this problem to occur, but as you point out it is also possible to resolve
the problem by re-designing the protocol you control. I'm inclined to think
(without evidence, admittedly) that re-designing to remove the conflict,
where possible, would actually be the superior option in most cases.

My question was: have you actually run into a scenario that necessitates
the feature you propose because you controlled neither conflicting
protocol? I think it would strengthen the proposal greatly to have a
concrete, uncontrived example.

Take a look at Eiffel’s ‘rename’ & ’select’ features for similar
> functionality and use-cases.
> Ultimately, this is a step in the direction of having true mixins.
Sure, maybe. I couldn't evaluate that claim. I'm inclined to favor the
proposal, but it'd have to stand on its own merits, not as a step to an
as-yet undesigned feature.

   I am fairly certain this affects the ABI though, so I thought I would
> bring it up now.
>> If two protocols have methods/properties with the same name, but
>> different signatures, we need a way to distinguish between them when
>> attempting to conform to both.
>>         protocol A {
>>                 var x:Int {get set}
>>         }
>>         protocol B {
>>                 var x:Double {get set}
>>         }
> Methods can be overloaded that differ in arguments or return type, so it
> seems like this problem mainly exists with *properties* that differ in
> type--am I wrong?
> There is also the case of functions with the same name and signature, but
> different semantics.  There may be no single implementation which
> simultaneously satisfies the semantics for both protocols. By renaming one
> of the functions, we are able to provide separate implementations for each
> requirement (which allows both protocols to function as intended).

True. However, putting on my critical hat, this seems like we're stretching
to provide support for an anti-pattern. It'd be nice to have an example
where one runs into the motivating problem *and* where the proposed feature
promotes a _better_ design than is currently possible, rather than making a
bad design compile.

At this point, I'm imagining scenarios where a user is trying to conform a
type MyAnimal to both Biped and Quadruped, then worrying that `walk()` has
two semantics: something has already gone deeply wrong IMO.

[Yes, I know there are animals that can sometimes walk on two or four legs.
The point here is that the protocols were clearly designed to model animals
at a certain level of detail, while it appears that the user writing
`MyAnimal` wants to model the animal at a different level of detail than
either protocol was designed to handle. You might have specific qualms
about this particular hypothetical, but I think you can pick out the
general point that there is a much larger problem inherent in the design
than the specific problem regarding two colliding method signatures.]

There may also be functions in different protocols with different names but
> the same semantics and signature.  This will allow a single implementation
> to satisfy both protocols without duplication.

This is a poor argument IMO. You can already implement foo() and then have
bar() forward to foo() with trivial effort and really minimal boilerplate.
It's an existing solution, and a more general solution because it doesn't
require matching signatures. Also, it's a better solution IMO because it
preserves the notion that a type T : Fooable, Barrable provides the full
API guaranteed by Fooable and Barrable.

Finally, we may want to rename an inherited default implementation to avoid
> conflicting with another protocol's default implementation in cases where
> we don’t want to override it.

Yes, I think this would be handy. I can't think of an existing way to do
this, and I expect it might make a big difference in designing good
protocols. So here, I think we have a strong argument.

Again, though, could we find a concrete example of how this feature would
promoter a better design of an actual type and/or protocol?

 One possibility is to allow a struct/class/enum to conform to the protocol
> while renaming one (or both) of the clashing methods:
>>         struct C: A,B {
>>                 var x:Int
>>                 var y:Double implements B.x
>>         }
>> The conforming method/property would still have to have the same
>> signature, but could have a different name (and parameter labels).  It
>> would also allow protocol methods which have identical signatures and
>> semantics, but different names to be implemented using the same method (i.e
>> ‘implements D.z & E.w’).
>> When something is cast to the protocol (say ‘as B’), then calling the
>> property (e.g. ‘x’) would end up calling the implementation of the renamed
>> property ( ‘y’ in this example) on the conforming type.
> Reflecting on this proposed change, it occurs to me that something of
> value would be lost, and I think that this something is actually rather
> valuable:
> Today, when I see that a type conforms to (for example) Sequence, I know
> that certain methods and/or properties exist on that type. Protocol
> conformance guarantees a certain API, not just certain semantics.
> It isn’t actually lost, however.  When working with it as a Sequence (for
> example), that API would be intact using the original names.  It is only
> when working with it as its own type that the renaming would have an effect.

That is not my point. With this proposal, knowing that MyGreatType conforms
to Sequence would no longer yield any information as to the MyGreatType
API. That is definitely something lost and we should acknowledge that.

Also, recall that Sequence has Self or associated type requirements. So:

let m = MyGreatType()
// There is nothing I can write here to use the Sequence API with `m`, IIUC;
// however, depending on how this feature is designed, I *might* be able to
// call a generic function that operates on a type `T : Sequence` and work
// with `m` that way.

Perhaps one way to mitigate this loss would be to have any renamed members
> listed *in the declaration of conformance*, something like this (with some
> additional bikeshedding):
> ```
> struct MyGreatType : Sequence (count => length) {
>   // MyGreatType conforms to Sequence but renames `count` to `length`
> }
> ```
> Yes, putting it in the conformance declaration is a definite possibility
> we should consider.
> I think we would also want a way to retroactively conform using existing
>> properties/methods in an extension declaring conformance.  Not sure what
>> the best syntax for that would be.  Off the top of my head (though I would
>> love to have something with less cruft):
>>         extension D:B {
>>                 @conform(to: B.x, with: D.y)
>>         }
>> or maybe just:
>>         extension D:B {
>>                 D.y implements B.x
>>         }
> If renamed members are declared along with protocol conformance, then the
> syntax for retroactive modeling follows naturally:
> ```
> extension D : B (x => y) { }
> // again, the actual notation here is ugly
> // but the underlying idea, I think, is worth considering
> ```
> Yup
> One thing I like about this is that it helps to solve the diamond problem.
>  ‘x’ could be a default implementation in B which D does not override.  I
> think this is an important case which my original proposal didn’t address
> fully.
> We should keep bikeshedding the syntax though...
> Thanks,
> Jon
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