[swift-evolution] Renaming for Protocol Conformance

Fri Sep 2 10:56:42 CDT 2016

> Am 24.08.2016 um 21:35 schrieb Xiaodi Wu via swift-evolution <swift-evolution at swift.org>:
> 
>> On Wed, Aug 24, 2016 at 1:59 PM, Jonathan Hull <jhull at gbis.com> wrote:
>> 
>>> On Aug 24, 2016, at 7:48 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> 
>>>> On Wed, Aug 24, 2016 at 3:39 AM, Jonathan Hull <jhull at gbis.com> wrote:
>>>> 
>>>>> On Aug 23, 2016, at 8:35 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>>> 
>>>>>> 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.
>>>> 
>>>> Right now I commonly have to hand-namespace protocol methods/properties to avoid conflicts.  So instead of having ‘var image:UIImage’ (which is the name which makes the most sense in the protocol’s context), I have ‘var protocolNameImage:UIImage’.  There are lots of things which have common properties like ‘count’ which have to be called ‘somethingCount’ or ‘countOfSomething’.  In the context of the protocol, these names are full of redundant words (especially when measured against the new naming guidelines).  We are all used to doing this for Objective C, but it feels out of place in Swift.
>>>> 
>>>> This will become a much more serious issue as the third-party code ecosystem grows.  Without some capability like this, you will have frameworks which can’t be used together (or at least with the same object).  I would hate to see a ‘best practice’ emerge of adding 3 letter prefixes to all protocol methods to get around compatibility issues.
>>> 
>>> Ah, well this isn't exactly the diamond problem you're talking about here. Instead, I think, we have a fundamental disagreement. I think I've been told that this opinion of mine is 'insane'--but I hold to it:
>> 
>> Well you asked for an additional example besides the diamond problem… so no it isn’t.  I did include a diamond problem example further down though…
>  
> Sorry, I wasn't asking for an example _besides_ the diamond problem. I was asking for more information about a concrete scenario, diamond problem or not, where an existing technique could not resolve the conflict (for instance, a scenario when you controlled neither of two conflicting protocols, and where no satisfactory alternative design existed that could avoid conforming a single type to both protocols).
> 
>> 
>>> Protocols are not merely a vehicle for delivering a reusable bag of code. One of its most essential purposes is to constrain the shape or API of its conforming types. Therefore, it is a feature, not a bug, that with every choice of name in a protocol you foreclose the possibility of composing that protocol with others that might have colliding names.
>>> 
>>> Currently, if you the protocol vendor have made the decision that `image` "makes the most sense in the protocol's context", you must have considered whether it would be absurd for a conforming type to have another use for `image`. If it would be absurd, then `image`

I find it a little bit strange to require from a protocol designer to foresee all future uses of a protocol. IMO protocols are not bags of code but encapsulate a certain (typically fine grained) semantic. How this semantic will be coupled with other semantics (i.e. protocols) is widely open.

>>> is the appropriate name for your protocol requirement and any other word would truly be redundant. But, if this is only one of many plausible images, then `somethingImage` or `imageOfSomething` *is* the appropriate name, and trying to shorten the name isn't at all consistent with Swift guidelines but rather an incorrect attempt to prioritize brevity over clarity.
>> 
>> Most things that conform would have ‘image’, and it would have exactly the same semantics as my protocol. Thus their ‘image’ would provide conformance without additional work.  But I have to worry about name collisions, so now I have to defensively call it ‘imageOfSomething', which they now have to implement to call their ‘image’ method.
>> 
>> 
>>> What you're arguing is that protocol designers should be able to design protocols without regard for how they will compose with others in conforming types, relying on a new member-renaming feature instead. But, as you point out, you can already use a protocol as a mere bag of code by naming all members with unique, prefixed names, then have conforming types forward their own choice of names to these.
>> 
>> No, I am arguing that protocol authors should design protocols in the way which makes the behavior/semantics of the protocol the most obvious to the caller.  95% of the time there won’t be collisions, but occasionally there will be and we have to have a plan for that.
>> 
>> 
>>> This member-renaming feature you propose would enhance the aesthetic pleasure of the protocol designer, allowing simple names that don't ever have to appear in the public API of a concrete type to be used for a protocol member without placing any restrictions on the API of conforming types. However, I don't see anything wrong with the current hand-prefixing method being enshrined as "best practice" for the bag-of-code approach to protocols. If, as you predict, a growing third-party code ecosystem makes name collisions worse, then in fact having uniquely distinguishable prefixed members would be less confusing than having conforming types renaming protocol members as a matter of course.
>> 
>> You are arguing that namespace collisions are a feature instead of a bug?  Did you feel that way about ObjectiveC’s lack of name spacing?
> 
> My argument is about protocols specifically: I understand that a major feature of protocols is that they make guarantees regarding the API of conforming types. In rare cases, two guarantees may conflict, but I do not consider that conflict to be a bug per se, as it is the inevitable result of what it means to have guarantees, i.e. it is part and parcel of the feature. In order to provide a way of resolving conflicting requirements in protocols, your solution eliminates the API-guaranteeing feature of protocols altogether.
> 
> I can't comment about Objective-C, because I've never written a single line of it.
>  
>> I don’t think it is anywhere near as confusing as you suggest. As I mentioned before, if you cast it to the protocol, then the original names will still work.
> 
> Except when you can't cast to a protocol existential, as is the case with any protocol with Self or associated type requirements.

This is a separate problem which will be solved once we have existential types.
Furhermore the argument still holds when the protocol is being used as type constrained.

>  
>> If you are trying to type the original name on the typed conformer, then showing the renamed version (with an indication of the renaming) in autocomplete should teach the change and clear up any confusion.
> 
> Mine isn't an argument about usability or learnability. It's a philosophical/design point: what are protocols for? My answer: among other uses, for constraining the API of conforming types. Perhaps this view is incompatible with the view that protocols should support additional mixin-like features.

I don't think this has anything to do with mixins. It is just the general problem of being able to combine protocols which have been designed independently from each other.

> 
>>>> To take your example of walk().  Perhaps we have a protocol ‘Walkable’ which refers to any data structure where the nodes can be walked using the ‘walk()’ function.  It is easy to imagine two different protocols A & B which specialize on this in different ways (say LinearWalkable & RandomWalkable), and both add some methods/properties and use those to provide efficient default implementations.  At some point, you may run into a data structure which could easily be walked in both ways.
>>>> 
>>>> As things are right now, you couldn’t inherit from both protocols.  While you could add new ‘linearWalk()’ & ‘randomWalk()’ to the protocols respectively (cluttering their interface), there is still the issue of what to do when 'walk()’ is called.  You can’t rename walk() in the originating protocols because it comes from their common ancestor.  Much better to force one (or both) of the methods to be renamed on the conforming data structure.  That keeps the interfaces of the protocols clean and makes the options available on the data structure clearer (e.g. ‘walk()’ & ‘randomWalk()’ )
>>>> 
>>>> What I have had to do in the current version is inherit from the original protocol and then copy and paste the default implementations from the specialized versions.  Now my code has been duplicated and is harder to maintain.  We can do better.
>>> 
>>> I think Charles's solution is pretty nice, but he's right that the API surface area will have to grow. I don't know his original use case, so I don't know how ugly I'd find the final solution to be in that scenario. In this particular example, I'd say that having `linearWalk()` and `randomWalk()` distinguished seems pretty sensible and an overall win for clarity. If the same vendor controlled all three protocols, then `Walkable` could have the `walk()` requirement removed altogether for even more clarity.
>> 
>> So you would remove 'walk()' from Walkable to avoid the name collision in this one case, when ‘walk()’ is Walkable’s entire reason for being?
> 
> No, `walk()` is not Walkable's entire reason for being. Protocols guarantee semantics also. A `Walkable` protocol without any required members would still have a reason for being: conforming types are walkable.
> 
> On the other hand, given that Walkable certainly would have associated type requriements, if you considered that `walk()` _was_ Walkable's entire reason for being *and* you could rename `walk()` in any conforming type, how is that different from not having a `walk()` requirement at all?

Generic methods using a type constrained by Walkable can call `walk()` on it. The same will be possible for variables declared with existential types in the future and it already applies for protocols without associated type or Self requirements.

-Thorsten 

> 
>> Also, you would lose polymorphism.
>> 
>> 
>> 
>>> Also, just a hunch, but I suspect your hypothetical would never hold. Could you envision how the requirements for RandomWalkable might be such that it's possible to implement an efficient _default_ implementation of a random walk for any of several conforming data structures, but only one of these data structures is LinearWalkable, _and_ such a linear walk is efficient using another _default_implementation for an overlapping but not identical set of data structures? It's not mere trivia here, because the crux of your argument is that there exist default implementations that require copying and pasting into conforming types (and sufficiently efficient default implementations so that copying and pasting is appropriate rather than implementing a more efficient version). More likely in diamond problem scenarios, I think, colliding members are going to be properties or methods either without default implementations or than need to supply more efficient versions of default implementations anyway.
>> 
>> Based on what?  There is a reason the diamond problem has a name (and a wikipedia entry).  Charles just said he ran into a problem like this.  I have run into it in the past as well.
> 
> Sure, and returning to my question above: could you share details about where you've run into this?
> 
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