[swift-evolution] Overriding protocol default implementation

Xiaodi Wu xiaodi.wu at gmail.com
Fri Feb 10 21:20:00 CST 2017

On Fri, Feb 10, 2017 at 6:59 PM, Rod Brown <rodney.brown6 at icloud.com> wrote:

> I don't believe these two worlds are in conflict at all.
> We currently have the default only state (without overrides), plus a
> conflict. We would simply be adding "override" functionality in part to
> clear the conflict.
> With POP the idea is that the protocol, in knowing about how it is
> constituted, has a clear pattern for how it implements its behaviours.
> Therefore you can leverage the protocol to bear the brunt of the work, just
> as we do now.
> The additional tweak to the design of protocols is to allow a user to
> define their own implementation where the default is not appropriate, or is
> incomplete for the use case.

I think I've written a poor explanation of my point. In Swift, it is the
library _author_, not the _user_, who gets the final say as to the upper
limits on what users can do with the author's types, by using modifiers
such as `final` and `open` (or the lack thereof). This has been the subject
of vehement opposition but, nonetheless, it is a clear and opinionated
decision on the part of the language. What you are critiquing as a bug is
regarded as a feature. That is to say, it is a way for the author of a
protocol extension method to deny to the user a customization point (in
other words, to disallow the outright overriding of the "default" behavior).

> This doesn't work against POP - it simply observes that implementations at
> times may need to be customised to the use case, and allows that, as an
> optional override.

Again, the status quo in Swift is that it is up to the protocol's author to
determine which methods are overridable by the user and which are not. The
idea is that protocol extension methods that are not protocol requirements
are the intended way for disallowing such overriding.

As to its practical use: this guarantees that if protocol `P` has a method
`foo()`, it is possible to invoke `foo()` on an instance of existential
type `P` knowing that you will invoke the intended method. A concrete type
`T` that conforms to `P` may have its own `foo()` with totally different
semantics. After all, if `foo()` is not a protocol requirement, then
conforming types can have their own `foo()` do anything at all, with the
collision in name being mere coincidence and no guarantee of similar
semantics. And, with extensions, some third party can implement such a
`foo()` on `T` that the library author has no way of reasoning about. By
having a "shadowing" feature for `foo()`, I can know that no matter how
anyone in the future extends type `T`, I will invoke the intended `foo()`
with the right semantics on an instance of existential type `P`.

> It's clear people are trying to do this already because they already have
> overrides that are causing this conflict, and thus we are having the
> discussion.

It sounds like what you are saying is that users of libraries are trying to
"override" protocol extension methods that authors of libraries have
designed not to be overridden. That this causes problems is, afaict, the
intended consequence of this feature and not an overlooked bug. For maximum
flexibility, however, Swift allows you to "shadow" the non-overridable
method with an identically named method of your own design that can have
different semantics.

> In this case, I don't see overriding the protocol "default" as working
> against this world - I think it clarifies it.
> Rod
> On 11 Feb 2017, at 11:39 am, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> In a vacuum I'd have thought your idea to be superior in terms of
> consistency and user-friendliness. In that world, protocols are strictly a
> list of requirements and (optionally) defaults for those requirements.
> In _this_ world, I think the idea is that protocols provide both behaviors
> and customization points; not all customization points have default
> behaviors, of course, but also not all behaviors are customizable defaults
> (what we're discussing here). I can see how that's more sophisticated but
> also more tricky. I don't see how such a model can be rolled back now that
> we already have it, though.
> On Fri, Feb 10, 2017 at 17:50 Rod Brown <rodney.brown6 at icloud.com> wrote:
>> I agree this is a very... tricky part of Swift, and that many have stated
>> it is expected behaviour.
>> I think this is fundamentally incorrect from a design standpoint with the
>> concept of a protocol as an agreed interface, rather than an agreed
>> implementation. By adding a Protocol Extensions, we have started to mix
>> implementation with interface, which is understandable as those with the
>> same interface often conform to the same behaviour.
>> That said, I think if my conforming to the protocol means I need to do
>> extra work for my specific implementation case, then letting someone
>> circumvent that by simply referring to me as the protocol is both dangerous
>> and lets the protocol's extension implementation have mixed priority. My
>> unique use case should be more important than the implementation that makes
>> sense for the generic case.
>> My view is protocol extensions should a "default", nothing more. If you
>> implement, you override. This behaviour preserves protocols as an agreed
>> interface with additional default logic rather than a conflicting set of
>> implementations.
>> I think I've stated this view before and been shot down, but I'm not sure.
>> Rod
>> On 11 Feb 2017, at 9:54 am, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> Robert, this is a tricky part of Swift. I raised the same point in my
>> very first email to this list a year and a half ago, and the response was
>> that no default/implement/override keyword was desired and that this
>> behavior is intentional. It has since been raised multiple times on this
>> list to the same conclusion.
>> The idea here is that protocol extension methods that are not protocol
>> requirements are statically dispatched and can be shadowed but not
>> overridden. If you read through some of the Swift-Evolution proposals,
>> you'll see that there are several designs that actually seek to take
>> advantage of this behavior. I can't recall which of these were actually
>> implemented in that way, but it does indeed show that the behavior has its
>> uses.
>> Many have suggested some sort of keyword to distinguish overriding and
>> shadowing. However, every such proposed design thus far breaks retroactive
>> conformance in some subtle way. It's hard to search the archives, but there
>> are hundreds of emails on this topic.
>> On Fri, Feb 10, 2017 at 15:57 Goffredo Marocchi via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> I remain very unconvinced that type casting should change code being
>> executed...
>> Sent from my iPhone
>> On 10 Feb 2017, at 21:04, David Waite via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> On Feb 10, 2017, at 12:41 PM, Robert Ryan via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> I’m hard pressed to think of a situation where you’d want the current
>> Swift 3 behavior of the first example (where you can override a protocol
>> default implementation but you only want that override used when you
>> reference the class itself, but not when you reference the protocol as a
>> type).
>> Thats easy - the extension methods are not related to protocol
>> conformance. The type implementing the protocol may not even know there
>> *is* an extension method, or that method could be added either later or by
>> yet other third party code.
>> As such, the type may not be overriding the extension method at all - the
>> protocol-implementing types may have their own method with a conflicting
>> name doing similar (or not) logic. It would not be appropriate for someone
>> calling the protocol extension method to get different behavior simply
>> because a type declared a similar named method.
>> In other words, without the method being declared on the protocol,
>> override behavior between an extension and type would be similar to unsafe
>> “duck” typing. There is no agreement by the type that they are conforming
>> to extension behavior, only to the protocol’s behavior.
>> IMHO the issue actually goes the other way
>>  - there is no way to indicate that an extension method to a protocol is
>> meant to declare ‘default’ behavior, e.g. represents a partial
>> implementation for types conforming to the protocol.
>>  - there is no way to indicate that a type property/method ‘implements’
>> the functionality of a protocol.
>> The lack of a distinction between extensions declaring random methods and
>> declaring default implementation behavior is obviously an element of
>> confusion. Both points become an issue if the protocol method signatures
>> ever changes - the extension could no longer be providing a default
>> implementation, or a type may silently switch from their own implementation
>> to the default implementation provided by the extension.
>> -DW
>> If there are such examples, then add a “build setting” to allow you to
>> turn off this warning, or add some keyword to the declaration of the
>> default implementation that indicates that you’re allowing it to be
>> overridden, but protocol types won’t use it (e.g. nondynamic).
>> Personally, I’d just add the warning and call it a day (because I don’t
>> know why you’d ever want the current Swift 3 behavior).
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