[swift-evolution] [Proposal] Explicit Non-Default-Implemented Protocol Requirements

[Xiaodi Wu]

Besides the comments below on consensus about how to deal with the
documentation aspect, I should point out that your example is incorrect. !=
is not a protocol requirement of Equatable with a default implementation,
but a protocol extension method that cannot be overridden which is *not* a
protocol requirement. That is by design.

Additionally, it is unclear to me why default implementations should need
be provided in the same file, as your solution requires. There are good
reasons why library A that builds upon library B might supply a default
implementation in an extension for a protocol B.P that refines B.Q. To the
end user, it is just as much of a default that doesn’t need to be
implemented when conforming to P, but your proposed solution does nothing
to address this issue.


On Wed, Aug 2, 2017 at 03:54 Gor Gyolchanyan via swift-evolution <
swift-evolution at swift.org> wrote:

> Good day, swift community!
> This time, I'd like to bring your attention to an everyday problem which,
> I'm sure, you've all dealt with at some point.
>
> *Motivation*
>
> Many protocols have a lot of default-implemented requirements so that they
> can be customized, which would otherwise be impossible to do.
> For example, the Comparable protocol, which (considering its inheritance
> of Equatable) states a requirement of ==, !=, <, <=, >, and >=, only
> requires implementation of == and <, because all others have default
> implementations in terms of these two. This forces the user to either
> memorize the exact set of mandatory requirements or, in case of more
> complicated protocols like Collection, constantly look up the
> documentation and hope that it has a section outlining how to conform to it.
> Conversely, the author of the protocol needs to manually keep track of the
> requirements and their default implementations to ensure that the ones that
> have to be mandatory stay mandatory and the rest of the requirements stay
> default-implemented, which is also a tedious and error-prone work.
>
> *Proposed Solution*
>
> I'd like to propose the ability to mark select protocol requirements as
> default, which would cause the following effects:
>
>    - The compiler will emit a compile-time error if a default
>    implementation is not available in a non-constrained protocol extension in
>    the same file and provide a fix-it for inserting a template.
>    - If a conforming type does not implement all requirements of the
>    protocol, the compiler will not produce fix-its for inserting a template
>    for any default requirements.
>
>
> *Example*
>
> // Equatable.swift
>
> protocol Equatable {
>
> static func == (_ some: Self, _ other: Self) -> Bool
>
> default static func != (_ some: Self, _ other: Self) -> Bool
> *^ error: missing implementation of a default requirement `!=`; fix-it:
> insert a default implementation*
>
> }
>
> extension Equatable {
>
> static func != (_ some: Self, _ other: Self) -> Bool {
> *[Your Code Here]*
> }
>
> }
>
> // Something.swift
>
> struct Something: Equatable {
> *^ error: missing implementation of a non-default requirement
> `==`; fix-it: insert an implementation*
>
> }
>
> extension Something {
>
> static func == (_ some: Something, _ other: Something) -> Bool {
> *[Your Code Here]*
> }
>
> }
>
> Cheers,
> Gor Gyolchanyan.
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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