[swift-evolution] Require use of override keyword to override dynamically dispatched methods defined in a protocol with a default implementation

Xiaodi Wu xiaodi.wu at gmail.com
Wed Jan 6 11:01:38 CST 2016

> Another option might be to allow imported definitions to be used by a
> conformance without the `override` marking to support retroactive modeling
> while requiring definitions in the same module as the conformance to
> explicitly specify the `override`.

That's an interesting suggestion. I don't think I'd prefer that
solution, though, because I would imagine that it's during retroactive
modeling of someone else's stuff that ambiguity regarding what's
overridden or not might occur.

I've been slow in fully understanding Greg Parker's feedback, but on
reflection it may be the only way to satisfy all use cases. In
Thorsten's scenario where neither the protocol and its extension nor
the type is under the control of the user, making the type conform to
the protocol would either require no keyword requirements regarding
overriding methods (as it is, and as you suggest) or else a syntax to
indicate an overriding method must exist *within the extension that
conforms a type to a protocol* (as Greg suggests).

Can I propose an amended solution then?

(1) When a protocol requires a method (or property getter/setter,
etc.) but doesn't provide a default implementation, no keyword is used
anywhere since all conforming types must provide their own
implementation--I suppose a requirement could be made for a keyword,
but I don't know that it adds much in terms of guarding against
unintended behavior; I guess that can be a continued point of

(2) When a protocol doesn't declare a method but an extension to the
protocol provides one, types implementing a method with the same
signature *must not* declare it to be overriding; such protocol
extension methods are not overridden because they can be invoked after

(3) When a protocol does declare a method, and an extension to the
protocol provides a default implementation, then to override that
implementation *either* the implementing type *or* an extension must
use the keyword `override`

(3a) In the case of an implementing type, `override func` is used
instead of `func`, just as in the case of a class overriding a
superclass method

(3b) In the case of an extension to a type (this is the syntax I could
come up with, but maybe it'll be objectionable in other ways), a
method in an existing class can be retroactively made overriding by
declaring `override [method signature]` with no body, similar to the
way that a method is declared inside a protocol; by analogy, an
overriding getter might use the syntax

extension Int: BoundedType {
    static var min { override get }

I think the syntax proposed in (3b) has the virtue of not requiring
additional keywords, being sufficiently similar to (3a) so that it's
not surprising, but still sufficiently unique in that the syntax is
not currently valid code and thus isn't currently used to mean
anything else.

On Wed, Jan 6, 2016 at 8:21 AM, Matthew Johnson <matthew at anandabits.com> wrote:
> On Jan 6, 2016, at 3:48 AM, Greg Parker via swift-evolution
> <swift-evolution at swift.org> wrote:
> On Jan 5, 2016, at 8:50 PM, Brent Royal-Gordon via swift-evolution
> <swift-evolution at swift.org> wrote:
> Taking inspiration from syntax used for methods in classes that override
> methods in superclasses, require methods that override dynamically
> dispatched default implementations in protocol extensions to use the
> override keyword. Likewise, forbid the override keyword if the method being
> implemented instead 'masks' (would that be the right word?) a statically
> dispatched method in a protocol extension which can nonetheless be invoked
> by upcasting to the protocol.
> This has been suggested before, usually in the form of a separate
> `implement` keyword. The main problem is that it makes it impossible to
> write a protocol after the fact which formalizes some existing pattern in
> the types.
> What do I mean by that? Well, imagine you need generic access to the `min`
> and `max` static properties of the various integer types. There's no
> existing protocol that includes those members. But you can write one and
> then extend the integer types to conform to your new protocol:
> protocol BoundedIntegerType: IntegerType {
> static var min: Self { get }
> static var max: Self { get }
> }
> extension Int: BoundedType {}
> extension Int8: BoundedType {}
> extension Int16: BoundedType {}
> extension Int32: BoundedType {}
> extension Int64: BoundedType {}
> func printLowestPossibleValueOfValue<Integer: BoundedIntegerType>(x:
> Integer) {
> print(Integer.min)
> }
> This only works because `min` and `max` *don't* need any special marking to
> be used to satisfy a requirement. Requiring a keyword like you suggest would
> remove that feature.
> Possible solution: if you want a new protocol adoption to map to some
> existing method or property then you must explicitly write that. You can't
> just adopt the protocol in an empty extension.
>    extension Int: BoundedType {
>        static var min = Int.min
>        static var max = Int.max
>    }
> but with some other syntax that isn't ambiguous. Code completion and
> compiler fix-its could suggest this when the class already implements
> something suitable.
> Another option might be to allow imported definitions to be used by a
> conformance without the `override` marking to support retroactive modeling
> while requiring definitions in the same module as the conformance to
> explicitly specify the `override`.
> --
> Greg Parker     gparker at apple.com     Runtime Wrangler
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