[swift-evolution] [Pitch] Requiring proactive overrides for default protocol implementations.

Vladimir.S svabox at gmail.com
Thu Apr 28 12:52:12 CDT 2016

In general I fully support this proposal. IMO Swift should be more explicit 
on what method in class/struct is required by protocol and if it overrides 
default implementation in protocol extension.

Actually, personally I like the idea that we need even separate base class 
inheritance declaration and protocol conformance like in Java:
class A:B implements C {..} or class A:B::C {}.. or even specify which 
protocol exactly requires this method like
func f() requiredBy SomeProto {} ...
but never mind :-)

Also your proposal should protect from such situations:
1. We use some 3rd party class AClass that implements AProtocol
2. We write extension to AProtocol, added new method ANewMethod
3. Then, we update the source of that AClass, and in new version of it the 
same ANewMethod was introduced(like in our AProtocol)
4. Now we have methods with same name in AClass and in protocol extension 
without any error/warning

Small fix(I think):
protocol A..
protocol B..
extension A { override required func foo() {...A extension...} }
Type CType: A, B {}
 >>> don't you want to write just 'required' here, not 'override required' ?

Right now we have protocol extension's priority over type definition:
protocol A {}

class C:A {
     func y() { print("Y in C") }

extension A { func y() {print("Y in A")} }

var c : A = C()
c.y() // Y in A here
so, it seems like you need to add this to "Impact on Existing Code"
I.e. now, when someone declared an protocol extension, it will get this 
method in class instance. Your proposal says we'll have class ext -> 
class-> protocol ex. -> protocol. Or I missed something?

Also, can't we just use 'override' instead of 'required'?
I.e. in protocol extension 'override' means - implements something, that 
was declared already in protocol. In class methods declaration 'override' 
will mean 'overrides base class'es implementation, or method in protocol 
extension, or implements protocol requirement'. I don't see the clear need 
of 'required' keyword here. I mean for me just 'override' will say 'this 
method is already declared somewhere'.

On 28.04.2016 19:53, Erica Sadun via swift-evolution wrote:
> Draft. Criticism and suggestions both welcome. -- E
>   Requiring Proactive Overrides for Default Protocol Implementations
>   * Proposal: tbd
>   * Author(s): Erica Sadun <http://github.com/erica>
>   * Status: tbd
>   * Review manager: tbd
>     <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#introduction>Introduction
> This proposal enhances protocol implementation safety. It incorporates two
> keywords that cooperate with compiler checks to limit "near miss"
> implementation errors and accidental member overrides.
> /This proposal was discussed on the Swift Evolution list in the [Pitch]
> Requiring proactive overrides for default protocol implementations.
> <http://thread.gmane.org/gmane.comp.lang.swift.evolution/15496> thread/
>     <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#motivation>Motivation
> The proposal introduces a mandatory |required| keyword that marks members
> as fulfiling protocol requirements. This expansion reduces the risk of
> near-miss implementations (for example, adding |thud(x:
> Double)| when |thud(x: Float)|is required), provides in-line documentation
> of why the member has been included, thereby enhancing the code-level
> documentation at the implementation point, and supports compile-time checks
> for protocol conformance.
> This proposal extends the |override| keyword to protocol conformance. The
> Swift Programming Language describes the way subclass methods must override
> implementations established in superclasses. /Methods on a subclass that
> override the superclass’s implementation are marked with
> */|override|*/—overriding a method by accident, without override, is
> detected by the compiler as an error. The compiler also detects methods
> with override that don’t actually override any method in the superclass./
> Adding an |override| requirement expands this cautious approach to
> protocols. Developers must override implementations inherited from protocol
> extensions with the |override| keyword. And the compiler will flag uses
> of |override| where member implementations do not, in fact, override an
> existing implementation. The keyword prevents accidental overrides, where a
> sensible member name conflicts with signatures established in the protocol
> conformance and forces users to proactively select a version in favor of
> existing protocol extensions.
>     <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#detail-design>Detail
>     Design
>   * The |override| keyword is extended to protocol inheritance, and when
>     used prefers the overridden behavior to the default behavior.
>   * Swift will prefer an overridden implementation in preference in reverse
>     hierarchical order: type extensions take precedence over type
>     declarations over protocol extensions over protocol declarations
>     (assuming protocol declarations eventually adopt default implementations).
>   * The |required| keyword marks a member as satisfying a protocol
>     requirement, whether in protocol extensions, type declarations, or type
>     extensions.
>         <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#required-protocol-members>Required
>         Protocol Members
> Protocol requirements are marked with |required| for compile-time checks of
> intentional conformance.
> protocol A {
>     func foo()
>     func bar()
>     func blort()
>     func gar()
> }
> extension A {
>     required func blort() {} // Correct, required by `A`
>     func womble() {} // Correct, new method in extension
>     func gar() {} // Incorrect: Compiler says: add `required` keyword or remove implementation
> }
> struct B: A {
>     required func foo() {} // Correct
>     required func far() {} // Near miss. Compiler: rename method or drop required keyword
>     func bar() {} // Possible accidental name match. Compiler: rename method or add required
> keyword
> }
>         <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#member-overrides>Member
>         Overrides
> Overrides are marked with |override| to ensure intent.
> protocol A {
>     func foo()
>     func bar()
>     func blort()
>     func gar()
> }
> extension A {
>     required func foo() {} // correct
>     func womble() {} // correct
> }
> struct B: A {
>     required func bar() {} // correct
>     required func foo() {} // incorrect: Compiler says: add `override` keyword or remove implementation
>      func womble() {} // incorrect: Compiler says add `override` keyword or remove
> implementation. `required` is not needed as `womble` is not a required
> protocol member.
> }
>         <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#handling-changes>Handling
>         Changes
> Default implementations can be added or removed at any time, as can type
> conformance implementations:
> **Original**	**Change**	**Outcome**
> Some member implemented in type	Protocol adds that member	Must add
> `required` to type implementation or rename member to avoid conflict
> Some member implemented in type, marked as `required`	Protocol removes that
> member or it never existed	Must remove `required` from type implementation
> Some member implemented in type, marked as `override`	Protocol extension
> removes that member or it never existed	Must remove `override` from type
> implementation
> Some member implemented in typed, member not mentioned in protocol
> Extension adds default version of member	Type implementation must add
> `override` keyword
> `required` member implemented in type	Default member added	Must add
> `override` or remove type implementation
> `override required` member implemented in type	Remove default member	Must
> remove `override` in type implementation
> `override required` member implemented in type	Remove type member
> implementation	Default implementation now used
> Type member uses `required` keyword	Protocol removes requirement or never
> had it	Type implementation must remove `required` keyword
> Protocol declares required member	Extension implements default
> implementation	Extension must add `required` keyword, differentiating
> default implementations from added behavior
> Swift adds default implementations to protocols as well as extensions
> Protocol adds default implementation	Type implementation must use both
> `required` and `override` keywords. Protocol extension must use `override`
> keyword. Order of preference goes: overriden member, overriden extension,
> protocol default implementation
>         <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#multiple-conformance-conflict>Multiple
>         Conformance Conflict
> Consider the following situation. For the sake of future-proofing, this
> example includes default protocol implementations although they do not yet
> exist in Swift.
> protocol A { func foo() {...default...} }
> protocol B { func foo() {...default...} }
> extension A { override required func foo() {...A extension...} }
> Type CType: A, B {}
> In this example, the compiler emits a warning that "CType cannot
> unambiguously differentiate which version of |foo| to use
> for |CType| instances". If the CType type were to be removed or either of
> its conformances erased, there would be no compiler issues.
> To fix this scenario, CType must implement a version of foo that resolves
> the conflict:
> Type CType: A, B { override required func foo() {
>     // either
>     A.foo(self)() // uses the A extension default implementation
>     // or
>     B.foo(self)() // uses the B protocol default implementation
>     // or both, one after the other, etc.
> }
> In this rewrite, |foo| is unambiguously referenced for |CType| instance
> members.
>     <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#impact-on-existing-code>Impact
>     on Existing Code
> These changes introduce mandates that do not exist in today's Swift code
> and will require migration. The migrator (and compiler) must detect both
> scenarios: that a member satisfies a protocol requirement and needs
> the |required| keyword, and that a member overrides a default
> implementation (in current Swift, only in extensions) and needs
> the |override|keyword.
> In the degenerate case that protocol extensions provide two distinct
> default implementations of the same member (whether required or not),
> the |override| version should always be preferred. When
> multiple |override| versions exist, the compiler should emit a warning
> about ambiguous resolution.
> Using type currying, e.g. |A.foo(self)| should always resolve using the
> rules enumerated earlier in this proposal, moving from type extensions to
> types to protocol extension to protocols.
>     <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#alternatives-considered>Alternatives
>     Considered
> Not at this time.
>     <https://gist.github.com/erica/fc66e6f6335750d737e5512797e8284a#acknowledgements-and-thanks>Acknowledgements
>     and Thanks
> Thanks, Doug Gregor, Jordan Rose, and Joe Groff
>> On Apr 27, 2016, at 6:07 PM, Douglas Gregor <dgregor at apple.com
>> <mailto:dgregor at apple.com>> wrote:
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> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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