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Requiring Proactive Overrides for Default Protocol Implementations

Requiring Proactive Overrides for Default Protocol Implementations

  • Proposal: tbd
  • Author(s): Erica Sadun
  • Status: tbd
  • Review manager: tbd


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 is a strictly syntactic system intended to provide greater safety at compile time, and would not affect existing compiled code bases.

This proposal was discussed on the Swift Evolution list in the [Pitch] Requiring proactive overrides for default protocol implementations. thread


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.

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.

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

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.

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 annotated with protocol Protocol adds that member Must add `required` to type implementation or rename member to avoid conflict
Some member implemented in type not annotated with protocol, even if separate extension conforms to protocol Protocol adds that member No change
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 type, 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

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 // uses the A extension default implementation
    // or // uses the B protocol default implementation
    // or both, one after the other, etc.

In this rewrite, foo is unambiguously referenced for CType instance members.

Overriding Conformance Defaults with Unowned Code

Consider the following situation:

// Code you do not own
Type DType { func foo() }

// Protocol you may or may not own
protocol A { func foo() }
extension A { func foo () { default implementation } }

// And you conform the imported type
extension DType: A {...something...}

How do you specify that DType instances should prefer or use the native third party implementation in favor of the default implementation? In this case, there must be some way to either assign the requirement:

extension DType: A {
    override required func foo =

Or refer to the original implementation:

extension DType: A {
    override required func foo() {

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.

Futrue directions

Using type-style currying for protocols, e.g. should resolve conflicts using the rules enumerated earlier in this proposal. The general flow of precedence would move up from type extensions to types to protocol extension to protocols.

Alternatives Considered

Not at this time.

Acknowledgements and Thanks

Thanks, Doug Gregor, Jordan Rose, and Joe Groff


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@therealbnut therealbnut commented Apr 28, 2016

Hey, how about:


Type AType { func foo() { ... } }


import Moduel1
protocol A { func foo() {...default...} }
extension AType: A {}
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