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
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub
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
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub
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.
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub 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.
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub 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
}
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub 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.
}
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub 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
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub 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.
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub 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 overridekeyword.
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.
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub Considered
Not at this time.
<Requiring Proactive Overrides for Default Protocol Implementations · GitHub and Thanks
Thanks, Doug Gregor, Jordan Rose, and Joe Groff
···
On Apr 27, 2016, at 6:07 PM, Douglas Gregor <dgregor@apple.com> wrote: