[swift-evolution] [swift-dev] RFC: "Near-miss" checking for defaulted protocol requirements

Douglas Gregor dgregor at apple.com
Mon Apr 25 15:50:20 CDT 2016 

> On Apr 25, 2016, at 1:13 PM, Erica Sadun <erica at ericasadun.com> wrote:
> 
> With apologies, I do not see near miss checks as a pressing need:
> 
> * Quick Help lists of required members (including associated types and inherited members) would be far more valuable to me than "near miss" detection.

Understood.

> 
> * I'd like the compiler to check for unsatisfied conformances and emit a list of required conformances including whether they are found and their origin. This would tangentially address the "near miss" case but only for unsatisfied conformances. It would not help when conformances are satisfied.

Yes, the experience here is terrible. What the compiler *should* do is give a serious of errors, each of which corresponds to a single requirement and has a Fix-It to stub out the appropriate method/property/etc. 

> * I believe "near miss" is less important than "intentional override", requiring a signature of intent as in inheritance. 

> Here's what you said previously on the topic of default implementation overrides that brought me here:
>> 
>> This is a commonly-requested feature that I don’t think we need. The TL;DR version is that I feel like specifying the conformance explicitly (my type Foo conforms to protocol P) already expresses intent, and the compiler should help with the rest. I’ve recently been working on providing better warnings for cases where one has tried to implement an optional requirement for a protocol (but got the declaration wrong), and I think we can turn it on for cases where one got a default implementation instead:
>> 
>> 	http://thread.gmane.org/gmane.comp.lang.swift.devel/1799 <http://thread.gmane.org/gmane.comp.lang.swift.devel/1799>
My opinion is that the signature of intent is the explicitly-stated conformance to the protocol, but this is not universally agreed upon (even within the core team). It suffices to say that a proposal to add some kind of “implements” keyword won’t come from me :)

	- Doug

> 
> -- Erica
> 
> 
>> On Apr 22, 2016, at 4:33 PM, Douglas Gregor via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>> 
>> Hi all,
>> 
>> A common complaint with protocol conformance checking is that it’s easy to make a little mistake when trying to implement a protocol requirement that has a default implementation. Here is a silly example:
>> 
>> protocol P { 
>>   func foo(value: Float)
>> }
>> 
>> extension P {
>>   func foo(value: Float) { } // default implementation of P.foo(value:)
>> }
>> 
>> struct S { }
>> 
>> extension S : P {
>>   func foo(value: Double) { } // intended-but-incorrect attempt to satisfy the requirement P.foo(value:)
>> }
>> 
>> S satisfies the requirement for P.foo(value:) using the implementation from the protocol extension, although it certainly looks like the user intended to provide a different implementation.
>> 
>> This kind of problem has prompted repeated calls for some kind of “implements” keyword to indicate that one is intending to implement a protocol requirement. I, personally, *really* don’t want yet another decorator keyword to indicate the intent here, because I believe the user has already indicated intent by stating conformance to the protocol P. I’ve recently committed a number of changes that provide “near-miss” checking for optional requirements of @objc protocols (which have the same problem).
>> 
>> It might be worth enabling this functionality for cases like the above as well. The Swift compiler patch to do so is attached, and will produce the following warning for the code above:
>> 
>> t2.swift:12:8: warning: instance method 'foo(value:)' nearly matches optional requirement 'foo(value:)' of protocol 'P'
>>   func foo(value: Double) { }
>>        ^
>> t2.swift:12:8: note: candidate has non-matching type '(value: Double) -> ()'
>>   func foo(value: Double) { }
>>        ^
>> t2.swift:12:8: note: move 'foo(value:)' to another extension to silence this warning
>>   func foo(value: Double) { }
>>        ^
>> t2.swift:12:8: note: make 'foo(value:)' private to silence this warning
>>   func foo(value: Double) { }
>>        ^
>>   private 
>> t2.swift:2:8: note: requirement 'foo(value:)' declared here
>>   func foo(value: Float)
>>        ^
>> 
>> It’s unfortunate that it’s a lot of notes. The first says what is wrong with the candidate (and there is much we could do to improve the precision of this diagnostic!), while the next two are mitigation strategies: move it to another extension (which implies that it’s not part of the conformance to P) or explicitly mark it as having less visibility than the conformance (in this case, private), which feels like a good way to state “I intend this to be a helper”. This might nudge Swift developers toward a style where they write one conformance per extension, but I don’t think that’s necessarily a bad thing: it’s a fine way to organize code.
>> 
>> Naturally, this handles typos as well, e.g.,
>> 
>> t2.swift:12:8: warning: instance method 'foob(value:)' nearly matches optional requirement 'foo(value:)' of protocol 'P'
>>   func foob(value: Float) { }
>>        ^
>> t2.swift:12:8: note: rename to 'foo(value:)' to satisfy this requirement
>>   func foob(value: Float) { }
>>        ^~~~
>>        foo
>> 
>> Running this on the standard library produces a number of results:
>> 
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: warning: instance method 'removeLast()' nearly matches optional requirement 'removeFirst()' of protocol 'RangeReplaceableCollection'
>>   public mutating func removeLast() -> Element {
>>                        ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: note: rename to 'removeFirst()' to satisfy this requirement
>>   public mutating func removeLast() -> Element {
>>                        ^~~~~~~~~~
>>                        removeFirst
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: note: move 'removeLast()' to another extension to silence this warning
>>   public mutating func removeLast() -> Element {
>>                        ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/RangeReplaceableCollection.swift:158:17: note: requirement 'removeFirst()' declared here
>>   mutating func removeFirst() -> Iterator.Element
>>                 ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: warning: instance method 'removeLast()' nearly matches optional requirement 'removeFirst()' of protocol 'RangeReplaceableCollection'
>>   public mutating func removeLast() -> Element {
>>                        ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: note: rename to 'removeFirst()' to satisfy this requirement
>>   public mutating func removeLast() -> Element {
>>                        ^~~~~~~~~~
>>                        removeFirst
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: note: move 'removeLast()' to another extension to silence this warning
>>   public mutating func removeLast() -> Element {
>>                        ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/RangeReplaceableCollection.swift:158:17: note: requirement 'removeFirst()' declared here
>>   mutating func removeFirst() -> Iterator.Element
>>                 ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: warning: instance method 'removeLast()' nearly matches optional requirement 'removeFirst()' of protocol 'RangeReplaceableCollection'
>>   public mutating func removeLast() -> Element {
>>                        ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: note: rename to 'removeFirst()' to satisfy this requirement
>>   public mutating func removeLast() -> Element {
>>                        ^~~~~~~~~~
>>                        removeFirst
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: note: move 'removeLast()' to another extension to silence this warning
>>   public mutating func removeLast() -> Element {
>>                        ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/RangeReplaceableCollection.swift:158:17: note: requirement 'removeFirst()' declared here
>>   mutating func removeFirst() -> Iterator.Element
>>                 ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/HashedCollections.swift.gyb:1077:10: warning: subscript 'subscript' nearly matches optional requirement 'subscript' of protocol 'Collection'
>>   public subscript(key: Key) -> Value? {
>>          ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/HashedCollections.swift.gyb:1077:10: note: candidate has non-matching type 'Key -> Value?' [with Iterator = DictionaryIterator<Key, Value>, SubSequence = Slice<Dictionary<Key, Value>>]
>>   public subscript(key: Key) -> Value? {
>>          ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/HashedCollections.swift.gyb:1077:10: note: move 'subscript' to an extension to silence this warning
>>   public subscript(key: Key) -> Value? {
>>          ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Collection.swift:147:3: note: requirement 'subscript' declared here
>>   subscript(bounds: Range<Index>) -> SubSequence { get }
>>   ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Range.swift:116:10: warning: subscript 'subscript' nearly matches optional requirement 'subscript' of protocol 'Collection'
>>   public subscript(_: Element._DisabledRangeIndex) -> Element {
>>          ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Range.swift:116:10: note: candidate has non-matching type 'Element._DisabledRangeIndex -> Element' [with Iterator = RangeIterator<Element>, SubSequence = Slice<Range<Element>>]
>>   public subscript(_: Element._DisabledRangeIndex) -> Element {
>>          ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Range.swift:116:10: note: move 'subscript' to an extension to silence this warning
>>   public subscript(_: Element._DisabledRangeIndex) -> Element {
>>          ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Collection.swift:147:3: note: requirement 'subscript' declared here
>>   subscript(bounds: Range<Index>) -> SubSequence { get }
>>   ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/ExistentialCollection.swift.gyb:508:15: warning: instance method '_distance(to:)' nearly matches optional requirement 'distance(to:)' of protocol 'ForwardIndex'
>>   public func _distance(to other: AnyForwardIndex) -> AnyForwardIndex.Distance {
>>               ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/ExistentialCollection.swift.gyb:508:15: note: rename to 'distance(to:)' to satisfy this requirement [with _DisabledRangeIndex = _DisabledRangeIndex_]
>>   public func _distance(to other: AnyForwardIndex) -> AnyForwardIndex.Distance {
>>               ^~~~~~~~~
>>               distance
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/ExistentialCollection.swift.gyb:508:15: note: move '_distance(to:)' to an extension to silence this warning
>>   public func _distance(to other: AnyForwardIndex) -> AnyForwardIndex.Distance {
>>               ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Index.swift:180:8: note: requirement 'distance(to:)' declared here
>>   func distance(to end: Self) -> Distance
>>        ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/ExistentialCollection.swift.gyb:508:15: warning: instance method '_distance(to:)' nearly matches optional requirement 'distance(to:)' of protocol 'ForwardIndex'
>>   public func _distance(to other: AnyBidirectionalIndex) -> AnyBidirectionalIndex.Distance {
>>               ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/ExistentialCollection.swift.gyb:508:15: note: rename to 'distance(to:)' to satisfy this requirement [with _DisabledRangeIndex = _DisabledRangeIndex_]
>>   public func _distance(to other: AnyBidirectionalIndex) -> AnyBidirectionalIndex.Distance {
>>               ^~~~~~~~~
>>               distance
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/ExistentialCollection.swift.gyb:508:15: note: move '_distance(to:)' to an extension to silence this warning
>>   public func _distance(to other: AnyBidirectionalIndex) -> AnyBidirectionalIndex.Distance {
>>               ^
>> /Users/dgregor/Projects/swift/swift/stdlib/public/core/Index.swift:180:8: note: requirement 'distance(to:)' declared here
>>   func distance(to end: Self) -> Distance
>>        ^
>> 
>> 
>> It’s somewhat frustrating that these are *all* false positives. However, they seem like “reasonable” false positives, in the sense that they’re close enough to the requirement to be justifiable, and the suggested recovery strategies look acceptable.
>> 
>> Thoughts? Should we turn this on?
>> 
>> 	- Doug
>> 
>> <default-impl-near-miss.patch>
>> _______________________________________________
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>> swift-dev at swift.org <mailto:swift-dev at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-dev
> 

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