<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div><blockquote type="cite" class=""><div class="">On Apr 22, 2016, at 3:33 PM, Douglas Gregor via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:</div><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi all,<div class=""><br class=""></div><div class="">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:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">protocol P { </font></div></div><div class=""><div class=""><font face="Menlo" class=""> func foo(value: Float)</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">extension P {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> func foo(value: Float) { } // default implementation of P.foo(value:)</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">struct S { }</font></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">extension S : P {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> func foo(value: Double) { } // intended-but-incorrect attempt to satisfy the requirement P.foo(value:)</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div></blockquote><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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).</div><div class=""><br class=""></div><div class="">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:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(255, 255, 255); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">t2.swift:12:8: </span><span style="font-variant-ligatures: no-common-ligatures; color: #d53bd3" class="">warning: </span><span style="font-variant-ligatures: no-common-ligatures" class="">instance method 'foo(value:)' nearly matches optional requirement 'foo(value:)' of protocol 'P'</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> func foo(value: Double) { }</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(52, 189, 38); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(255, 255, 255); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">t2.swift:12:8: </span><span style="" class="">note: </span><span style="font-variant-ligatures: no-common-ligatures" class="">candidate has non-matching type '(value: Double) -> ()'</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> func foo(value: Double) { }</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(52, 189, 38); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(255, 255, 255); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">t2.swift:12:8: </span><span style="" class="">note: </span><span style="font-variant-ligatures: no-common-ligatures" class="">move 'foo(value:)' to another extension to silence this warning</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> func foo(value: Double) { }</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(52, 189, 38); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(255, 255, 255); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">t2.swift:12:8: </span><span style="" class="">note: </span><span style="font-variant-ligatures: no-common-ligatures" class="">make 'foo(value:)' private to silence this warning</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> func foo(value: Double) { }</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(52, 189, 38); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> private </span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(255, 255, 255); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">t2.swift:2:8: </span><span style="" class="">note: </span><span style="font-variant-ligatures: no-common-ligatures" class="">requirement 'foo(value:)' declared here</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> func foo(value: Float)</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(52, 189, 38); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class="">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.</div><div class=""><br class=""></div><div class="">Naturally, this handles typos as well, e.g.,</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(255, 255, 255); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">t2.swift:12:8: </span><span style="font-variant-ligatures: no-common-ligatures; color: #d53bd3" class="">warning: </span><span style="font-variant-ligatures: no-common-ligatures" class="">instance method 'foob(value:)' nearly matches optional requirement 'foo(value:)' of protocol 'P'</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> func foob(value: Float) { }</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(52, 189, 38); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(255, 255, 255); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">t2.swift:12:8: </span><span style="" class="">note: </span><span style="font-variant-ligatures: no-common-ligatures" class="">rename to 'foo(value:)' to satisfy this requirement</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> func foob(value: Float) { }</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(52, 189, 38); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^~~~</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> foo</span></div></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class="">Running this on the standard library produces a number of results:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">/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'</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> public mutating func removeLast() -> Element {</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">/Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: note: rename to 'removeFirst()' to satisfy this requirement</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> public mutating func removeLast() -> Element {</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^~~~~~~~~~</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> removeFirst</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">/Users/dgregor/Projects/swift/swift/stdlib/public/core/Arrays.swift.gyb:726:24: note: move 'removeLast()' to another extension to silence this warning</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> public mutating func removeLast() -> Element {</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">/Users/dgregor/Projects/swift/swift/stdlib/public/core/RangeReplaceableCollection.swift:158:17: note: requirement 'removeFirst()' declared here</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> mutating func removeFirst() -> Iterator.Element</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(196, 196, 196); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> ^</span></div></div></div></div></blockquote><div><br class=""></div>Would a word-by-word edit-distance heuristic work better? That is, removeFirst is not a plausible typo for removeLast because First is not a plausible typo for Last.</div><div><br class=""></div><div>Admittedly, you might need to do something else if the word counts don't match.</div><div><br class=""></div><div>John.</div></body></html>