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> On Jun 5, 2016, at 5:02 AM, Patrick Pijnappel via swift-evolution > <swift-evolution@swift.org> wrote: > > This has actually been proposed before, see SE-0073: > https://github.com/apple/swift-evolution/blob/master/proposals/0073-noescape-once.md Actually that proposal was for noescape closures and this suggestion is for escaping closures. I don't think the compiler can verify this for noescape closures. If it is possible it would be far more complicated. > > >> On Sun, Jun 5, 2016 at 11:37 AM, Charles Srstka via swift-evolution >> <swift-evolution@swift.org> wrote: >> MOTIVATION: >> >> As per the current situation, there is a pitfall when writing asynchronous >> APIs that does not occur when writing synchronous APIs. Consider the >> following synchronous API: >> >> func doSomething() -> SomeEnum { >> if aCondition { >> if bCondition { >> return .Foo >> } else { >> return .Bar >> } >> } else { >> if cCondition { >> return .Baz >> } >> } >> } >> >> The compiler will give an error here, since if both aCondition and >> cCondition are false, the function will not return anything. >> >> However, consider the equivalent async API: >> >> func doSomething(completionHandler: (SomeEnum) -> ()) { >> dispatch_async(someQueue) { >> if aCondition { >> if bCondition { >> completionHandler(.Foo) >> } else { >> completionHandler(.Bar) >> } >> } else { >> if cCondition { >> completionHandler(.Baz) >> } >> } >> } >> } >> >> Whoops, now the function can return without ever firing its completion >> handler, and the problem might not be discovered until runtime (and, >> depending on the complexity of the function, may be hard to find). >> >> PROPOSED SOLUTION: >> >> Add a @required attribute that can be applied to closure arguments. This >> attribute simply states that the given closure will always be eventually >> called, and the compiler can enforce this. >> >> DETAILED DESIGN: >> >> - The @required attribute states in our API contract that a given closure >> *must* be called at some point after the function is called. >> >> - Standard API calls like dispatch_async that contractually promise to >> execute a closure or block get @required added to their signatures. >> >> - When the compiler sees a @required closure in a function declaration, it >> checks to make sure that every execution path either calls the closure at >> some point, or sends a @required closure to another API that eventually ends >> up calling the closure. >> >> - If there’s a way for a @required closure not to be called, the compiler >> emits an error letting the developer know about the bug in his/her code. >> >> IMPACT ON EXISTING CODE: >> >> None. This is purely additive. >> >> ALTERNATIVES CONSIDERED: >> >> I got nothin’. >> >> Charles >> >> _______________________________________________ >> swift-evolution mailing list >> swift-evolution@swift.org >> https://lists.swift.org/mailman/listinfo/swift-evolution > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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