On Fri, Feb 17, 2017 at 4:46 PM Joe Groff via swift-evolution < [email protected]> wrote:
> On Feb 17, 2017, at 11:03 AM, Adrian Zubarev via swift-evolution < > [email protected]> wrote: > > I suggest we need to find a way to shorten the list of the possible error > types with a the help of typeallias > > extension MyError1: Error { ... } > extension MyError2: Error { ... } > extension MyError3: Error { ... } > > typealias MyErrors = MyError1 | MyError2 | MyError3 > > func foo() throws(MyErrors) -> MyResult > func bar<T : Error>(_: () throws(T) -> Void) rethrows(MyErrors, T) -> MyResult > > Do you actually need that? Experience in other languages like Rust and > Haskell that use Result-based error propagation suggests that a single > error type is adequate, and beneficial in many ways. If nothing else, you > could `Either` your way to multiple errors if you really needed to. > > IMO, if we accept a single error type per function, there could be a > simpler model for this. We could say that the `throws` type is a generic > parameter of all function types, and it defaults to the uninhabited `Never` > type for functions that don't throw. > > () -> () == () throws Never -> () > () throws -> () == () throws Error -> () > > In this model, you'd get many benefits: > > - `rethrows` could become first-class, reducing down to just polymorphic > `throws`: > > func foo(_: () throws -> ()) rethrows // Swift 3 > func foo<T: Error>(_: () throws T -> ()) throws T // Swift X > func foo<T: Error>(_: () throws T -> ()) throws Either<MyErrors, T> > > - Protocols could abstract over error handling; for instance, we could > support throwing sequences: > > protocol IteratorProtocol { > associatedtype Element > associatedtype Error: Swift.Error = Never > > mutating func next() throws Error -> Element? > } > > I really dig this proposal, Joe. Matches my experience from other languages as well. -Colin
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