[swift-evolution] [Pitch] Typed throws

[Colin Barrett]

On Fri, Feb 17, 2017 at 4:46 PM Joe Groff via swift-evolution <
swift-evolution at swift.org> wrote:

> On Feb 17, 2017, at 11:03 AM, Adrian Zubarev via swift-evolution <
> swift-evolution at swift.org> 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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