[swift-evolution] [Pitch] Typed throws

[Brandon Knope]

I really like this. Seems much more elegant and simple this way 

> On Feb 17, 2017, at 4:45 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?
> }
> 
> Separate of the type system model, the type *checking* model also deserves thorough consideration. Propagating the effects of possibly multiple error types propagating within a `do` block is much trickier than doing so as a single "throws" or not bit, especially if you want to be able to use type context in `catch` patterns or to implicitly propagate a narrower `throws` type out of the enclosing function.
> 
> -Joe
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