[swift-evolution] [Pitch] Typed throws

[Joe Groff]

> 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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