[swift-evolution] [Pitch] Typed throws

[Matthew Johnson]

Sent from my iPad

> On Feb 19, 2017, at 11:34 AM, Brandon Knope via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I really like this. Seems much more elegant and simple this way 

+1

> 
>> 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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