[swift-evolution] [Proposal] Typed throws

[Martin Waitz]

> Am 18.02.2017 um 17:37 schrieb Matthew Johnson via swift-evolution <swift-evolution at swift.org>:
> 
> Thank you for taking the time to put this proposal together Anton!  I really want to see typed throws make it into Swift 4.  This will be a very nice feature to have.
> 
> I noticed that you included Joe Groff’s idea of replacing `rethrows` by making every function have an error type which is by default `Never` for non-throwing functions and `Error` for throwing functions that do not specify an error type.  
> 
> I want to urge you to consider updating the proposal to take this direction now rather than later.  This is a breaking change which means the longer we wait the harder it is to justify.  In fact, I think incorporating the breaking change could increase the chances of it being accepted for Swift 4.  Without that it is a purely additive change and those are not being given priority in the Swift 4 release.

Seconded.
With typed throwing function parameters, it makes a lot of sense to be able to specify the rethrown type, based on the function given as parameter.

Now some bike-shedding:
I’m not really happy with the `throws(Type)` syntax, as it is too close to function parameters.
Why exactly is `throws Type` ambiguous?
The proposal mentions `Type -> Result` as potential thrown type, but functions cannot conform to `Error`.
Maybe we can instruct the parser to just allow simple type names between `throws` and the arrow `->`.

If that is not possible, we should at least try to find some visual hints to separate Error type from function parameters.

E.g. we could use brackets (think of: we are specialising the `throws`):

	func foo() throws<MyError> { … }

 — Martin
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