[swift-evolution] Move placement of 'throws' statement

[Jeremy Pereira]

> On 9 Jan 2017, at 22:11, Anton Zhilin via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I also thought about sum types as implementation of errors, but selecting between Tyler’s and John’s syntaxes, I would pick the latter. Compare:
> 
> let x: (_ a: Int) -> (Error | (_ b: Float) -> (Error | Double
> ))
> 
> 
> let x: (_ a: Int) throws(Error) -> (_ b: Float) throws(Error) -> Double
> 
> 
> 
> let x: (_ a: Int) -> (Error | Double
> )

This is returning an error (or legitimate value) not throwing, yes? Because to me, and most other people probably, throwing is different to returning a legitimate value, not least because the normal control flow is interrupted. Making it look like a return value is confusing and wrong IMO.


> 
> 
> let x: (_ a: Int) throws(Error) -> Double
> Granted, the version with sum types contains less characters and leads to more minimalistic type system. But | on itself does not mean error handling. It’s used for creation of sum types, without error handling semantics. So it’s easier to grasp the meaning of type containing throws than anything else. If Swift had a special symbol as related to errors, as ? relates to optionals, then we could use it there. Unfortunately, there isn’t anything like that.
> 
> What would it look like if the function returns nothing but can throw an error?
> 
> let x: (_ a: Int) -> (Error
>  | ())
> 
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