[swift-evolution] [Proposal] Typed throws

[Matthew Johnson]

> On Feb 22, 2017, at 7:10 AM, Anton Zhilin <antonyzhilin at gmail.com> wrote:
> 
> How about this:
> 
> func bypassRethrows<E: Error>(_ f: () throws(E) -> ()) throws(E) {
>     if let e = (MyError() as Error) as? E {
>         throw e
>     }
> }
> I obviously can’t “test” this right now, but should work?
> 
> 

No because there are many types that conform to `Error` but are not `E` and the signature says you only throw `E`.  You have no way to get an instance of `E` except by catching one thrown by `f` because `Error` does not have any initializers and you don’t have any visibility to anything that provides an `E` in any way except when `f` throws.

I am hoping to have time to write up my analysis of generic rethrowing functions later today.  There are some very interesting tradeoffs we need to make.

> 2017-02-22 3:42 GMT+03:00 Colin Barrett <colin at springsandstruts.com <mailto:colin at springsandstruts.com>>:
> 
> 
> 
> On Sun, Feb 19, 2017 at 2:34 PM Anton Zhilin via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Now that I think about it, generic throws does not exactly cover rethrows.
> Firstly, rethrows has semantic information that function itself does not throw—it would be lost.
> 
> That's not true. Parametric polymorphism guarantees that rethrows and polymorphic throw are the same. 
> 
> For example, you can prove that as a consequence of parametricity that there is only one (pure) function in the of the set of all functions with the type ``forall A. A -> A'' and furthermore that it is the identity function.
> 
> The intuition behind this is that you (meaning the fiction; imagine being a function!) cannot construct your own value of "A" since you don't have any information about what "A" is. The only place to get an "A" is from your argument.
> 
> 

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