[Pitch N+1] Typed Throws

[michelf (Michel Fortin)]

@dmt

The lack of knowledge "How to make an instance of E" is the key here. The only possible source of it is the failure of the call to body.

Not really: you could store a E somewhere else and throw that stored E instead. For instance:

var storedError: (any Error)?

func f<E: Error>(body: () throws(E) -> ()) throws(E) {
    if let e = storedError as? E {
       throw e // and never call body()
    }
    try body()
}

And it's a silly example written like that, but as a cache of some Result<Any, Error> that you cast back to the original return type and error type it makes a lot of sense.