[swift-evolution] Proposal: Allow Type Annotations on Throws
David Owens II
david at owensd.io
Tue Dec 22 12:09:50 CST 2015
> On Dec 22, 2015, at 9:50 AM, Matthew Johnson <matthew at anandabits.com> wrote:
>
> Unfortunately, I don’t see a way to make it safe. You had to use fatalError in a default case to make it work. An alternative would have been to include an ‘UnknownError’ case in ‘PublishedError’. Neither is not an acceptable solution IMO.
I need the fatalError() because the sample is a working example in Swift today. If we had typed errors, this would simply work:
static func from<T>(@autoclosure fn: () throws InternalError -> T) throws PublishedError -> T {
do {
return try fn()
}
catch InternalError.Internal(let value) {
throw PublishedError.Converted(value: value)
}
}
This states that the only closure accepted is one that throws an InternalError.
> This top level `from` example also brings up a couple of points that I don’t recall being addressed in your proposal. Specifically, the interaction of typed errors with generics and type inferrence.
I call out in the proposal that errors work with generics no differently than other types.
> I still consider this to be an unresolved concern. I would like to have a safe way to perform error conversion during propagation without cluttering up my control flow and seriously degrading readability. This is a problem that can and has been solved in other languages. IMO it is should be considered an essential element of a proposal introducing typed errors.
When Swift has a macro as powerful as Rust, then this is a solved problem as well. However, Swift isn’t there yet.
-David
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