[swift-evolution] [Pitch] Typed throws

Matthew Johnson matthew at anandabits.com
Tue Feb 28 12:51:23 CST 2017

> On Feb 28, 2017, at 12:27 PM, Tino Heth via swift-evolution <swift-evolution at swift.org> wrote:
>> I disagree with that, it works if you only have a single function parameter type that throws an error, but if there are more than one inferring the type won’t be possible anymore: func foo(_: () throws(T) -> Void, _: () throws(S) -> Void) rethrows(S) (here, we’re assuming that T is handled inside foo).
> Well, isn't it actually incorrect that "rethrows" is a property of the function, and not of its parameters?
> Just imagine this
> func catches(catchMe: () throws -> Void, throwMe: () throws -> Void) rethrows {
> 	try? catchMe()
> 	try throwMe()
> 	print("Nothing thrown")
> }
> func doesThrow() throws {
> 	throw NSError()
> }
> func doesntThrow() {
> 	print("I'm a nice function")
> }
> try catches(catchMe: doesThrow, throwMe: doesntThrow)
> The last call can't throw, but the compiler still needs the "try" on it.
> I guess in real-world code, this isn't an issue — but still, it's wrong… so if we want a really sophisticated model for error handling, imho this should be addressed.
> Maybe we should also take into account that a higher-order function might catch some errors, and only rethrow for special cases, and this adds complexity as well?
> In this case, each function-parameter would require a full list a errors that are filtered out, or even a translation-table… I guess most developers would agree that this is overkill, and that keeping things simple has merit as well.

This is something I have discussed extensively in the thread titled “Analysis of the design of typed throws”.  I am interested if you have any comments on that discussion.

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