[swift-evolution] [Pitch] Typed throws

Chris Lattner clattner at nondot.org
Mon Feb 20 17:04:51 CST 2017

> On Feb 20, 2017, at 9:57 AM, John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> On Feb 19, 2017, at 3:04 PM, Anton Zhilin via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> It’s expected that if you need resilience, then you will throw an “open” enum. Essentially, we pass resilience of typed throws on to those who will hopefully establish resilience of enums.
>> If you prefer separate error types, then declare a base protocol for all your error types and throw a protocol existential. You won’t even need default case in switches, if closed protocols make it into the language.
>> I don’t like any solution that is based on comments. I think that compiler should always ignore comments.
> I agree.  And in general, this sort of thing is exactly my core concern about adding typed throws to the language: I am completely certain that many programmers will add typed throws annotations because they're programmers and thus, well, probably a little obsessive/compulsive, and they're trying to precisely document the behavior of their function without necessarily thinking about the usefulness of that information for their clients and (if they're writing a library; and really you should almost always be writing code as if you're writing a library) whether they're actually willing to commit to that behavior in their interface.  For those programmers, typed throws is just going to box them in and force them into anti-patterns in the long run.

As you know, I still think that adding typed throws is the right thing to do.  I understand your concern about “the feature could be misused” but the same thing is true about many other language features.

One thing you didn’t mention is that boxing thrown values in an existential requires allocation in the general case.  This may be unacceptable for some classes of Swift application (in the embedded / deep systems space) or simply undesirable because of the performance implication.


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