> On Feb 27, 2017, at 5:01 PM, Dave Abrahams <dabrah...@apple.com> wrote: > > > on Mon Feb 27 2017, Matthew Johnson <matthew-AT-anandabits.com> wrote: > >>> On Feb 27, 2017, at 4:20 PM, Dave Abrahams <dabrah...@apple.com> wrote: >>> >>> >>> I'm sorry, I don't see any substantive difference, based on what you've >>> written here, between this feature and const. >> >> Let me give it one more shot and then I’ll drop it. :) >> >> Const is viral because if an API does not declare its arguments const >> it cannot be used by a caller who has a const argument. > > Unless the caller casts away const, thus erasing information that was > previously encoded in the type system.
Yes, of course. > >> It is required in order to make an API as generally useful as >> possible. >> >> Typed errors are not viral in this way because no callers are >> prevented from calling an API regardless of whether it declares error >> types or just throws Error like we have today. > > Unless the caller can recover (which is *very* rare) or it catches and > rethrows one of the errors *it* declares, thus erasing information that > was previously encoded in the type system. I view this as being fundamentally different than casting away const. Casting away const says “I know better than the types”. Converting an error to a different type is extremely different. It much more similar to any other kind of value wrapper a library might create in order to shield its users from being coupled to its implementation details / dependencies. This is not a way *around* the type system in the sense that casting away const is. It is a way of *using* the type system (hopefully) to your advantage. > >> Pressure to declare error types in your signature in order to make >> your function as generally useful as possible does not exist. Each >> function is free to declare error types or not according to the >> contract it wishes to expose. >> >> An argument can be made that community expectations might develop that >> good APIs should declare error types and they could be considered >> viral in this sense because any API that is simply declared `throws` >> is dropping type information. But I think this overstates the case. >> The community appears to be very sensitive to the problems that can >> arise when error types are too concrete, especially across module >> boundaries. I think we can learn to use the tool where it works well >> and to avoid it where it causes problems. >> >>> >>> -- >>> -Dave >> > > -- > -Dave _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
