[swift-dev] What can you change in a non-exhaustive enum?

Greg Titus greg at omnigroup.com
Thu Oct 12 17:20:49 CDT 2017

I like Joe’s idea here, with the extension that the client should have just one of these arrays that contains all the symbols that it knows about at the time it was compiled:

I.e. in the client:

static myKnownOpaqueEnumCases = [MyOpaqueEnum.intCase, MyOpaqueEnum.middleCase, MyOpaqueEnum.stringCase];

switch indexForMyOpaqueEnumTag(&myOpaqueEnum, myKnownOpaqueEnumCases) {
case 0: //…
case 2: //…
default: //...

This optimizes for space in the client, because you have one array instead of one per potentially-different-sets-of-cases switch, but more importantly this allows for an optimization inside indexForMyOpaqueEnumTag(). If the count of the array passed in from the client is equal to the count of all known cases in the callee, then you can immediately return the internal enum tag value instead of performing a binary search.

(If the client expects cases that the callee doesn’t have, the link would have failed for a missing symbol, if the callee has more cases the count won’t match, so if the count is equal the cases in both object files have to be identical.) This returns the common runtime case (when the client is up to date with the callee) to being O(1).

The cost being, if you don’t take that fast path, maybe you have a few more entries in the cases array to binary search over than that particular switch statement needed.

- Greg

> On Oct 12, 2017, at 2:25 PM, Jordan Rose <jordan_rose at apple.com> wrote:
> So, an update! This came up while I was talking to members of the core team, and ChrisL felt very strongly that restricting reordering of enum elements was a no-go, since it would be the only part of the language that worked this way (even if it only mattered for binary frameworks). Ted also rightly pointed out that any such language-level restriction would have to be reviewed by the core team.
> So where does that leave us?
> - The naive implementation is to turn a switch into an if-else chain, unfortunately requiring one function call per case to match.
> - A slightly more complex solution keeps a single 'getMyOpaqueEnumTag' entry point (see original email), but exposes symbols for every tag. The values of the symbols would be kept in alphabetical order, which allows the generated code to do a binary search over the cases they care about. This still means N symbols, but a switch that involves several of them doesn't necessarily have to take linear time.
> - Joe Groff came up with this idea that also involves sorted symbols:
> switch indexForMyOpaqueEnumTag(&myOpaqueEnum, [MyOpaqueEnum.intCase, MyOpaqueEnum.stringCase]) {
> case 0:
>   var payload: Int
>   getMyOpaqueEnumPayload(&myOpaqueEnum, MyOpaqueEnum.intCase, &payload)
>   doSomething(payload)
> case 1:
>   var payload: String
>   getMyOpaqueEnumPayload(&myOpaqueEnum, MyOpaqueEnum.stringCase, &payload)
>   doSomethingElse(payload)
> default:
>   print("unknown case")
> }
> In this example, the actual tag values for 'intCase' and 'stringCase' might not be 0 and 1, but 'indexForMyOpaqueEnumTag' can do the binary search to find out which enum we're asking for. Like the previous solution, you only have to check the cases you care about, but this time the binary search is in the callee, rather than the client.
> - Use availability ordering, plus some kind of explicit annotation for when multiple cases are added within the same release. (In this thread people have suggested dates, ad-hoc sub-version numbers, and plain integer values.)
> I appreciate everyone's creativity with solving the availability ordering problem, but I don't want to tie us to a checker that will tell you if you screw up or a history scraper that will implicitly add the right annotations. (I don't think those are bad ideas, but they're a whole extra pile of work on top of the main implementation!) That leaves explicit annotations of some kind, and that leaves us in a worse place than Objective-C. Which is permitted, but not desirable.
>  At this point in time I think the second option is the best one we have: it's relatively simple to implement, it supports everything Objective-C does, and it doesn't make the availability model even more complicated. It is going to be less efficient than actually knowing the case numbers at compile time, though. Still, as Slava's pointed out, we can still change this after we go ABI-stable; doing something more efficient will just be limited based on deployment target.
> Jordan

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