> On Aug 16, 2016, at 7:08 PM, Xiaodi Wu <[email protected]> wrote: > > On Tue, Aug 16, 2016 at 6:59 PM, Karl <[email protected] > <mailto:[email protected]>> wrote: > >> On 17 Aug 2016, at 00:34, Charles Srstka via swift-evolution >> <[email protected] <mailto:[email protected]>> wrote: >> >>> On Aug 16, 2016, at 5:30 PM, Xiaodi Wu <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>> On Tue, Aug 16, 2016 at 5:19 PM, Charles Srstka via swift-evolution >>> <[email protected] <mailto:[email protected]>> wrote: >>>> On Aug 16, 2016, at 5:13 PM, David Sweeris <[email protected] >>>> <mailto:[email protected]>> wrote: >>>> >>>> Any proposal that expands the power of generic programming gets an almost >>>> automatic +1 from me. >>>> >>>> I can't think of any circumstances in which I wouldn't want to use ":==" >>>> instead of ":". Are there any downsides to expanding ":" to mean what >>>> ":==" does? >>>> >>>> Incidentally, I kinda thought things either already worked like this, or >>>> would work like this after generics were "completed", but I can't tell you >>>> why I thought that. >>> >>> Me neither, but the last time I proposed that, people stated that there >>> were some cases where this could not work. No concrete examples were given, >>> but I assume it probably has something to do with associated type >>> wackiness. :== seems like a workable compromise to me. >>> >>> If an existential of a protocol P doesn't conform to itself, what can you >>> do inside the body of a generic function that has a generic constraint >>> specified with `:==`? In other words, what would we know about what's in >>> common between such an existential of a protocol and types that conform to >>> the protocol? >> >> My proposal is that in such cases, using :== would lead to a compiler error. >> >> Charles >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] <mailto:[email protected]> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> <https://lists.swift.org/mailman/listinfo/swift-evolution> > I think the point is that existentials not conforming to their protocols is > the underlying reason this doesn’t work already. > > From the previous thread: >> There are a couple of reasons this is the case. IIRC in some cases it >> actually isn't possible for the existential to conform to the protocol in a >> sound way. And even when it is possible, I believe it has been said that it >> is more difficult to implement than you might think. Hopefully the >> situation will improve in the future but I'm not aware of any specific plans >> at the moment. > > It seems like a reasonably straightforward axiom. I would be interested to > learn more about those edge-cases. > > Hmm, re-reading that makes me worry about this proposal at a practical level. > IIUC, this is something that is pretty much desired across the board, and if > we could have it for `:` it'd be best. > But, it sounds like the reason `:` doesn't work that way isn't that the core > team has a different opinion, but rather that it's very difficult to > implement. And, IIUC, the situation is that no protocol existential currently > conforms to itself, not that some do and some don't. The implementation work > wouldn't be any easier if we called the operator `:==`...
There are various ways to work around the problem using a new operator, though, depending on how much work you wanted to do in implementing it. At the very least you could take the ‘preprocessor’ approach and turn one function using :== into two separate functions, one using == and one using :, with both functions sharing the same body. This would still have the code bloat in the binary, but at least it wouldn’t be bloating up the source. We could then solve some of the binary bloat by spinning off the function body into a third function and having both the two original functions call that. Or maybe we could have the : variant reinterpret the sequence as an == sequence and send it to the == variant. There are multiple ways that this could be handled without forcing protocols to conform to themselves. Charles
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