> On May 30, 2017, at 14:53, Dave Abrahams <dabrah...@apple.com> wrote: > > > on Tue May 30 2017, Jordan Rose <jordan_rose-AT-apple.com> wrote: > >> My knee-jerk reaction is to say it's too late in Swift 4 for this kind >> of change, but with that out of the way, I'm most concerned about what >> it means to have, say, a UTF-8 index that's not on a UTF-16 boundary. >> >> let str = "言" >> let oneUnitIn = str.utf8.index(after: str.utf8.startIndex) >> let trailingBytes = str.utf8[oneUnitIn...] > > This is not new; it exists today.
Yes, I think that’s valuable. What’s different is that it’s not a String.Index. > >> What can I do with 'oneUnitIn'? > > All the usual stuff; we're not proposing to change what you can do with > it. By changing the type, you have increased the scope of where an index can be used. What happens when I use it in one of the other views and it’s not on a boundary? (I suspect the answer is “it traps” but the proposal should spell that out explicitly.) > >> How do I test to see if it's on a Character boundary or a >> UnicodeScalar boundary? > > as noted, > > Replacing the failable APIs listed [above](#motivation) that detect > whether an index represents a valid position in a given view, and > enhancement that explicitly round index positions to nearby boundaries > in a given view, are left to a later proposal. For now, we do not > propose to remove the existing index conversion APIs. > > That means you can use oneUnitIn.samePosition(in: str) or > oneUnitIn.samePosition(in: str.unicodeScalars) to find out if it's on ta > character or unicode scalar boundary. I’m sorry, I completely missed that. This part of the question is withdrawn. I’m also concerned about putting “UTF-16” in the documentation for encodedOffset. Either it’s a ‘utf16Offset’ or it isn’t; if it’s an opaque value then it should be treated as such. (It’s also a little disturbing that round-tripping through encodedOffset isn’t guaranteed to give you the same index back.) Jordan
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