Offset Indexing and Slicing

[Michael_Ilseman]

Specifically:

@John_McCall

That said, it's important that abstract indices compose nicely with concrete indices. Programmers should feel like they can reliably use abstract indices anywhere they could use concrete indices. That doesn't appear to be true of the current proposal: in particular, programmers can slice between two abstract indices or between two concrete indices, but not between a mix of either. The Core Team feels it's important to avoid this kind of inconsistent-feeling experience.

Along the same vein, abstract indices can only be anchored relative to the start and end of a collection. It is frequently useful to be able to anchor relative to a concrete index, e.g. to slice from an index returned by firstIndex(where:) to the fifth element past that. This pattern is not addressed by the current proposal, and it's not clear whether the proposal can be extended this way in the future.

If necessary, the Core Team is willing to consider new language support for slicing subscripts, either through new syntax or through new interpretations of existing syntax.

In effect, it would be really nice to be able to mix an OffsetBound with an Index in some kind of range expression. But doing so may require re-thinking everything from Swift's subscript syntax to new hierarchies of range expressions. This sorta blew open the investigation area to a degree where I was unable to make progress at that time, and it could be a while before I can re-visit this.