> On Sep 29, 2017, at 3:48 PM, Taylor Swift <kelvin1...@gmail.com> wrote: > > > > On Fri, Sep 29, 2017 at 4:13 PM, Andrew Trick <atr...@apple.com > <mailto:atr...@apple.com>> wrote: > > >> On Sep 29, 2017, at 1:23 PM, Taylor Swift <kelvin1...@gmail.com >> <mailto:kelvin1...@gmail.com>> wrote: >> >> Instead of >> >> buf.intialize(at: i, from: source) >> >> We want to force a more obvious idiom: >> >> buf[i..<n].intialize(from: source) >> >> >> The problem with subscript notation is we currently get the n argument from >> the source argument. So what would really have to be written is >> >> buf[i ..< i + source.count].initialize(from: source) >> >> which is a lot more ugly and redundant. One option could be to decouple the >> count parameter from the length of the source buffer, but that opens up the >> whole can of worms in which length do we use? What happens if n - i is less >> than or longer than source.count? If we enforce the precondition that >> source.count == n - i, then this syntax seems horribly redundant. > > Sorry, a better analogy would have been: > > buf[i...].intialize(from: source) > > Whether you specify the slice’s end point depends on whether you want to > completely initialize that slice or whether you’re just filling up as much of > the buffer as you can. It also depends on whether `source` is also a buffer > (of known size) or some arbitrary Sequence. > > Otherwise, point taken. > > -Andy > > After thinking about this more, one-sided ranges might provide just the > expressivity we need. What if: > > buf[offset...].initialize(from: source) // initializes source.count elements > from source starting from offset > > buf[offset ..< endIndex].initialize(from: source) // initializes up to > source.count elements from source starting from offset > > > The one sided one does not give a full initialization guarantee. The two > sided one guarantees the entire segment is initialized.
In every other context, x[i...] is equivalent to x[i..<x.endIndex] I don't think breaking that precedent is a good idea. > For move operations, the one sided one will fully deinitialize the source > buffer while the two sided one will only deinitialize endIndex - offset > elements. — -Dave
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