[swift-evolution] [Review] SE-0065 A New Model for Collections and Indices

Nate Cook natecook at gmail.com
Tue Apr 12 10:02:16 CDT 2016

> On Apr 12, 2016, at 4:15 AM, Dmitri Gribenko via swift-evolution <swift-evolution at swift.org> wrote:
> On Mon, Apr 11, 2016 at 9:56 PM, Brent Royal-Gordon via
> swift-evolution <swift-evolution at swift.org> wrote:
>> (On the other hand, it might be that I'm conceiving of the purpose of `limitedBy` differently from you—I think of it as a safety measure, but you may be thinking of it specifically as an automatic truncation mechanism.)
> Hi Brent,
> Could you explain what kind of safety do you have in mind?  Swift will
> guarantee memory safety even if you attempt to advance an index past
> endIndex using the non-limiting overload.

One challenge that I've run into is that the `limitedBy` methods throw away one bit of information—namely, did I move as far as I requested or not? Example:

let j = c.index(10, stepsFrom: i, limitedBy: c.endIndex)

There's no way to interpolate the answer to that question efficiently in a non-random-access collection. If `j` is equal to `c.endIndex`, that could be because `c.endIndex` is ten steps after `i` *or* because the limit kicked in, and without checking `c.distance(from: i, to: j)` there's no way to know for sure.

If the `limitedBy` methods returned an optional index, we'd get all the information that the index-moving algorithm finds (let's hear it for the Law of Useful Return!). With that API, we could decide whether to use the returned index or not:

// Use the resulting index no matter what:
let i = c.index(10, stepsFrom: c.startIndex, limitedBy: c.endIndex) ?? c.endIndex
let prefix = c.prefix(upTo: i)

// Only use the result if it wasn't limited:
if let j = c.index(10, stepsFrom: i, limitedBy: c.endIndex) {
    let sub = c[i..<j]      // sub.count == 10
} else {
    // not enough elements...

// "Safe" successor:
if let j = c.index(1, stepsFrom: i, limitedBy: c.endIndex) {
    // ...


> Dmitri
> -- 
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
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