> On Feb 10, 2017, at 5:49 PM, Jonathan Hull via swift-evolution
> <[email protected]> wrote:
>
> An easier to implement, but slightly less useful approach, would be to have
> methods which take an array of indexes along with the proposed change, and
> then it adjusts the indexes (or replaces them with nil if they are invalid)
> as it makes the update. For example:
>
> func append(_ element:Element, adjusting: [Index]) -> [Index?]
> func appending(_ element:Element, adjusting: [Index]) -> (Self,
> [Index?])
This is a very interesting idea. A couple observations:
1. The problem of adjusting indices is not just a String one. It also applies
to Array, among other things.
2. This logic could be encapsulated and reused in a separate type. For
instance, imagine:
let myStringProxy = IndexTracking(collection: myString, trackedIndices:
[someIndex, otherIndex])
myStringProxy.insert("foo", at: otherIndex)
(someIndex, otherIndex) = (stringProxy.trackedIndices[0],
stringProxy.trackedIndices[1])
Or, with a helper method:
myString.withTracked(&someIndex) { myStringProxy in
myStringProxy.insert("foo", at: otherIndex)
}
3. An obstacle to doing this correctly is that a collection's index
invalidation behavior is not expressed in the type system. If there were a
protocol like:
protocol RangeReplaceableWithEarlierIndexesStableCollection:
RangeReplaceableCollection {}
That would help us here.
--
Brent Royal-Gordon
Architechies
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