[swift-evolution] [Review] SE-0074: Implementation of Binary Search functions

[Joe Groff]

> On May 9, 2016, at 6:23 PM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> * Operations that depend on sorted-ness and use binary predicates should
>> not be available on all Collections; they're too easy to misuse,
>> they're hard to name well, and as Nicola Salmoria has noted, they
>> would not make any sense at all for a Set<T>.
>> 
>> * They should be scoped to a kind of collection that bundles
>> the predicate with the elements, e.g.
>> 
>>   let x = Sorted([3, 4, 1, 5, 2], >)          // stores a sorted copy of the array
>>   let y = Sorted(preSorted: 0..<100, <)  // stores a copy of the range
>> 
>> Maybe there should also be protocols for this; CountableRange<T> would
>> already already conform to the immutable version.  We might want a
>> mutable form of the protocol for sorted collections with
>> insertion/removal methods.  This whole area needs more design.
> 
> I agree with both of these statements, but not with your conclusion.
> 
> There are three classes of collections:
> 
> 1) Those which are always sorted, like a SortedSet.
> 2) Those which may or may not be sorted, like an Array.
> 3) Those which are never sorted, like a Set.
> 
> These APIs are useless on a #3, but #2 is still a valuable use case to support. In particular, it's quite common to use sorted `Array`s, and these APIs would help you do that.
> 
> What I might consider doing is tying this to `RangeReplaceableCollection`. That protocol is applied only to types which allow insertion at arbitrary indices, which is a good, though not perfect, proxy for types which might allow you to manually maintain a sort order. `Array`, `ArraySlice`, `ContiguousArray`, and the mutable `String` views would get these methods, while `Set` and `Dictionary` would not.

We could also introduce a new OrderedCollection protocol. (This would also be useful in the future for supporting `case` pattern matching on collections. It makes sense to pattern-match arrays and other ordered collections in order by element, but you'd expect very different semantics pattern-matching an unordered Set.)

-Joe