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

[Joe Groff]

> On May 13, 2016, at 7:30 AM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
> 
> on Mon May 09 2016, Joe Groff <jgroff-AT-apple.com> wrote:
> 
>>> 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.)
> 
> What do you mean by “Ordered” here?  Please note that when Cocoa uses
> “Ordered” it means something very different from “Sorted.”  I don't find
> the Cocoa usage intuitive myself, but it might be best to avoid that
> term to avoid confusion.

By "ordered", I only mean "ordering is significant to the value of the collection", so Array is ordered but Set is not.

-Joe