[swift-evolution] [Review] SE-0074: Implementation of Binary Search functions
brent at architechies.com
Mon May 9 20:23:19 CDT 2016
> * 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.
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