SE-0270 (Review #2): Add Collection Operations on Noncontiguous Elements

[CTMacUser (Daryle Walker)]

@dabrahams

So, if we are going to solve this problem with one method taking a predicate argument, it should be a binary predicate over elements and indices:

/// Collects the elements satisfying the given predicate over their
/// values and positions just before the element at the specified index,
/// preserving their relative order.
@discardableResult
public mutating func gather(
    at gatheringPlace: Index, 
    where predicate: (Element, Index) -> Bool
) -> Range<Index>

As I said in another post on discontinuous element manipulation, (having only) a predicate-based method is inherently flawed. It must test all elements, even when only a relatively few will be targeted. Imagine touching only 10 elements out of 1000. An sibling function that can specify its targets by exact indices stored in ranges, (sorted) sets, etc. is still needed.