[swift-evolution] Dropping Comparable requirement for indices

[Nate Cook]

> On Jul 5, 2016, at 9:39 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I've already raised the question here of whether we should weaken the
> requirement that Collection.Index be Comparable to merely being
> Equatable.  
> 
> Part of the motivation was to support data structures that otherwise
> would be expensive or impossible to implement.  For example, with
> Comparable indices, you can't build a linked list that supports
> restructuring (e.g. insert, delete, splice) in O(1) without invalidating
> indices... not even an unsafe linked list with reference semantics.
> [While I don't think linked lists are terribly good data structures for
> most things, they are still useful in teaching, and it bothered me that
> we were ruling them out.]  Even if you take away the index invalidation
> restriction, you have to store a counter in the index, which is an awkward inefficiency.
> Supporting Comparable indices for a tree data structure requires
> encoding the path from the root to the node.  It's only one or two words
> in practice, but it's another awkward inefficiency.

So far in Swift 3 I've enjoyed the Comparable requirement on indices—it's much easier to perform bounds checks in collection algorithms as a result. However, I haven't tried implementing linked-lists or other non-linear collections since the Comparable requirement was added. I can see that requirement being a significant burden, particularly in a structure like a self-balancing tree, where managing index comparability could grow quite cumbersome.

> Over the weekend, I tried lifting the restriction to see what kind of
> effect it would have on the standard library.
> https://github.com/apple/swift/pull/3325
> 
> Although I *had* been leaning strongly toward making this change, having
> looked at the effects, I am now more ambivalent:
> 
> * A Range<T>, where T is not Comparable, could be constructed with any
>  pair of Equatable T instances.  We'd only detect that the Range may be
>  invalid when T happened to also be Comparable.  However, the fact that
>  you are creating a Range already sort of implies an underlying
>  ordering.

This was exactly how Ranges worked from Swift's public launch up until the collections-move-indices changes landed, so it doesn't seem like a major problem to bring this behavior back. If we can promise better/faster bounds checking for Comparable indices, hopefully most collection authors would opt into that whenever possible.

> * A Collection with non-Comparable indices still imposes an ordering
>  upon the indices.
> 
> * In a Collection with Comparable indices, the ordering implied by <
>  needs to match the ordering of indices in the collection.  Otherwise,
>  there would be no way to form Ranges (for use in slicing, for example)
>  without causing a trap.  This is *weird*!  There's little precedent
>  for imposing stronger semantic requirements on an associated type if
>  it conforms to a particular protocol (Comparable in this case), except
>  where the requirement is part of the protocol.

This sounds like an argument *for* removing the Comparable requirement. Is the issue that it's difficult to guarantee that the < ordering matches the collection, or is the issue that even if you drop the Comparable requirement, there's an implicit comparability anyway?

> Also, Dmitri reminded me that even with a Comparable requirement on
> indices, there is still no problem converting an equatable iteration
> state into an index; you just need to augment it with an integer
> counter.  So it's still trivial to create a Collection from nearly
> everything that is today a multipass Sequence.  It does make indexing
> slightly more expensive, but it's likely we'd optimize that overhead
> away in many critical situations.
> 
> Anyway, the thoughts of the community on this topic would be interesting
> to us.  We need to make a decision about this very soon.

I don't know that I've helped at all, but those are my thoughts. Thanks!
Nate