"I think it's irrelevant which fold you use for best because the answer will only diverge if all the elements in the list are 'equal' according to the metric."
If anyone called 'best on an infinite list, I'd expect it to diverge regardless of how it was implemented. You never know if something's the maximal/minimal element unless you've checked all the others.
(If 'best operated on heaps or something, then you might be able to say "this potentially infinite substructure has no better candidates" because the heap structure makes guarantees like that.)
Anyway, #2 is still irrelevant because it's trivial to fix:
(def best (f seq)
(reduce (fn (a x)
- (if (f a x) a x))
+ (if (f x a) x a))
seq))
Even if we used 'rreduce instead of 'reduce here, this fix would still be necessary (if we consider #2 important to fix at all).
Here's a fix for #1 as well, along with a return to the original variable names:
This still leaves one inconsistency with the original: This version behaves more intuitively in the presence of continuations, because it doesn't use mutation.
"If anyone called 'best on an infinite list, I'd expect it to diverge regardless of how it was implemented."
Oops, I didn't consider that 'best in Arc actually does operate on a data structure that has enough information to tell you whether an infinite substructure can contain a better candidate: If any tail of the list is 'is to one of the tails you've already encountered, its length is infinite but it certainly has no better candidates (since you've already seen them all). All infinite lists in Arc have tails that contain themselves as tails, so it's possible to make an Arc 'best that terminates on any infinite list.
