I should clarify: "iteration" is a bit of a fuzzy concept in Clojure that encompasses things like "for", "reduce", "some", "map", etc., some of which have diffe

I should clarify: "iteration" is a bit of a fuzzy concept in Clojure that encompasses things like "for", "reduce", "some", "map", etc., some of which have differing implementations under the hood. If you're leveraging InternalReduce you're likely to be pretty darn efficient, especially because under the hood it's allowed to multithread with Java fork/join (which you would have to implement much more verbosely if you wanted to use it with an ArrayList). But in a lot of cases, you scan through a collection by taking the first element, then the first of the rest (which is generated by returning a "modified" copy of the original collection), etc. See for instance, the "some" implementation: https://github.com/clojure/clojure/blob/c6756a8bab137128c8119add29a25b0a88509900/src/clj/clojure/core.clj#L2434 https://github.com/clojure/clojure/blob/c6756a8bab137128c811...