[h_r]

I don't want to discuss this forever but I have a couple of comments:

Your points about efficiency are a separate topic entirely from the original claim that manually writing out an imperative solution makes it easier to see the algorithmic complexity. That was a surprising claim to me because, in my experience, if I understand what some HOF is doing, reading the code is even easier because there is less of it to wade through (and mental exhaustion doesn't make one easier to read vs the other).

> In every mainstream language I can think of `map foo myCollection` creates an intermediary map

You need to build up the final result, sure. Not an intermediary map but whatever structure (or functor) you're mapping over. That is the whole point of immutable data structures. Also when you're using persistent data structures, which all modern FP languages do, the cost of constructing the result can be far less than what you expect, especially if the result is lazy and you need only some of the results. There is a cost to immutability and if it's unbearable in some situation, fall back to in-place mutation but the semantics of these two approaches are definitely not the same.

> But I've also seen `sum map foo myCollection` so many times

Yeah... that should be a fold (reduce, whatever). :)