Again, it depends. A left fold might be fine (and likely will be). Lists might be fine (and likely are). I don't know anything about the size of your data, or a

Again, it depends. A left fold might be fine (and likely will be). Lists might be fine (and likely are). I don't know anything about the size of your data, or about the business you're writing software to support (who knows, maybe you're writing a FPS game, or some real-time aviation software!). At this point (taking into account the extent of your knowledge as you yourself described it), my advice would be to just use basic folding and lists. At some point in the future, if you observe some performance issue in exactly that area, you might remember this and think "hmm, memory consumption is bit excessive here; maybe I'll try a foldr instead", or "finding the intersection of these two big lists is a little slow; maybe I'll convert them both to sets instead?"