It depends on your needs. This is neither constrained to Haskell specifically nor functional programming more generally.
If you were building a website for your local Italian restaurant, what would your needs be? Do you need an ElasticSearch cluster to handle customer menu item queries? Do you need a database at all?
In Haskell's case it's best to avoid lists entirely, as they're _usually_ not the optimal data structure. But best for whom? Does the beginner care that a set operation would be more effective than a list operation for their given use-case?
In my day job, I frequently generate records returned from a database along with local changes to be posted later, and say compute the sum of one of the columns. That sounded like something I'd use folding for, with my limited knowledge, so I was just curious at which point (order of magnitude) I'd have to worry about doing it this way or that.
But if lists are not to be used, what should I use for the above? And will the data structure you propose be fine with either fold?
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?"
If you were building a website for your local Italian restaurant, what would your needs be? Do you need an ElasticSearch cluster to handle customer menu item queries? Do you need a database at all?
In Haskell's case it's best to avoid lists entirely, as they're _usually_ not the optimal data structure. But best for whom? Does the beginner care that a set operation would be more effective than a list operation for their given use-case?