FSet fully supports nesting of collections. Sets of sets, maps keyed by seqs, … all these combinations Just Work⢠with no further effort on your part. Such types don’t necessarily arise frequently during ordinary application programming, but they do show up in compiler and static analysis work. The state table of an LALR parser generator, for instance, is basically a map keyed by sets of sequences (the LR(0) items); such a type can be manipulated quite directly and elegantly in FSet.
Here’s a hypothetical example of where such nesting might come in handy. Say you’re organizing a huge dinner party with over 200 guests, to be held in a large ballroom with many circular tables. You have some data about each guest — their personality, their profession, their interests, their tastes — and a model that tells you how compatible two people are based on that information. You want to organize them into tables such that the people at each table are all fairly compatible, but also to have enough variety at each table to keep the conversation interesting.
So you have some code that can take a set of people and assign a score, and you are going to do some search to try to make it so all the tables score fairly high (no point in going for the absolute optimum, since your scoring model isn’t perfect anyway). To do that, you’re going to want to keep track of the score for each set that you construct, and the natural way to do that is with a map whose keys are the sets.
Admittedly, this isn’t exactly difficult to do with Common Lisp’s builtin types. You could
keep the sets as lists, each sorted in a canonical order so there’s only one possible list
representation for each set, and use a CL hash table with equal as its test function. But
with FSet, you don’t even have to think about it; just build the sets and put them in the map. FSet
handles the rest.