[rklaehn]

Nothing wrong with using incredibly un-fancy data structures, like arrays, in a FP way. You just need to make sure that you don't do operations that modify individual elements, but work in large batches. So no for loops inserting elements one by one, but map, filter, flatmap...

Most good FP data structures will be tree like, but with a low branching factor and chunky leafs. Now of course that does not save you from lots of memory consumption if you use a language/platform where even primitives are boxed, like the JVM. But that is a completely separate topic.