Languages with multidimensional arrays (APL, BQN, J, but not K) have trouble with dicts because an index into an array is a list of numbers, and an index into a dict is an arbitrary value. Many primitives, and especially selection, are designed around lists of numbers and don't transfer to dicts. In K, where the index into a list is one number, there's still a requirement that the keys in a dict all have the same level of nesting, but this isn't bad in practice. BQN will eventually have hashmaps implemented as in a more mainstream/conventional way, as objects. There's a model at https://github.com/mlochbaum/bqn-libs/blob/master/hashmap.bq... .

I don't think studying the compiler is a very good way to learn BQN, but I would like to write up parts of it (limited by time and motivation of course). I did some chat sessions on this sort of compilation during early development; see the links at the bottom of https://mlochbaum.github.io/BQN/implementation/ .