What was the motivation behind Julia's staging (specialization, macro, generated function, etc.) systems design?

▶ Background (what I learned trying to make a language even though it didn't work out due to unforeseen difficulty my naive mind didn't see coming.) So, the key to performance in a dynamic language is staging. Runtime code generation, dynamic compilation, and so on are normally slow unless you stage them carefully so you don’t end up generating/interpreting code in the hot loop. Julia does make use of a lot of these. However, Julia has AT LEAST 3 staging mechanisms, and they don’t always interac...