I guess my confusion arises because I mistook the operation of generators based on this comment, understanding generators basically to provide a sugary lazy interface to map and filter, and because when writing my_merge_names using generators as above it was compiled away and ran fast. Yet here in Base.keys(::MyStruct) the generator does exactly the same thing yet allocates at runtime and runs slow. It seems like a challenge to figure out when or whether a generator will be unrolled by the compiler.