@ilya.chernikov would be good to have a hook in the REPL's after classes are generated before anything is done with them so verification can happen. I can do this in a clone of the REPL's, but then I get back to internal API's. So post compile immediately before classes go anywhere, and another hook pre-eval before any classes get added to a classloader or loaded. I'll want access to the classes as bytes in both cases, in the second assuming it is the Repl classloader I think they are loaded just before eval yes? If not, then being able to fetch bytes from that classloader if not handed off in the callback. want a PR for this?