jdb: https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jdb.html
javap: https://docs.oracle.com/javase/8/docs/technotes/tools/unix/javap.html
# compile
scalac -g:vars [source]
# run and it should ouput a port
env JAVA_OPTS="-agentlib:jdwp=transport=dt_socket,server=y,suspend=n" scala [class]
# attach to the port
jdb -attach [port]
# disassembles class file to assist set breakpoints
javap [class]