In python if I was trying to figure out something complex 20 levels deep, I would stick in an "import pdb; pdb.set_trace()" to get a REPL at that spot. There's

In python if I was trying to figure out something complex 20 levels deep, I would stick in an "import pdb; pdb.set_trace()" to get a REPL at that spot. There's probably an equivalent for Ruby? (Off-topic, the werkzeug debugger even lets you debug browser apps by jumping into a REPL anywhere in the stack trace, in the browser, after an exception is thrown. I think it also has a command that bakes you a pie.)