From the rr site: Remember, you're debugging the recorded trace deterministically; not a live, nondeterministic execution. The replayed execution's address spaces, register contents, syscall data etc are exactly the same in every run.
Elm's debugger (and similar projects like the Pux debugger from PureScript-land) are live systems. They are replaying all events deterministically but you can edit your code, reload it and then replay the exact same series of events or toggle events on/off and see what the resulting state is. rr is super cool and I'm very glad it exists but it's a static analysis tool where the Elm debugger is almost REPL-like in terms of how you can play with things and see the results live.
Elm's debugger (and similar projects like the Pux debugger from PureScript-land) are live systems. They are replaying all events deterministically but you can edit your code, reload it and then replay the exact same series of events or toggle events on/off and see what the resulting state is. rr is super cool and I'm very glad it exists but it's a static analysis tool where the Elm debugger is almost REPL-like in terms of how you can play with things and see the results live.