The REPL often is also the default debugger. On error one gets a debug REPL at the point of the error (no unwinding), one debug level deeper. The debug REPL is

The REPL often is also the default debugger. On error one gets a debug REPL at the point of the error (no unwinding), one debug level deeper. The debug REPL is basically a normal REPL, but with some debug features (stack movement, ...) and the ability to call predefined restarts in the code.