Debugging Lisp: fix and resume a program from any point in stack 🎥

You are doing god’s work on a time-intensive computation, but your final step errors out :S Are you doomed to start everything from zero, and wait again for this long process? No! Find out.

I show this with Emacs and Slime, then with the Lem editor (ready-to-use for CL, works with many more languages thanks to its LSP client).

(This video is so cool :D Sound on)

We use the built-in Common Lisp interactive debugger that lets us restart one precise frame from the call stack. Once you get the debugger:

press “v” on a frame to go to the corresponding source file
fix the buggy function, compile it again (C-c C-c in Slime)
come back to the debugger, place the point on the frame you want to try again, press “r” (sldb-restart-frame, see your editor’s menu)
see everything succeed. You did not restart everything from zero.
if you don’t see some function calls and variables in the debugger, compile your code with max debug settings (C-u C-c C-c in Slime).

For more:

https://lispcookbook.github.io/cl-cookbook/debugging.html - see how to break and step, how to trace function calls, etc.