Debug in Emacs

debug-on-entry

enter the debugger when a function is called

cancel-debug-on-entry

As the name suggests

toggle-debug-on-error

whether to enter the debugger if an error occurs

When you are in the debugger, you have following commands for use:

c	continue, but don’t stop next time.
d	continue and enter the debugger again next time.
e	evaluate expressions (better than M-:).
r	return a value from debugger.
l	list functions that will enter the debugger.
b	flag the current frame so we will enter debugger again when the frame exits.

Complete list of commands

edebug-defun / C-u eval-defun

instrument function to enter stepper when the function runs. To cancel the instrument, simply use eval-defun to reevaluate the function normally.

Some common commands in edebug:

n	step
q	quit
r	re-display last value in echo area
b	set break point, use C-u to set a one-time break point
x	set break condition
c	continue

Complete list of commands