17.5 Debugging Exercises

• Install the count-words-example function and then cause it to enter the built-in debugger when you call it. Run the command on a region containing two words. You will need to press d a remarkable number of times. On your system, is a ‘hook’ called after the command finishes? (For information on hooks, see Command Loop Overview in The GNU Emacs Lisp Reference Manual.)
• Copy count-words-example into the *scratch* buffer, instrument the function for Edebug, and walk through its execution. The function does not need to have a bug, although you can introduce one if you wish. If the function lacks a bug, the walk-through completes without problems.
• While running Edebug, type ? to see a list of all the Edebug commands. (The global-edebug-prefix is usually C-x X, i.e., CTRL-x followed by an upper case X; use this prefix for commands made outside of the Edebug debugging buffer.)
• In the Edebug debugging buffer, use the p (edebug-bounce-point) command to see where in the region the count-words-example is working.
• Move point to some spot further down the function and then type the h (edebug-goto-here) command to jump to that location.
• Use the t (edebug-trace-mode) command to cause Edebug to walk through the function on its own; use an upper case T for edebug-Trace-fast-mode.
• Set a breakpoint, then run Edebug in Trace mode until it reaches the stopping point.