Suppose you have written a function definition that is intended to
return the sum of the numbers 1 through a given number. (This is the
triangle function discussed earlier. See Example with Decrementing Counter, for a discussion.)
However, your function definition has a bug. You have mistyped ‘1=’ for ‘1-’. Here is the broken definition:
(defun triangle-bugged (number) "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (setq number (1= number))) ; Error here. total))
If you are reading this in Info, you can evaluate this definition in
the normal fashion. You will see
triangle-bugged appear in the
Now evaluate the
triangle-bugged function with an
argument of 4:
In a recent GNU Emacs, you will create and enter a *Backtrace* buffer that says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function 1=) (1= number) (setq number (1= number)) (while (> number 0) (setq total (+ total number)) (setq number (1= number))) (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) triangle-bugged(4)
eval((triangle-bugged 4)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
(I have reformatted this example slightly; the debugger does not fold long lines. As usual, you can quit the debugger by typing q in the *Backtrace* buffer.)
In practice, for a bug as simple as this, the ‘Lisp error’ line will
tell you what you need to know to correct the definition. The
1= is ‘void’.
However, suppose you are not quite certain what is going on? You can read the complete backtrace.
In this case, you need to run a recent GNU Emacs, which automatically starts the debugger that puts you in the *Backtrace* buffer; or else, you need to start the debugger manually as described below.
Read the *Backtrace* buffer from the bottom up; it tells you
what Emacs did that led to the error. Emacs made an interactive call
to C-x C-e (
eval-last-sexp), which led to the evaluation
triangle-bugged expression. Each line above tells you
what the Lisp interpreter evaluated next.
The third line from the top of the buffer is
(setq number (1= number))
Emacs tried to evaluate this expression; in order to do so, it tried to evaluate the inner expression shown on the second line from the top:
This is where the error occurred; as the top line says:
Debugger entered--Lisp error: (void-function 1=)
You can correct the mistake, re-evaluate the function definition, and then run your test again.