When we evaluated fill-column
to find its value as a variable,
we did not place parentheses around the word. This is because we did
not intend to use it as a function name.
If fill-column
were the first or only element of a list, the
Lisp interpreter would attempt to find the function definition
attached to it. But fill-column
has no function definition.
Try evaluating this:
(fill-column)
You will create a *Backtrace* buffer that says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function fill-column) (fill-column) eval((fill-column) nil) elisp--eval-last-sexp(nil) eval-last-sexp(nil) funcall-interactively(eval-last-sexp nil) call-interactively(eval-last-sexp nil nil) command-execute(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
(Remember, to quit the debugger and make the debugger window go away, type q in the *Backtrace* buffer.)