RE: Symbol's function definition is void: mapcan

[Drew Adams]

> I am using the bzr version of emacs, and I am seeing the error messages
> in the subject line in two separate instances. The first time was when I
> tried to get the GNUS email client to work. I wasn't able to and gave
> up. The second time i started playing around with the "eww" web browser
> and whenever I hit any link, this error is displayed. This must be a
> fundamental problem because it is happening with two unrelated
> applications, but I can't find out what it is.
> 
> I speculated that this is because of a dependency to the cl (common
> lisp) library so I have put (require 'cl) into my .emacs.el. This time,
> the error changed to:
> 
> Wrong type argument: listp, nil
> 
> So, what could be happening?

mapcan is a Common Lisp function.  For Emacs, you can either, as you
tried, load library cl.el (actually, cl-extra.el) or load cl-lib.el
and use cl-mapcan instead of mapcan.

If the code you are using is not yours, and it calls mapcan instead
of cl-mapcan, then presumably it should be requiring cl.el (cl-extra.el).

If your loading cl.el leads to a wrong-type-argument error, then it
sounds like the code has a bug.  Try loading the source code for the
function that calls mapcan, then `M-x debug-on-error THE-FUNCTION',
for THE-FUNCTION that calls mapcan.  Step through the debugger to
find out what the problem is.  Or maybe just inspect that function,
to see what it is expecting vs what it receives as arg.

If it seems to be a bug in the code that calls mapcan, consider
reporting it: to the author if not part of Emacs, or
`M-x report-emacs-bug' if part of Emacs.