Pop first element in a list using a macro

Question

I have the following code

(defmacro popm(l)
`(prog1 (car ,l)
    (setf ,l (cdr ,l))))

which is supposed to pop the first element out of the list. However, I don't know how to call the macro. I tried using (popm (2 3 4)) but I get the error "EVAL: 2 is not a function name; try using a symbol instead". Can anyone point me in the right direction?

Answer 1

The correct way to debug macros is to use macroexpand-1:

(macroexpand-1 '(popm (1 2 3)))
(PROG1 (CAR (1 2 3)) (SETF (1 2 3) (CDR (1 2 3))))

Now it should be clear why your macro does not work: it cannot be used with constant objects.

It does work with variables though:

(defparameter a '(1 2 3))
(popm a)
==> 1
a
==> (2 3)