How is my LISP function an unbound variable?

Question

I define a function in LISP, and it defines correctly. But whenever I try to call it, I get an error saying "The variable FACTORIAL is unbound."

I have tried this on both OS X and Windows 7, on LispWorks and Allegro. The function is -

(defun factorial (x)
   (if (= 1 x) 1
       (* x factorial (- 1 x))))

Any help is appreciated.

Answer 1

In the third line of your code, you're multiplying x times factorial times 1-x.

The first thing to notice is factorial isn't a variable: it's a function. As Common-Lisp is a Lisp-2, factorial isn't bound as a variable at all–it's bound as a function.

You need to be calling the factorial function on one less than x, not x less than one.

So:

(defun factorial (x)
   (if (= 1 x) 1
       (* x (factorial (- x 1)))))

…should do it.

Answer 2

It looks like you're missing a set of parentheses:

(defun factorial (x) 
   (if (= 1 x) 1 
       (* x (factorial (- 1 x))))) 

Without the () around factorial, Lisp thinks you're referring to a variable instead of a function.

Answer 3

To complete the answer of @Isaac Hodes, this show you that there is clearly 2 namspace for function and variable in CL. You wouldn't have the same error if you were in scheme. You can read more here.