Question

(defun recursive-sum (L)
  (if (null L)
  0
  (+ (first L) (recursive-sum L))))

What’s wrong with my code piece?

I got this error message:

*** - SYSTEM::READ-EVAL-PRINT: variable SUM.LISP has no value
The following restarts are available:
USE-VALUE      :R1      Input a value to be used instead of SUM.LISP.
STORE-VALUE    :R2      Input a new value for SUM.LISP.
ABORT          :R3      Abort debug loop
ABORT          :R4      Abort debug loop
ABORT          :R5      Abort main loop
Break 3 [5]> 

Answer 1

Considering the error-code you provided, it seems like you are using a variable called SUM.LISP which has no actual value/is not defined at a certain point. Search that variable name in your code and make sure it is always properly defined and within scope. For more information, more source code is needed.

---

As the function you quoted will most likely not work properly I implemented a fixed version:

(defun recursive-sum (L)
   (if L
      (+ (car L) (recursive-sum (cdr L))) 
      0))

where, as mentioned in the comments, you decrease the given list each call by one element.