How to sum a list of numbers in Emacs Lisp?

Question

This works:

(+ 1 2 3)
6

This doesn't work:

(+ '(1 2 3))

This works if 'cl-*' is loaded:

(reduce '+ '(1 2 3))
6

If reduce were always available I could write:

(defun sum (L)
  (reduce '+ L))

(sum '(1 2 3))
6

What is the best practice for defining functions such as sum?

Answer 1

This ought to do the trick:

(defun sum-list (list)
  (if list
      (+ (car list) (sum-list (cdr list)))
    0))

[source]

Edit: Here is another good link that explains car and cdr - basically they are functions that allow you to grab the first element of a list and retrieve a new list sans the first item.

Answer 2

(apply '+ '(1 2 3))