Combinations of list elements

It was written in Emacs Lisp and requires Common Lisp loop facility.

Can this code be improved? Did I hit any anti-patterns along the way?

(defun combos (list)
  (let* ((a (car list))
         (d (cdr list))
         (s (and d (combos d)))
         (v (loop for c in s collect (cons a c))))
    (cons (list a) (append s v))))

I would suggest handling a nil input explicitly. Then you can:

• strip out the and (in s's binding), and just recurse directly

• strip out the cons at the bottom, leaving only the append

Here's the code I tested with (in Scheme, since I have neither Emacs nor a CL implementation installed):

;; Requires SRFI 26
(define (combos lst)
  (if (null? lst) '(())
      (let* ((a (car lst))
             (d (cdr lst))
             (s (combos d))
             (v (map (cut cons a <>) s)))
        (append s v))))

And if you really can't bear to see the empty list as one of the combinations (though it really is), just take the cdr of the resulting list. It's always the first element.

Update: I installed Emacs now, so I was able to port my Scheme version to elisp and test it:

(defun combos (list)
  (if (null list) '(nil)
      (let* ((a (car list))
             (d (cdr list))
             (s (combos d))
             (v (mapcar (lambda (x) (cons a x)) s)))
        (append s v))))

StackExchange Code Review Q#18398, answer score: 5