Recursion with let and cl-labels or just cl-letf?

Question

I found this code on stackoverflow (from Rainer Joswig):

 (defun listFormat ()
   (cl-labels ((place-index (idx l)
        (if (null l)
            nil
            (append (list (first l)) (list idx)
                  (place-index (+ idx 1) (rest l))))))
    (place-index 1 l))))

and I changed it by adding a let form and cl-labels:

(defun listFormat ()
   (let ((l '(a b c d)))
   (cl-labels ((place-index (idx l)
                         (if (null l)
                             nil
                           (append (list (first l)) (list idx)
                                   (place-index (+ idx 1) (rest l))))))
     (place-index 1 l))))

Is this a good practice? In the code I'm working on I need to establish variables with let, then do a recursive function, i.e., I don't want the stuff inside the let to be repeated on each recursion. I'm wondering if there is a way to combine this under cl-letf. I try this, but it's not liking it:

(defun listFormat ()
   (cl-letf ((l '(a b c d))
             (place-index (idx l)
                          (if (null l)
                              nil
                            (append (list (first l)) (list idx)
                                    (place-index (+ idx 1) (rest l))))))
     (place-index 1 l)))

complaining of void function idx. So, is there a way to combine let content that shouldn't be resursed with an inner recursion function?

Answer 1

Recursion as general looping construct is discouraged in Emacs Lisp. Don't. This is not Scheme. Why don't you use one of the following instead?

dash.el:

(defun list-format (list)
  (-flatten (--map-indexed (list it (1+ it-index)) list)))

cl-lib:

(defun list-format (list)
  (cl-mapcan 'list list (number-sequence 1 (length list))))

LOOP:

(defun list-format (list)
  (cl-loop for item in list
           for i from 1
           append (list item i)))

Vanilla:

(defun list-format (list)
  (let ((i 1)
        (list list)
        result)
    (while list
      (push (pop list) result)
      (push i result)
      (setq i (1+ i)))
    (nreverse result)))