Collect a list into a list of pairs (an "alist")

collect-alist.lisp

; https://stackoverflow.com/a/40028542/2954547
(defun collect-alist-loop (items)
  (unless (evenp (length items))
    (error "Items must have an even number of pairs!"))
  (loop
    :for (a b)
    :on items
    :by #'cddr
    :collect (cons a b)))

(defun collect-alist-mut (items)
  (unless (evenp (length items))
    (error "Items must have an even number of pairs!"))
  (do ((accum '()))
    ((null items)
     (nreverse accum))
    (let ((key (pop items))
          (val (pop items)))
      (push (cons key val) accum))))

(defun collect-alist-rec (items)
  (unless (evenp (length items))
    (error "Items must have an even number of pairs!"))
  (labels ((recur (&optional (items items) (accum '()))
             (if (null items)
               (nreverse accum)
               (let* ((key (car items))
                      (val (cadr items))
                      (pair (cons key val)))
                 (recur (cddr items) (cons pair accum))))))
    (recur)))

(format t "~A~%" (collect-alist-loop '(a b c d e f)))
(format t "~A~%" (collect-alist-mut '(a b c d e f)))
(format t "~A~%" (collect-alist-rec '(a b c d e f)))

collect-alist.scm

(define (collect-alist-rec items)
  (unless (even? (length items))
    (error "Items must have an even number of pairs!"))
  (let loop ((items items)
             (accum '()))
    (if (null? items)
      (reverse accum)
      (let* ((key (car items))
             (val (cadr items))
             (pair (cons key val)))
        (loop (cddr items) (cons pair accum))))))

(print (collect-alist-rec '(a b c d e f)))