(defun -sort-keyed (key-computer comparator list)
  "Sort LIST by given keys, comparing them using COMPARATOR.
First KEY-COMPUTER is used to compute key value for each list
item.  Then resulting keys are sorted (stably) according to
COMPARATOR.  Finally, a copy of the original LIST is built and
rearranged in the same way.

LIST is not modified by side effects.  KEY-COMPARATOR is called
once for each element of LIST, i.e. with one argument.
COMPARATOR is called with two keys, and should return non-nil if
the first one (and thus its associated LIST element) should sort
before the second."
  (if (cdr list)
      (let (keyed-list)
        (--each list
          (!cons (cons (funcall key-computer it) it) keyed-list))
        ;; Reversing before sorting seems unnecessary, but it ensures
        ;; stability by restoring original order.  We don't use
        ;; '-call' because there's no need to copy argument.
        (--map (cdr it)
               (sort (nreverse keyed-list)
                     (lambda (a b) (funcall comparator (car a) (car b))))))
    (when list
      (list (car list)))))

(let ((comparator (lambda (a b) (let (comparator) (funcall comparator a b)))))
  (sort '(1 2 3 4) comparator))