Format a string using named placeholders in #emacs-lisp

format-spec+.el

(defun format-spec+ (fmt specs)
  (with-temp-buffer
    (insert fmt)
    (goto-char (point-max))
    (let ((objs '()))
      (while (re-search-backward "%\\([-0-9\\-\\.]*\\){\\([^}]+\\)}\\([a-zA-Z]\\)" nil t)
        (let ((num (match-string 1))
              (var (match-string 2))
              (typ (match-string 3)))
          (replace-match (concat "%" num typ))
          (setq objs (append (list (cdr (assq (intern var) specs))) objs))
          ))
      (apply 'format (buffer-string) objs)
      )))

(princ (format-spec+ ":%-10{key}s %{val}d\n" '((key . "foo") (val . 100))))
(princ (format-spec+ "%10{val}d :%{key}s\n" '((key . "bar") (val . 200))))
(princ (format-spec+ ":%-10{key}s %{val}d, :%-10{key}s %{val}d\n" '((key . "quux") (val . 300))))  
;; =>
;; :foo        100
;;        200 :bar
;; :quux       300, :quux       300