version 3 !

En tenant compte des différentes remarques, voici une nouvelle version

(setq t-start nil)
(setq t-end nil)
(setq t-list (list))
(setq t-data-num 9)
(defun tap ()
 "Record a tap"
 (interactive)
 (setq t-end (float-time()))
 (if t-start (push (- t-end t-start) t-list))
 (if (> (length t-list) t-data-num)
 (progn
 (message "BPM %S" (* 60 (/ 1. (/ (float (apply '+ t-list)) (length t-list)))))
 (setq t-list (reverse (cdr (reverse t-list))))
 )
 (message "Result available in %S tap"(- t-data-num (length t-list)))
 )
 (setq t-start t-end)
 )
(global-set-key [C-f2] 'tap)