;;; this would go into slime.el
(defun slime-eval-custom-expand (expander exp-str package buffer-name buffer-mode printer)
  (lexical-let ((package package)
                (buffer-name buffer-name)
                (buffer-mode buffer-mode)
                (printer printer))
    (slime-eval-async
     (list 'swank:eval-and-grab-output (format "(%s %s)" expander exp-str))
     (lambda (expansion)
       (slime-with-popup-buffer (buffer-name package t)
         (funcall buffer-mode)
         (setq buffer-read-only nil)
         (erase-buffer)
         (insert (funcall printer (second expansion)))
         (setq buffer-read-only t)
         (font-lock-fontify-buffer)))
     package)))

(defun* slime-add-custom-expander (key expander buffer-name &optional (buffer-mode 'slime-mode) (printer #'identity))
  (define-key slime-mode-map (vector ?\C-c key)
    (lexical-let ((expander expander)
		  (buffer-name buffer-name)
		  (buffer-mode buffer-mode)
		  (printer printer))
      (lambda (&rest _)
	(interactive "P")
	(slime-eval-custom-expand expander
				  (first (slime-sexp-at-point-for-macroexpansion))
				  (slime-current-package)
				  buffer-name
				  buffer-mode
				  printer)))))

;;; This stuff would go into .emacs
(slime-add-custom-expander ?j 'ps:ps "*ParenScript generated Javascript*" 'js-mode #'read)

(provide 'js-expander)