┌─[phoe][phoetower][~]
└─▪ cat /tmp/b.lisp 
(in-package #:cl-user)
 
;; Read a symbol, then a lisp term.  Sometimes the symbol is NIL, instead of FOO
#.(set-dispatch-macro-character #\# #\{
                                (lambda (stream char n)
                                  (declare (ignore char n))
 
                                  (let ((langname (read stream t nil t)))
                                    (format t "Language ~s at ~s~%" langname (file-position stream))
                                    (unless langname
                                      (format t "   It was NIL!~%")))
                                 
                                  ;; Return the read term
                                  (read stream t nil t)))
#{foo (format t "1~%")
#{foo (format t "2~%")
;; If this line is commented out, then everything works.  The unused var warning mungs up the read order
#{foo (let ((a 1)) (format t "3~%"))
#{foo (format t "4~%")

┌─[phoe][phoetower][~]
└─▪ sbcl --script /tmp/b.lisp 
Language FOO at 717
1
Language FOO at 740
2
Language FOO at 868
3
Language FOO at 905
4