formatlispcommon-lispbufferedoutputstream

LISP: (format) prints only after successful (read)

I'm going through the book ANSI Common Lisp by Paul Graham, and there's this example:

(defun ask-number ()
  (format t "Please enter a number. ")
  (let ((val (read)))
       (if (numberp val)
           val
           (ask-number))))

It should behave like this:

$ (ask-number)

Please enter a number. a

Please enter a number. (ho hum)

Please enter a number. 52

52

But when I try it (SBCL 1.0.55), it doesn't print the format string until successful read:

$ (ask-number)

a

(ho hum)

52

Please enter a number. Please enter a number. Please enter a number.

52

Where's the error? How to make it behave the intended way?

Solution

  • This is an often asked question. There are possibly duplicates of this on Stackoverflow.

    The output can be buffered.

    You then need to call the standard Common Lisp function FINISH-OUTPUT to force the IO system to write any pending output.

    After that, read.