Question

I want to get output like 0 1, but the code below just print nil. I use type-of to test the (first hex), it is integer. %d should work, right? If I use message, it works in Emacs.

(defun draw-board (board)
  (loop for x below 2
        for hex = (aref board x)
        do (format "%d " (first hex))))

(draw-board [(0 2) (1 2) (0 3) (0 2)])

Answer 1

1- emacs lisp format is not Common Lisp format. Notice the missing
argument!

(format "%d" 42) == (cl:format NIL "~D" 42)

2- therefore the only things your loop does are:

 - to check that board is a vector with at least two slots. (aref
   signals an error if the index is out of bound).

 - to check that each of those two slots are lists (first signals an
   error when passed a non list).

 - to check that the first element of each each of those two slots
   are numbers. (format signals an error when you pass a non number
   for %d).

That’s all.

You never said you wanted to print anything.

To print something, you must put it in a buffer, and use
ps-print-buffer:

(defun draw-board (board)
  (with-temp-buffer
    (loop for x below 2
          for hex = (aref board x)
          do (insert (format "%d " (first hex))))
    (ps-print-buffer)))