circular-code.lisp

; Implementing a loop via circular code in Common Lisp.
;
; This code could work with a Lisp interpreter, but not a Lisp compiler.

; #n= is a label for a s-expression
; #n# references a labeled s-expression

; note that we can use SBCL for this, too. We just have to switch to its interpreter.
#+sbcl (setf sb-ext:*evaluator-mode* :interpret)

(setf *print-circle* t)

(defun circular-code (f n)
  #1= (progn
        (if (zerop n)
            (return-from circular-code 'done)
          (progn
            (funcall f n)
            (decf n)))
        #1#))

#|

CL-USER > (circular-code 'print 10)

10 
9 
8 
7 
6 
5 
4 
3 
2 
1 
DONE

|#