Question

since I’m a newbie to Common Lisp I tried to solve problems on SPOJ by using Common Lisp (SBCL). The first problem is a simple task of reading numbers until number 42 is found. Here’s my solution:

(defun study-num ()
  (let ((num (parse-integer (read-line t))))
    (when (not (= num 42))
      (format t "~A~%" num)
      (study-num))))
(study-num)

The solution is accepted. But when I looked into the details of the result I found it used 57M of MEM! It’s bloody unreasonable but I can’t figure out why. What can I do to make an optimization?

Answer 1

You are making repeated recursive calls, without enough optimization switched on to enable tail-call elimination (SBCL does do this, but only when you have “optimize for speed” set high and “optimize for debug info” set low).

The Common Lisp standard leaves tail-call elimination as an implementation quality issue and provides other looping constructs (like LOOP or DO, both possibly suitable for this application).

In addition, a freshly started SBCL is probably going to be larger than you expect, due to needing to pull in its runtime environment and base image.