for/continue in scheme/lisp

Question

I'm writing a small interpreter for a C-like language in Scheme (R5RS) and trying to convert something like:

for (i = 0; i < 100; i++)
{
    if (isprime(i)) continue;
    else /* do something with i */
}

to valid Scheme (the isprime function is just an example and not important).

However, after trying for some time, I have not been able to find an efficient/simple way to add the equivalent of a continue statement to a do loop in Scheme. What would be even better would be a "for" macro which allows "continue" and "break" to be used.

I'm considering switching to Common Lisp. Would this sort of thing be any easier in CL?

Answer 1

We can write FOR as a macro. The Common Lisp version:

(defmacro for ((var start end) &body body)
  (let ((block-name (gensym "BLOCK")))
    `(loop for ,var from ,start below ,end
           do (block ,block-name
                (flet ((continue ()
                         (return-from ,block-name)))
                  ,@body)))))


CL-USER 2 > (for (i 10 20)
              (if (evenp i) (continue))
              (print i))

11 
13 
15 
17 
19 

Answer 2

CL's tagbody is a convenient target:

(let (i)
  (tagbody
     (setf i 0)
   body
     (if (isprime i)
         (go increment))
     (do-something-with i)
   increment
     (setf i (1+ i))
     (if (< i 100)
         (go body))))

Answer 3

I'd go for continuations like in this pseudo-scheme example.

Just store the current point of execution in a continuation and call it when appropriate.

(call/cc (lambda break ; jump outside the for
  (for 0 100 (lambda i 
    (call/cc (lambda continue ; jump to the next iteration
      (if (isprime i)
        (continue)
        (break))))))))