Question

Learning Common Lisp (using GNU CLISP 2.43) .. so might be a noob mistake. Example is the ‘print prime numbers between x and y’

(defun is-prime (n)   (if (< n 2) (return-from is-prime NIL))    (do ((i 2 (1+ i)))       ((= i n) T)     (if (= (mod n i) 0)          (return NIL))))  (defun next-prime-after (n)   (do ((i (1+ n) (1+ i)))       ((is-prime i) i)))  (defmacro do-primes-v2 ((var start end) &body body)   `(do ((,var (if (is-prime ,start)                   ,start                   (next-prime-after ,start))               (next-prime-after ,var)))        ((> ,var ,end))      ,@body))  (defmacro do-primes-v3 ((var start end) &body body)   (let ((loop-start (gensym))         (loop-end (gensym)))      `(do ((,loop-start ,start)           (,loop-end ,end)           (,var (if (is-prime ,loop-start)                     ,loop-start                     (next-prime-after ,loop-start))                 (next-prime-after ,var)))          ((> ,var ,loop-end))        ,@body ))) 

do-primes-v2 works perfectly.

 [13]> (do-primes-v2 (p 10 25) (format t '~d ' p)) 11 13 17 19 23 

Next I tried using gensym to avoid naming clashes in macro expansion – do-primes-v3. However I’m stuck with a

 *** - EVAL: variable #:G3498 has no value 

Tried using macro-expand to see if i could spot the mistake but I can’t.

 [16]> (macroexpand-1 `(do-primes-v3 (p 10 25) (format t '~d ' p))) (DO  ((#:G3502 10) (#:G3503 25)   (P (IF (IS-PRIME #:G3502) #:G3502 (NEXT-PRIME-AFTER #:G3502))      (NEXT-PRIME-AFTER P)))  ((> P #:G3503)) (FORMAT T '~d ' P)) ; 

Answer 1

Use DO* instead of DO.

DO Initializes the bindings in a scope where they are not yet visible. DO* initializes the bindings in a scope where they are visible.

In this particular case var needs to reference the other binding loop-start.