bug#14482: Macro not able to see a function

[Ludovic Courtès]

tags 14482 notabug
thanks

Hi,

Sanel Zukan <san...@gmail.com> skribis:

> ---------------------------------------
>
> (define (partition-two lst)
>   (if (< (length lst) 2)
>     '()
>     (cons
>       (list (car lst) (cadr lst))
>       (partition-two (cddr lst)))))
>
> (define-macro (letn bindings . body)
>   `(let* ,(partition-two bindings)
>      ,@body))
>
> (define (println v)
>   (display v)
>   (newline))
>
> (println
>   (letn (a 3
>          b 4
>          c 4
>          d a
>          f (+ a b))
>      (+ a b c d f)))
>
> ---------------------------------------
>
> It correctly prints result, but I'm getting also:
>
> ---------------------------------------
> ;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
> ;;;       or pass the --no-auto-compile argument to disable.
> ;;; compiling /home/sanel/letn.ss
> ;;; WARNING: compilation of /home/sanel/letn.ss failed:
> ;;; ERROR: Unbound variable: partition-two
> 21
> ---------------------------------------

That’s because ‘partition-two’ is only visible at run time, and not at
macro-expansion time.

To fix that, use the ‘eval-when’ form:

--8<---------------cut here---------------start------------->8---
(eval-when (load compile eval)
   (define (partition-two lst)
     (if (< (length lst) 2)
         '()
         (cons
          (list (car lst) (cadr lst))
          (partition-two (cddr lst))))))
--8<---------------cut here---------------end--------------->8---

(BTW, I recommend looking at R5RS ‘syntax-rules’ and R6RS ‘syntax-case’
hygienic macros.)

Ludo’.