GNU bug report logs - #14482
Macro not able to see a function

Previous Next

To add a comment to this bug, you must first unarchive it, by sending
a message to control AT debbugs.gnu.org, with unarchive 14482 in the body.
You can then email your comments to 14482 AT debbugs.gnu.org in the normal way.

Toggle the display of automated, internal messages from the tracker.

Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Sanel Zukan <sanelz <at> gmail.com>
To: bug-guile <at> gnu.org
Subject: Macro not able to see a function
Date: Mon, 27 May 2013 12:51:42 +0200

Hi,

Here is example code:

---------------------------------------

(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
---------------------------------------

I'm using guile: 2.1.0.180-9b977-dirty

Best.

Message #10 received at 14482-done <at> debbugs.gnu.org (full text, mbox):

From: ludo <at> gnu.org (Ludovic Courtès)
To: Sanel Zukan <sanelz <at> gmail.com>
Cc: 14482-done <at> debbugs.gnu.org, request <at> debbugs.gnu.org
Subject: Re: bug#14482: Macro not able to see a function
Date: Thu, 30 May 2013 23:21:59 +0200

tags 14482 notabug
thanks

Hi,

Sanel Zukan <sanelz <at> 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’.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.