Re: Four problems with Scheme 48 1.8

   Date: Wed, 15 Jul 2009 11:40:24 +0200
   From: Martin Ward <[email protected]>

   I was hoping that there would be a simple way to define defmacro using
   define-syntax and syntax-rules, but I can't find one.
   (There is something which lookd promising in SLIB, but I couldn't
   get it to work).

DEFMACRO is a bug.  Some Schemes attempt to mask the bug by providing
DEFMACRO, but fortunately for you Scheme48 doesn't do that.

Anyway, why write a macro?  Your macro -- as defined -- exhibits a
property that is at least confusing and that one might consider to be
a bug: (SUBSTR <string> <from> <count>) evaluates the expression
<from> twice.  Aside from that, though, SUBSTR macro does nothing that
a procedure couldn't do:

(define (substr string from . count)
  (substring string
             from
             (if (pair? count)
                 (+ from (car count))
                 (string-length string))))