Re: syntax-rules problem

   Date: Wed, 19 Nov 2008 08:03:00 +0100
   From: Helmut Eller <[email protected]>

   I have some trouble to understand what's wrong with the following macro:

     (begin
       (define-syntax define-a
         (syntax-rules ()
           ((_) (define a 0))))
       (define-a)
       a)

   I would expect that after evaluating (define-a), a is defined
   and that the entire form would return 0. 

   However, Scheme48 (version 1.8) signals an undefined variable error.
   Is this a bug or a feature?

This is the intent of hygiene.  The macro DEFINE-A cannot refer to
names in the user's environment, and the user cannot refer to names in
DEFINE-A's environment.  If you want your macro to define a binding of
a name in the user's environment, the user must supply that name.
SYNTAX-RULES can express only hygienic macros; you will need to use
explicit renaming macros if you want to break hygiene (or concatenate
names, &c.).

Also, by the way, DEFINE-SYNTAX is not valid in local scopes.  While
that program is valid (assuming it occurs in a top-level sequence), if
one were to substitute LET() for BEGIN, it would cease to be valid.
Local macros are written instead with LET-SYNTAX or LETREC-SYNTAX.