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Hygenic versus Unhygenic macros in Heron

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Christopher Diggins

Posts: 1215
Nickname: cdiggins
Registered: Feb, 2004

Hygenic versus Unhygenic macros in Heron (View in Weblogs)
Posted: Mar 21, 2005 3:23 PM
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Summary
Hygenic macros can not access the local scope where they are expanded, while unhygenic macros (as found in C) can. I want the best of both worlds!
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Unhygenic macros can access the local scope where they are expanded. This is a dangerous and often unwelcome thing because it can lead to very subtle bugs and name clashes. However, it makes it very easy to write clever AOP style code, I can use it to implement Programming with Contracts, and tons of other nifty things.

A lot of people gave me a kick in the pants for not having hygenic macros ala Common Lisp. So I thought how about both? A regular old hackish macro is defined in Heron as follows:

  define assert : 
  {
    if (#e) { 
      throw assertion_error($fxn_name);
    }     
  };
The dollar signs convert meta-values to values accessible at run-time, and # expands code (and macros).

Now the question is how should I syntactically differentiate a hygenic macro? Perhaps:

  1) safe_define : /* boring hygenic macro */;
  2) hygenic_define : /* boring hygenic macro */;
  3) clean_define : /* boring hygenic macro */;
Another option, which I kind of like, is a hybrid macro. This is a hygenic macro which has limited access to the local context (i.e. where it is expanded) through some kind of special context meta-variable (for instance _context). To illustrate what I mean, consider the pseudo-code:
 
  define assert : 
  {
    if ($e) {
      throw assertion_error($_context::fxn_name);
    }     
  };
Your comments and suggestions would be greatly appreciated!

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