Macros[by implementor of toy compiler]

Alan Bawden Alan@LCS.MIT.EDU
Tue, 8 May 2001 03:15:06 -0400 (EDT)

   Date: Mon, 7 May 2001 17:48:47 +0200 (CEST)
   From: Michal Gajda <>

   * Introduction of general hygienic macro's as you propose, forces us to
   cope with following problems:

   1. Full typechecking of macros(in place of definition) seems to need
   second-rank polymorphism. (Decidable, but harder to implement) Of course
   you can delay typechecking until you expand all macros, but then
   error-messages become unreadable.

An interesting option is to allow the macro writer to supply his own
type-checker that is then run before macro-expansion.  Type errors can then
be presented to the user using the pre-expansion source code.

I can hear you all boggling over the lack of safety in what I have just
proposed.  Suppose the macro-writer provides a bogus type-checker?  Ack!
Unsafe code!  Mid-air collisions!  Nuclear melt-downs!  Am I nuts!?
Actually no.  Just do a separate type-check of the fully expanded code
using only the built-in type-checkers.  If this verification check comes up
with a different answer, then one of your macros has a buggy type-checker.
(What you do about finding -that- bug and reporting it to the author of the
macro is another issue...)

A student did a Masters thesis for Olin Shivers and me at MIT last year
trying to work out the kinks in a macro facility that would work
approximately this way.  The results were interesting, but not yet ready
for prime time.