I'm writing a parser which allows Perl callbacks, and I want to make some macros available to the programmer writing the callbacks. An example probably explains most easily. For a grammar rule to do addition

Sum ::= Addend + Addend
[download]

the user might want to supply a callback like

sub { <first Addend> + <second Addend> }
[download]

In this example, the things in angle brackets are the macros. The idea is that the parser replaces these before the callbacks are compiled.

My question is what's the best syntax to use for macros? I want macros that are easy to use, but on the other hand don't get in the way of Perl constructs, and on the third hand (?) aren't impossible to parse out. Obviously there are tradeoffs. The angle brackets of the above example are, to my mind, not the right choice. They too easily are mistaken for other Perl syntax, both by the programmer and the parser. Note that my primary question is not how to program the macro substitution, but what syntax to use for the macros?

I'm sure other modules have tackled this same issue. Pointers to modules which monks feel did macros right would be very helpful.

thanks, jeffrey kegler

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