Debugging parsers

This parser subproject is the hardest programming I've done in twenty years. Well, wait - I took Compilers from Kent Dybvig in ... must have been 1995. So fifteen years, anyway, but I don't remember Compilers. That is, I remember Kent, and I remember the class, but I don't really remember the coding. This stuff is stored in a different place in my brain, I think, very intuitive and nonverbal - part of what makes it so exhilarating.

Anyway, debugging recursive descent compilers is hard, because they're declarative in nature. This is actually going to end up being a key insight (no, not hexapodia) - debugging an imperative program is far, far easier than debugging a declarative one, because you can step through what the program is doing. In a declarative program, not only do you not know when or where things are being done, it's exceptionally difficult to interpose a check point.

I wrote a small debug atom for parsers, though - when it's invoked, it always succeeds, consuming nothing, just like \&nothing - but it prints a message.

Using it felt like ... Prolog.