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State of the union

So the next "blocker" I want to tackle on the list is to tidy up the union so it contains only one instance of each type--it would be nice to support void at this point, but we'll see.

I suppose you could argue that this isn't really a blocker, since the current code works, and doesn't seem to run into any compiler limitations. It's just really ugly.

This is one of those cases where "n is usually small" - for example, pacc.pacc has just 5 different types (and will eventually add void, but that doesn't come into the union anyway). It's hard to imagine a grammar that would use many more. So we don't need any clever data structures--a simple vector with linear search will be perfectly adequate.

Done. What's next? I was thinking this wouldn't be a bad time to do void, since we're playing with types, but that's a "later" rather than a "blocker". I suppose detecting left recursion would be a good one. I've read a lot about left-recursion, and have decided to pin my colours to the masts of Dale Schumacher, who wrote Left Recursion Considered Harmful (he didn't quite call it that) and Ian Piumarta, who's peg / leg recursive-descent PEG parsers for C inspired me to start pacc.

Obviously, there's a huge advantage in not trying to handle left-recursion (other than to detect it), which is that I don't have to implement anything! Anyway, I'm supposed to be using test-based development, so let's start with a left-recursive test-case or two. Easy, and either direct or indirect left-recursion loops. (In fact, they run out of memory very quickly, which is a bit disturbing. Hmm... let's quickly look at that with a debugger. OK, well one thing that's going wrong is in int l = 2 * (p->stack_alloc - p->stack) + s; we only have 31 bits to play with. Obviously that should be a size_t. That's better! We now run for about a minute and grow to over 10G before the inevitable grisly death.)

OK so that's all done. Huge thanks again to Ian for idea liberation.

Last updated: 2015-05-24 19:45:25 UTC

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