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So, it's time I tackled feeding. This is, I believe, a unique feature of pacc, and makes it possible to use packrat parsing on the command line. I'm a bit hesitant to dive into this one: I had a proof-of-concept for peg / leg a couple of years ago, but I actually don't have that to hand at the moment. The concept isn't a tricky one. But the details will inevitably be tedious.

Let's start with the command line. --feed foo turns on feeding, and foo() is the feeder function (I don't think we'll allow a default feeder). --feed-rule x gives the name of the rule that appears at feed points; it defaults to __ (two underscores). That's easy enough.

I think I want to make another proof-of-concept in pacc this time. Let's take test/pacc/baf4.pacc as a starting point, and add feed markers. Hmm. Consider this pair of rules:

__ ← ("\n" / _)*
_ ← [ \t]*

pacc says left recursion in rule `__:*:0'. And it's right, up to a point (future work: report the error more meaningfully in terms of the user's sugar). The derived rules read:

__:*:0  ←  __:1:0  __:*:0  /  ε
__:1:0  ←  "\n" / _

So the desugared rule __:1:0, which matches a single instance of the subject of the * operator, may make no progress, because _ may make no progress. And so we may indeed left-recurse into __:*:0. Yeah, OK, pacc is right, I'm wrong. I love it when that happens! What we want here is:

__ ← ("\n" _)*

Hmm again. This grammar, or even the unmodified one, is apparently happy to parse 2 + (with a value of 2). How can that be: it includes an end-of-input marker? There's a much simpler eof test in test/pacc/not1.pacc, and that works as expected. It's unfortunately not all that easy to see how pacc parsed an input unless it built a syntax tree. Better try and get a smaller example that fails in the same way. Aha! Once again, pacc is right, and I'm wrong. We had this rule:

Decimal ← Digit* Space { atoi(ref_ptr(ref())) }

which allows an "empty" Decimal. Obviously it should be Digit+, and with that change it behaves as expected.

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


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