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Nullable left-recursive rules fail #1
Description
This grammar fails to parse "aaaa":
(gg-grammar (start n := x eof -> n) (x n := x a -> (+ 1 n) | -> 0) (a ?a))
The intent is similar to this grammar:
(gg-grammar (start n := y eof -> n) (y n := x -> n | -> 0) (x n := x a -> (+ 1 n) | a -> 1) (a ?a))
It seems nullable left-recursive rules fail, perhaps because they're not "making progress."
Activity
dominiccooney commentedon Nov 13, 2009
Interestingly this equivalent grammar succeeds:
dominiccooney commentedon Nov 14, 2009
I think this is a bug in the left-recursive PEG algorithm; pinged gurus at https://lists.csail.mit.edu/pipermail/peg/2009-November/000244.html
dominiccooney commentedon Sep 24, 2010
I think the best thing to do is to disallow nullable left-recursive rules, probably by warning about them and failing if they're activated.
dominiccooney commentedon Oct 15, 2010
After adding some tracing, the problem is the heuristic in gg-recall that doesn't activate rules which aren't involved in the recursion. In the first grammar above, rule a is not involved in the recursion (only a is) and so gg-recall eagerly memoizes failure for it.
If that heuristic is relaxed, the parser succeeds and computes the right result. However this has bad downstream effects when a is right-recursive. Laurence Tratt has drafted a paper about this at https://lists.csail.mit.edu/pipermail/peg/2009-November/000248.html