Spurious parses stemming from nullable nonterminals

[corwin-of-amber]

The following grammar:

@builtin "whitespace.ne"

d -> a

a -> b _ "&"
   | b

b -> letter
   | "(" _ d _ ")"

letter -> [a-z]

when run using nearley-test on (x), generates two (identical) parses [ [ [ '(', null, [ [ [ [ 'x' ] ] ] ], null, ')' ] ] ]. Since the above grammar is unambiguous, this is unexpected.

Notice that this does not occur if you omit the rule a -> b _ "&", which does not appear in the derivation, and this is even more unexpected. It has to do with the order of prediction.

I am preparing a pull request that suggests a fix.

[corwin-of-amber]

Tried 2.2.0 and it works 👍

[kach]

Awesome. Closing, then.