[antlr-interest] Difficult to explain prediction error

[Burton Samograd]

Re-sending in case this question was missed.  Any feedback or help appreciated.

--
Burton Samograd

-----Original Message-----
From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-bounces at antlr.org] On Behalf Of Burton Samograd
Sent: Tuesday, August 14, 2012 12:48 PM
To: antlr-interest at antlr.org
Subject: [antlr-interest] Difficult to explain prediction error

Hello,

I have the following rule in my tree walker:

filter
        : ( return_type[...] selection_logic[...] )* { return_type | { /* handle nothing */ } )

which causes no warnings or errors from the Antlr compiler.  It seems to be working well but in one case where I have a certain return_type.  return_type is a selection of rules similar to the following in the grammar:

return_type: (return_a|return_b|return_c|return_d) ;

return_a : RETURNA ;

return_b : RETURNB ;

return_c : RETURNC ( LPAREN float RPAREN )? -> ^(RETURNC float?)

return_d : RETURND ( LPAREN float COMMA float RPAREN )? -> ^(RETURNC float+)

return_c and return_d are tokens with optional parameters after them.

The problem is when I try a single return_c without the parameters.  A single return_d without parameters does not have a problem.

I've single stepped into the generated Antlr tree walker and have found that it is throwing an exception and giving the following error
message:

-unknown source-(0)  : error 3 : ()* loopback of 520:7: ( return_type[...] selection_logic[...] )*, at offset 0, near UP :
cannot match to any predicted input...

This is caused by an exception that is generated when stepping over a function called cdfa37.predict.

I'm having problems understanding why this exception is being generated because the * after the rule in the above error message should cause the rule to be skipped and move onto the next one, which is a single (optional) return_type.

And like I said, this rule passes pretty much all of the test cases except for one that I have found. Can anybody shed some light on why my grammar is behaving this way?

--
Burton Samograd

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