How to modify grammar to remove left recursive error in ANTLR4?

Question

I'm trying to parse a language. The follow ANTLR4 parser rules are directly copied from the language specification :

physical_value 
 : raw_value DIV factor MUL factor PLUS offset 
 ;

raw_value
 : (physical_value MINUS offset) DIV factor  
 ;

but antlr reports an error:The following sets of rules are mutually left-recursive I don't know how to modify the grammar, Hope someone can help me. Thanks.

http://en.wikipedia.org/wiki/Left_recursion#Removing_immediate_left_recursion — David Brabant, Sep 09 '14 at 08:59
@DavidBrabant ANTLR 4 removes immediate left recursion automatically. This is indirect left recursion (also appears on the page you linked to, but in a different section). — Sam Harwell, Sep 09 '14 at 11:17

score 1 · Accepted Answer · answered Sep 09 '14 at 16:19

1

You cannot eliminate the left recursion from the rules you posted, because the only string it matches is an infinite sequence.

physical_value always starts with a raw_value
raw_value always starts with a physical_value

...and repeat

answered Sep 09 '14 at 16:19

Sam Harwell

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Thanks for your answer, u mean there is no way to modify the rules? I cannot do anything? – sara Sep 10 '14 at 07:24
@sara: Could you post some sample input and what should be matched by the rules. Maybe there's a way to modify the rules. – Onur Sep 10 '14 at 16:03

How to modify grammar to remove left recursive error in ANTLR4?

1 Answers1