I am working on parsing a grammar using Antlr4 and running into a problem that I cannot understand. In a nutshell, the problem is that Antlr4 parser fails to fully parse a test string in my original grammar but when I add a superfluous rule , the parse is completed. I am providing a simplified version of my grammar to illustrate the issue.
grammar my;
st: 'H' hd | EOF ;
hd: 'D' d | 'C' c | st ;
d: hd ;
c: 'D' c | hd ;
s1: 'D' s1 | c ;
// p: hd ;
SKP: [ \t\r\n]+ -> skip
When provided with the input string: H C D C C D Antlr4 parser reports the error: line 2:0 no viable alternative at input 'DCCD' and the command grun st -gui shows the partial parse tree:
However, if the commented out rule (p: hd) is included in the grammar, Antlr4 parses the string completely and produces the following parse tree:
Note that the nonterminal p is not in the original grammar and cannot be reached from the start symbol st. As such, the added production is superfluous and should not affect the parsing of the grammar.