[antlr-interest] Is this a bug in Antlr?

[Duncan Booth]

I have a problem parsing a grammar, and it looks to me as though it 
may be a bug in Antlr, but then again maybe I'm just missing 
something obvious. I've cut the problem down to a (nearly) minimal 
grammar attached below.

Compile and run using:
   java antlr.Tool grammar.g
   csc GrammarTokenTypes.cs Grammar.cs /R:antlr.runtime.dll
   Grammar.exe

The output from the program is:
line 0:0: unexpected token [")",<5>,line=0,col=0]

The output I would expect from the program is:
 ( a )

I can get the expected output in a number of ways, e.g. removing 
either branch of the (unused) 'list_iter' rule, using the commented 
out line under the 'factor' rule instead of the current line. Any of 
these minor changes make the parser work the way I expect, but with 
the grammar as given below, the parser seems determined that an 
'expr' can never be followed by a ')'.

Any suggestions how to get this to parse the way I expect without 
breaking the grammar?

---- begin Grammar.g ----
header
{
        using antlr;
}

options
{
        language = "CSharp";
}

class Grammar extends Parser;
options
{
	k = 2;                           // two token lookahead
	buildAST = true;
	defaultErrorHandler = false;     // Don't generate parser error 
handlers
}

tokens
{
    LPAR    = "(";
    RPAR    = ")";
}

{
    class DummyTokenStream : TokenStream
    {
        protected Token[] tokens;
        protected int index;
        
        public DummyTokenStream(Token[] tokens)
        {
            this.tokens = tokens;
            index = 0;
        }
        
        public Token nextToken()
        {
            if (index < tokens.Length)
                return tokens[index++];
            return new CommonToken(EOF, "");
        }
    }
    public static void Main(string[] args) //throws Exception 
    {
        Token[] tokens = new Token[] {
            new CommonToken(LPAR, "("),
            new CommonToken(NAME, "a"),
            new CommonToken(RPAR, ")")
        };
        try
        {
            TokenStream s = new DummyTokenStream(tokens);
            Grammar parser = new Grammar(s);
            parser.expr();
            CommonAST t = (CommonAST)parser.getAST();
            Console.WriteLine(t.ToStringList());
        } catch(Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }
}


expr:
        term (("+" | "-") term)* ;

term:
        factor (("*"| "/") factor)* ;

factor:
      atom (trailer)?;
//    atom;

atom:
        "(" (expr)? ")" 
      | NAME | NUMBER ;

trailer:
        "(" (expr)? ")" ;

list_iter:
        list_for |
        list_if ;

list_for:
        "for" expr "in" expr (list_iter)? ;

list_if:
        "if" expr (list_iter)? ;
---- end Grammar.g ------ 
Duncan Booth                                     
duncan at rcp.co.uk
int month(char *p){return(124864/((p[0]+p[1]-
p[2]&0x1f)+1)%12)["\5\x8\3"
"\6\7\xb\1\x9\xa\2\0\4"];} // Who said my code was obscure?
http://dales.rmplc.co.uk/Duncan


 

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