[antlr-interest] [AST grammar] input null pointer exception

[maulattu]

Hi mates!
I'm using antlr to build a tree grammar used to parse this pseudo-language 
BUS_0 := EBUS_0 | EBUS_1;

in order to have a compiled code like this:
0; 0; 1; 128
that is:
    * 0 -> "or" (operator)
    * 0 -> ebus_0 (1st operand)
    * 1 -> ebus_1 (2nd operand)
    * 128 -> bus_0 (result)

I've an assignment operator (:=), or (|) and "and" (&). In future it will support shift operator.

I used a grammar to build an AST derived from the simpleCalc grammar available on the antlr book as well as on the wiki:
grammar canScript;

options {
  language = Java;
  output = AST;
  ASTLabelType=CommonTree; // type of $stat.tree ref etc...
}

prog:   ( stat {System.out.println($stat.tree.toStringTree());} )+ ;

stat:   busExpr ':=' expr ';'     -> ^(':=' busExpr expr)
    |   NEWLINE                    ->
    ;

expr:   andExpr ('|'^ andExpr)*
    ; 

andExpr
    :   atom ('&'^ atom)*
    ; 

atom:   INT
    |   ebusExpr
    |   '('! expr ')'!
    ;

busExpr
    :    'BUS_0'
    |    'BUS_1'
    |    'BUS_2'
    ;
ebusExpr
    :    'EBUS_0'
    |    'EBUS_1'
    |    'EBUS_2'
    ;

INT :   '0'..'9'+ ;
NEWLINE:'\r'? '\n' ;
WS  :   (' '|'\t'|'\n'|'\r')+ {skip();} ;

This is the walker grammar:
tree grammar canScriptEval;

options {
    language = Java;
    tokenVocab = canScript;
    ASTLabelType = CommonTree;
}

@header {
import java.util.Vector;
}

@members {
Vector<String> vect = new Vector<String>(50,50);
}

prog
    :    stat+
    ;

stat
    :   expr
    |   ^(':=' busExpr expr)
    ;

expr
    :   ^('|' a=expr b=expr)    {
                                    vect.add(new String("OR"));
                                    /*vect.add($a.text);
                                    vect.add($b.text);*/
                                }
    |    ^('&' a=expr b=expr)    {
                                    vect.add(new String("AND"));
                                    /*vect.add($a.text);
                                    vect.add($b.text);*/
                                }
    | ebusExpr
    | INT
    ;

busExpr
    :    'BUS_0'
    |    'BUS_1'
    |    'BUS_2'
    ;
ebusExpr
    :    'EBUS_0'
    |    'EBUS_1'
    |    'EBUS_2'
    ;

It works fine, since that if it receives as input
BUS_0 := (EBUS_0 | EBUS_1) & EBUS_2;
it will correctly build the ast:
(:= BUS_0 (& (| EBUS_0 EBUS_1) EBUS_2))

As you can see from the walker grammar, the commented rows cause a null-pointer exception in the resulting java code
vect.add(new String("OR"));
                                                        vect.add((a!=null?(input.getTokenStream().toString(
                      input.getTreeAdaptor().getTokenStartIndex(a.start),
                      input.getTreeAdaptor().getTokenStopIndex(a.start))):null));

In particular, input.getTokenStream() returns null and I can't understand why (it is in the "expr()" function)

A lot of thanks in advance!

Mau


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