[antlr-interest] better error messages in tree parsers

Terence Parr parrt at cs.usfca.edu
Tue Jan 26 14:57:40 PST 2010

Hi, a reminder that debugging tree grammars can be a bitch.  I like to override standard messaging to spew lots of stuff.  E.g., i like this kind of thing:

ASTVerifier.g: node from after line 150:17 [grammarSpec, rules, rule, altListAsBlock, altList, alternative, elements, element, ebnf, block, altList, alternative]  no viable alt; token=[@-1,0:0='ALT',<84>,0:-1] (decision=24 state 3) decision=<<>>

Here's my code:

    public String getErrorMessage(RecognitionException e,
                                  String[] tokenNames)
        List stack = getRuleInvocationStack(e, this.getClass().getName());
        String msg = null;
        String inputContext =
            ((Tree)input.LT(-3)).getText()+" "+
            ((Tree)input.LT(-2)).getText()+" "+
            ((Tree)input.LT(-1)).getText()+" >>>"+
            ((Tree)input.LT(1)).getText()+"<<< "+
            ((Tree)input.LT(2)).getText()+" "+
        if ( e instanceof NoViableAltException ) {
           NoViableAltException nvae = (NoViableAltException)e;
           msg = " no viable alt; token="+e.token+
              " (decision="+nvae.decisionNumber+
              " state "+nvae.stateNumber+")"+
              " decision=<<"+nvae.grammarDecisionDescription+">>";
        else {
           msg = super.getErrorMessage(e, tokenNames);
        return stack+" "+msg+" context=..."+inputContext+"...";
    public String getTokenErrorDisplay(Token t) {
        return t.toString();


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