[antlr-interest] better error messages in tree parsers

[Oliver Zeigermann]

As input.LT seems to return null values in case we are at the very
start/end of the node stream, I added this check which does the job
for me

           input.LT(-3) == null ? "" : ((Tree)input.LT(-3)).getText()+" "+
           input.LT(-2) == null ? "" : ((Tree)input.LT(-2)).getText()+" "+
           input.LT(-1) == null ? "" : ((Tree)input.LT(-1)).getText()+" >>>"+
           input.LT(1) == null ? "" : ((Tree)input.LT(1)).getText()+"<<< "+
           input.LT(2) == null ? "" : ((Tree)input.LT(2)).getText()+" "+
           input.LT(3) == null ? "" : ((Tree)input.LT(3)).getText();


2010/1/26 Terence Parr <parrt at cs.usfca.edu>:
> 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=<<>>
> context=...DOWN BLOCK DOWN >>>ALT<<< DOWN DOC_COMMENT...
>
> 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()+" "+
>            ((Tree)input.LT(3)).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();
>    }
>
> Ter
>
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