[antlr-interest] better error messages in tree parsers

[Terence Parr]

On Jan 26, 2010, at 6:00 PM, kferrio at gmail.com wrote:

> ROTFL!  Thanks for calling it as you see it.  I feel a little less naïve now, knowing that you have "issues" with debugging.  Thanks for the nice example too!

:) Added a faq entry.  yeah, it's tough for me right now because I'm debugging a tree grammar parsing an AST representing an ANTLR tree grammar. my brain hurts.

Ter
> 
> Kyle 
> 
> Sent from my Verizon Wireless BlackBerry
> 
> -----Original Message-----
> From: Terence Parr <parrt at cs.usfca.edu>
> Date: Tue, 26 Jan 2010 14:57:40 
> To: antlr-interest at antlr.org interest<antlr-interest at antlr.org>
> Subject: [antlr-interest] better error messages in tree parsers
> 
> 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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