[antlr-interest] customizing error messages for syntactic predicates

[Gavin Lambert]

At 08:49 25/01/2009, Carter Cheng wrote:
 >I apologize for the potential double post. But I meant something 

 >like this where I am trying to throw an error when a condition 
is
 >met.
 >
 >LPAREN LBRACE { $LPAREN.index == $LBRACE.index - 1 }?
 >arrayElementList? RBRACE RPAREN { $RBRACE.index == $RPAREN.index 
-
 >1 }?

When I needed to do something like this, I used the following 
method (C# code, but should be similar for Java):

tag[String name]
	:	TAB? t=TAG { $t.text.Equals("<" + $name + ">") }? NL
	;
catch [FailedPredicateException ex] { throw new 
CustomRecognitionException("Expected <" + $name + "> but found " + 
$t.text, ex, Input); }

Note: CustomRecognitionException is exactly what it sounds like -- 
a new exception class derived from RecognitionException.  This is 
required because the standard implementation of GetErrorMessage in 
the parser completely ignores the text in the exception.  And yes, 
this is completely dumb.  Because of that, I also had to use this 
override:

	public override string GetErrorMessage(RecognitionException e, 
string[] tokenNames)
	{
		if (e is CustomRecognitionException) {
			return e.Message;
		}
		return base.GetErrorMessage(e, tokenNames);
	}