[antlr-interest] validating semantic predicates

[Ari Steinberg]

I had a similar problem in my grammar.  To get around it, I caught the semantic exception and then throw a new one with a better error message.  My parser has a reference to the lexer so I created getLine() and getColumn() functions which return the current line and column the lexer so that the exception returns valid data.

myRule:
...
;
exception
catch[SemanticException ex]
{
    throw SemanticException( "Syntax error in Explain statement", getFilename(), getLine(), getColumn());
}

Cheers,

Ari

-----Original Message-----
From: Matt Benson [mailto:gudnabrsam at yahoo.com]
Sent: Wednesday, June 18, 2003 2:45 PM
To: Antlr List
Subject: [antlr-interest] validating semantic predicates


I have Tooled my grammar and it has generated a
parser.  I have written the grammar to process the
different elements of the file by calling methods on a
registered implementation of a handler object.  Thus,
I am currently concerned only with content.  I am
providing the handler a Stack of relevant tokens in
case something farther away than the current context
is needed.  So every time I enter a rule, I push the
primary token onto the stack (remember I'm dealing
with XML so this is pretty straightforward), and pop
it before I return from the rule.  I figure while I'm
doing the pop, I might as well do a kind of sanity
check by way of a validating semantic predicate:

{t == tokenStack.pop()}?

One thing I noticed right away is that the
SemanticException thrown in the event that this is not
true contains the exact text.  I don't see in the docs
whether and how to customize this text; I am currently
doing this:

{t == tokenStack.pop() /*unexpected token on the
stack*/}?

which is good enough, but I just wondered if there was
a better way.

TIA,
Matt


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