[antlr-interest] First and Last Token of a Rule

[Jim Idle]

The upcoming token at any point is returned by input.LT(1), the previous token by input.LT(-1)

So:

foo
@init {
 CommonToken sToken = input.LT(1);
}
: A bar* D { doStuff(sToken, input.LT(-1)); }
;

And so on. Also look at things like $start depending on what the output is etc.

However, you will be much better off building an AST then walking the tree to do your actions. 

Jim

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> bounces at antlr.org] On Behalf Of Zachary Palmer
> Sent: Friday, January 15, 2010 2:03 PM
> To: antlr-interest at antlr.org
> Subject: [antlr-interest] First and Last Token of a Rule
> 
> All,
> 
> I think this is a pretty simple operation, but I have no idea how to
> execute it.  Suppose I'm in some action code and have a reference to
> the
> parser.  Is there a way for me to obtain the most recently used token?
> How about the token that started the most recent grammar rule?
> 
> For instance, consider the following grammar (using a Java target
> language):
> 
> foo: 'a' bar* 'd' { doStuff(); };
> bar: ('b' | 'c') { doStuff(); };
> 
> Let's assume we are feeding this grammar the string "abcd".  In that
> case, doStuff is called three times: once after the token 'b' is
> matched
> in the bar rule, once after the token 'c' is matched in the bar rule,
> and once after the tokens 'a' through 'd' are matched in the foo rule.
> I would like, from within the body of the doStuff method, to obtain the
> first and last token of each rule matched.  So, for instance, if my
> doStuff method looked like this:
> 
> void doStuff() {
>     Token first = ...; // first token of the current rule
>     Token last = ...; // token most recently used
>     System.out.println(first.getText() + ", " + last.getText());
> }
> 
> then the output to the above grammar when provided the input "abcd"
> should be
> 
> b,b
> c,c
> a,d
> 
> This is, of course, a representative example; the real situation is a
> bit more complicated.  The catch is that I don't want to add any
> arguments to the doStuff method or do anything else that would require
> me to change every rule in this 3,000 line grammar.  Is there a way
> that
> I can get the first token of the current rule and the most recently
> used
> token without tweaking every single grammar rule?
> 
> Many thanks for reading!
> 
> Zachary Palmer
> 
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