[antlr-interest] Passing arguments to rules

[Gavin Lambert]

At 22:41 31/10/2008, Hendrik Maryns wrote:
 >I have a rule as follows:
 >
 >firstOrderAtomic returns [Formula result] :
 >firstOrderAtomicHead[$first.result, $second.result] { $result =
 >$firstOrderAtomicHead.result;}
 >first=firstOrderVariable
 >second=firstOrderVariable ;
 >
 >I hoped the rule firstOrderAtomicHead would receive the results
 >from first and second, but looking at the produced code, it
 >doesn't.  How would I do this to be able to pass the two
 >variables to the head.

I'm fairly certain that you can't do that, 
because things are called in the order they're 
found, so first and second aren't populated when 
firstOrderAtomicHead is called.  (Given that 
firstOrderAtomic has no idea how much of the 
input stream firstOrderAtomicHead will consume, 
it couldn't work any other way.)

You could use a post-processing action, though:

firstOrderAtomic returns [Formula result]
   : head=firstOrderAtomicHead
     first=firstOrderVariable second=firstOrderVariable
     { $result = CreateFormula($head, 
$first.result, $second.result); }
   ;

(You might need to specify an attribute on $head, 
eg. $head.text or $head.result.)

 >I think probably moving to an AST instead of trying to do this
 >directly in the parser would be a solution, but you probably
 >can judge this better, so that's why I'm asking


That's another option.  It'd probably lead to 
slightly cleaner code, and it'd be handy if you 
wanted to do multiple passes or different sets of 
actions for the same basic input -- but it is a 
little extra overhead.  Up to you to decide which you prefer :)