[antlr-interest] StringTemplate 2.3 regions and ANTLR v3 code gen

[Terence Parr]

Ok, another just amazing ST moment.  Imagine the following template  
that was previously used to generate validation code for a predicate:

/** Every predicate is used as a validating predicate (even when it is
*  also hoisted into a prediction expression).
*/
validateSemanticPredicate(pred) ::= <<
if ( !(<if(debug)>evalPredicate(<pred>,"<pred>")<else><pred><endif>) ) {
     throw new FailedPredicateException(input, "<ruleName>", "<pred>");
}
 >>

So when I see

a : {p}? A ;

I want to generate

if ( !(p) ) {
   throw new ...
}

and

if ( !(evalPredicate(p,"p")) ) {
   throw new ...
}

when debugging.

The template is not only hard to read, but you have the debug stuff  
mingled with regular code; bad separation of concerns.  What you  
really want is the normal template to have code that might need  
replacing marked as a region via <@eval>...<@end>:

group Java;
...
/** Every predicate is used as a validating predicate (even when it is
*  also hoisted into a prediction expression).
*/
validateSemanticPredicate(pred) ::= <<
if ( !(<@eval><pred><@end>) ) {
     throw new FailedPredicateException(input, "<ruleName>", "<pred>");
}
 >>

Then, in a subgroup, you can override/specify the code needed to do  
the actual eval:

group Dbg;
...
/** Force predicate validation to trigger an event */
@validateSemanticPredicate.eval() ::= <<evalPredicate(<pred>,"<pred>")>>

This avoids the need to override the *entire*  
validateSemanticPredicate template, thus, duplicating the generation  
code (introducing the usual maintenance nightmare).  You override a  
"snippet" or _region_. :)

Hooray!

Coming soon to a theatre near you. :)

Ter
PS	Thanks again to Ric Klaren for helping invent this!