Subj : Re: Adding 'goto' statement to Rhino
To   : Josh Gertzen <junk@truecode.org>
From : Igor Bukanov <igor@fastmail.fm>
Date : Wed Apr 07 2004 11:23 pm

Josh Gertzen wrote:
> Hi,
> 
> The company I give blood to :) is in the process of revamping most of
> there core technology, which currently includes a less-than-stellar
> proprietary scripting language.  I have successfully made the case
> that Rhino can serve as a replacement for the in-house language as
> well as provide the company with many additional benefits.
> 
> However, since we have many scripts written in the in-house language,
> a conversion process is being put together which will parse the old
> scripts and write them out in JavaScript.  (Obviously there is more to
> it than that, but work with me here)
> 
> Unfortunately, 'goto' statements are used excessively in the scripts. 
> Therefore, for the conversion to work I have to add 'goto' & 'label'
> statement support to Rhino.

Are you sure that is impossible to replace the goto with break? "break" 
in ECMAScript can be used outside loops as well so in many cases I think 
it should be possible to replace forward goto with

label: {
         ....
	break label;
	....
}

and a reasonable backward goto can be written as

label: for(;;) {
	...
	continue  label;
	...
	break;
}

If your conversion tool has to do some parsing it perhaps can generate 
such transformations automatically.

> 
> So I opened up the Rhino code, but before I go any further I just
> wanted to get some input from the group:
> 
> 1. Has this already been done by someone? If so are the changes
> available somewhere?
> 
> 2. Since there is already support for loop labels and break's with
> labels such as [restart: while(true) break restart;], could I simple
> piggy-back on the interals of this?
> 
> 3. What files/areas should I be looking to make such a change in?

If it is really impossible to live without goto you may have a look at 
transformCompilationUnit_r method in 
org/mozilla/javascript/NodeTransformer.java that replaces break and 
continue parse nodes (Token.BREAK/Token.CONTINUE) with explicit jumps. 
Be aware that code assumes that label are always defined prior their 
usage so for goto support you have to add a forward label search. Also 
note that any jump accross try/catch or with blocks has to insert scope 
recovery code.

Regards, Igor

.