[antlr-interest] Unfolding of loops with antlr?

[Harald M. Müller]

Hi -

Once again a blunt answer: No-one would (and should) do this voluntarily.
This is code which only works - but this is one of the worst things one can
say about code.

Under all normal circumstances, you would use rewriting to do this, i.e. you
would load the input into some intermediate form (ANTLR trees, e.g.
CommonTree; your own homegrown tree; even an XML structure), then work on
this intermediate structure (using ANTLR rewrite rules; or your own manual
visitor; or XSLT). Only such an architecture yields the necessary
"separations of concerns" or "assignment of responsibilities".

What you do is "only possible," but nothing more.

If for some reason (usually if you are in a very big hurry; and no real
quality needs to be delivered) you want to deliver this code (after it
miracuously survives some code review), you should at least write a comment
of >= 30 lines where you explain
* why you did not choose a standard rewriting solution;
* what should happen if the code fails (or why you can prove that it will
not fail; even when the implementation of the ANTLR runtime changes so that
usual lexers and parsers will still work)
* and label this with "HACK" (or at least "HACK?").
A bunch of unit tests tailored to this specific code should also be
provided, so that in the case that suddenly this does not work any longer,
someone can run those tests and conclude "aha, the tricky code with the
stream rewinding and direct calling is broken, after all ... let's now
either throw away the software or get the budget to write it according to a
standard design."

This is about how people on my projects have to handle such "I figured it
out" solutions.

Regards
Harald

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org 
> [mailto:antlr-interest-bounces at antlr.org] On Behalf Of Jon Schewe
> Sent: Friday, December 21, 2007 9:00 PM
> To: Antlr List
> Subject: Re: [antlr-interest] Unfolding of loops with antlr?
> 
> I figured it out.  Can someone let me know if I'm doing 
> something inherently bad here by mucking with the input 
> stream directly and calling the rule method directly?
> loop
> @init {
>   Map<String, Integer> loopScope = new HashMap<String, Integer>();
>   pushScope(loopScope);
> }
> :
> ^(
>   'LOOP'
>   ^(index=ID lb=numint ub=numint) {null == getValue($index.text) }? {
>       System.out.println("Do loop with index: " + $index.text 
> + " from "
> + $lb.value + " to " + $ub.value);
>       // save the place we are at just before the constraints
>       final int marker = ((CommonTreeNodeStream)input).mark();
>       for(int counter=$lb.value; counter <= $ub.value; ++counter) {
>         loopScope.put($index.text, counter);
>         // jump to just before the constraints
>         ((CommonTreeNodeStream)input).rewind(marker);
>         constraints();
>       }
>   }
>   //constraints
> ) /*{
>     System.out.println("End of loop");
>     for(int i=$lb.value; i<=$ub.value; ++i) {
>       loopScope.put($ID.text, i);
>       constraints(); // invoke constraints rule
>     }
>   }*/
> ;
> finally {
>   // remove loop scope
>   popScope();
> }
> 
> Jon Schewe wrote:
> > I've got something I'd like to do with antlr and I'm not sure it's 
> > possible.  I want to unfold loops with antlr inside the 
> parser.  The 
> > idea is this, say I've got an input file like below.  Now antlr has 
> > given me the ability to write nice rules to parse this 
> language and to 
> > do various actions on each rule.  What I'd really like to 
> do is to be 
> > able to tell antlr to repeat a rule action on the same input stream 
> > multiple times while I change some data outside.  Another 
> option would 
> > be for me to be able to duplicate a set of tokens during 
> parsing.  Say 
> > if I know the set of tokens between LOOP and END, could I 
> tell antlr 
> > to duplicate that set of tokens 10 times (since the loop 
> goes from 1 
> > to 10) with an extra token added in to tell me the loop 
> index during 
> > the initial parse and then let the tree parser just walk the tree?
> >
> > I'm sure I could hand the AST off to some custom method to 
> do it, but 
> > that seems like I'm duplicating what antlr is already doing in the 
> > tree parser.
> >
> > Params
> > i 5;
> >
> > Variables
> > x[i];
> > y;
> >
> > Constraints
> > LOOP index 1...10:
> >   loop[index]:
> >   f[index] = 10 * x[index];
> > END
> >
> > simple:
> > g = 5 * y;
> >
> >   
> 
> --
> Jon Schewe | http://mtu.net/~jpschewe
> If you see an attachment named signature.asc, this is my 
> digital signature.
> See http://www.gnupg.org for more information.
> 
> For I am convinced that neither death nor life, neither 
> angels nor demons, neither the present nor the future, nor 
> any powers, neither height nor depth, nor anything else in 
> all creation, will be able to separate us from the love of 
> God that is in Christ Jesus our Lord. - Romans 8:38-39
>