[antlr-interest] LPG WAS Retaining comments

[siemsen at ucar.edu]

On Mar 13, 2008, at 11:06 AM, Jim Idle wrote:

>> Andy Tripp wrote:
>> The "Trust me, I translate from one non-trivial language to  
>> another without needing an AST,
>> but I wont give any details at all" thread from someone else is  
>> really bugging me, too.
>
> Well, all the poster was saying (I think) is that he was able to  
> write a translator by just using actions in the parser. I have done  
> the same thing in the past. However, what Loring was trying to say  
> (and most would agree I think), is that as soon as you get to a non- 
> trivial case, while you can probably find a way to do without, you  
> will find it better on many levels to construct an AST. I don't  
> think that that is in any way a contentious statement.

That poster was me.  To give Andy some details, my translator reads  
~1250 files written in an abstract language called MOF and writes  
~1250 Java files.  MOF is Managed Object Format, a language used to  
define the Common Information Model, which is a object-oriented  
standard for representing computers and networks.  The Common  
Information Model is defined by the Distributed Management Task  
Force.  The MOF files are available at http://www.dmtf.org/standards/ 
cim/cim_schema_v2171/.  The DMTF wanted to define an object-oriented  
model while staying language-agnostic, so they use MOF.  MOF isn't  
wildly different than Java, hence my translator qualifies as  
"trivial" as Loring defined trivial:

"Trivial translations--A source to B source for languages in which  
language B can straightforwardly express the paradigms of language A-- 
can be done without intermediate analysis.  Even there, an AST is  
helpful and aids maintainability."

Perhaps the negative connotation of the word "trivial" has made this  
into more of an argument than it's worth.  At first I assumed that  
"trivial" meant "small" or "for teaching purposes" or something. My  
translator seems nontrivial to me.  The grammar has ~900 lines, yet  
no AST.  I built it by reading the ANTLR book, learning as I went,  
and didn't encounter a need for an AST.  Others might have a similar  
experience, so I posted to learn why ASTs should be used.  As a  
result of this thread, I now think an AST might improve my translator  
somewhat.  I plan to add one to get separation-of-concerns and to  
perhaps discover the "helpful and aids maintainability" benefits that  
Loring mentioned.

So Andy, I hope you aren't still bugged by this, but I remain  
unconvinced that every useful (nontrivial) translation needs an AST.

-- Pete







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