[antlr-interest] Fwd: De-emphasizing tree grammars?

[Raju Bitter]

Interesting, Galvin. Is there any document available describing the
approach, and the problems you ran into with the ANTLR syntax tree
approach?

And are you talking about these classes?
https://github.com/ceylon/ceylon-spec/tree/master/src/com/redhat/ceylon/compiler/typechecker/tree

Oh, and regarding Lisp and Java: Did you check Clojure?
http://clojure.org/ That's a Lisp written in Java. :-)

- Raju

On Fri, Dec 23, 2011 at 6:01 AM, Gavin King <gavin.king at gmail.com> wrote:
> Ooops, I meant to send this to the list rather than to Terence
> privately. Been happening to me a lot recently since gmail changed
> their UI. Grrr.
>
>
> ---------- Forwarded message ----------
> From: Gavin King <gavin.king at gmail.com>
> Date: Tue, Dec 20, 2011 at 3:51 PM
> Subject: Re: [antlr-interest] De-emphasizing tree grammars?
> To: Terence Parr <parrt at cs.usfca.edu>
>
>
> On Tue, Dec 20, 2011 at 3:25 PM, Terence Parr <parrt at cs.usfca.edu> wrote:
>
>> The AST/ tree grammar mechanism also is something I've been pushing for 20 years
>> without success.  I started out in the compilers world where you really do need an AST,
>> to avoid all of the noise of rule names inside your tree.
>
> So here's a little bit of feedback on that. I've used ANTLR very
> seriously on two projects now, first for the JPA-QL to SQL translation
> of the Hibernate ORM library, and now on the compiler and IDE for the
> Ceylon programming language.
>
> For the query translator in Hibernate, we used a 3-stage translator
> (parser/tree builder, followed by tree transformer, followed by
> renderer) where the second two grammars were tree grammars. This
> worked out somewhat well, though debugging was quite difficult and
> managing less 1-to-1 type transformations was very hard. We were
> successful, but I was left doubting that tree grammars had really made
> our life easier.
>
> For the Ceylon compiler, I did not even try to use a tree grammar,
> because for building a real compiler you want a proper typesafe syntax
> tree, not a bunch of untyped tuples. Java isn't lisp, and if you're
> trying to write lisp in Java you're unlikely to have much success ;-)
> I did initially try letting ANTLR build its untyped tree for
> subsequent translation to a typed syntax tree, but this approach was a
> failure for two reasons:
>
> 1. it's an additional unnecessary step in the pipeline, and, much more
> importantly,
> 2. ANTLR has an awful habit of just throwing away whole branches of
> its syntax tree when some rule fails to match far down the tree.
>
> In a real compiler, you *never* want to throw away things the
> programmer typed in. That's especially true if, as in our case, the
> compiler is also going to be the typechecker of your IDE. IDEs can
> simply never, ever throw away input, no matter how many syntax errors
> it has. Indeed, the hardest "bit" of building an IDE is doing stuff
> like auto-completion correctly while the user is typing, when you're
> almost guaranteed that the text is not well-formed.
>
> So basically my conclusion after all this is that ANTLRs tree stuff is
> pretty much a misfeature. It's cute for simpler problems, but once you
> get beyond hello world, you're better off just writing your own syntax
> tree using Java. Which is what I've done in Ceylon.
>
> Now, if ANTLR was able to produce a *typesafe* syntax tree, and not
> keep throwing bits of it away all the time, then that would be
> something really useful...
>
>
> --
> Gavin King
> gavin.king at gmail.com
> http://in.relation.to/Bloggers/Gavin
> http://ceylon-lang.org
> http://hibernate.org
> http://seamframework.org
>
>
> --
> Gavin King
> gavin.king at gmail.com
> http://in.relation.to/Bloggers/Gavin
> http://ceylon-lang.org
> http://hibernate.org
> http://seamframework.org
>
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