I am currently trying to build an ANTLR grammar for HQL. There is already an existing one for Hibernate 3, but that one is based on ANLTR 2.x and supports quite a bit more than NHibernate does at the moment (DML statements, for instance). After several failed attempts to port the grammar to ANTLR 3 and generate C# code out of it, I gave up and started building my own.
I have read the ANTLR book not that long ago, so I ought to have a known what was in store for me. I didn't. I found out that this require a totally different mode of thinking. My recursion muscle is very tired at the moment, but I managed to create a simple grammar for:
select x.y, z.b from Entity as e join e.Children as c
I kept having false starts with the thing, until I went and read Boo's source, and figure out how Boo's parser works. Basically, instead of letting the tool to generate the parser and work of the generated tree, this approach calls for constructing our own tree while we parse the source.
As you can see, we constantly pass the Query instance down to lower rules, so we can operate on it and build our tree. This is much easier than trying to handle the CommonTreeAdaptor [sic] and derivatives.
I want to make the parser and the resulting AST as smart as possible, before trying to plug it into NHibernate's itself. That is going to be a significant undertaking, and I would like to have help, so feel free to contribute.
Now that I have the initial stuff going, I am going to refactor it a bit to match this BNF (http://www.hibernate.org/89.html), and yes, I know it is outdated.
There are about a dozen tests for the syntax yet, so it is possible to just grab it and start working on it.
You can grab the source from: https://rhino-tools.svn.sourceforge.net/svnroot/rhino-tools/experiments/Hql