I am working with Antlr4, studying it also. It's possibilities are amazing. I think it is the best grammar environment, Terence Parr (rhe designer of it) explains why, and I think he is right. One important reason is that Antlr4 does not need code fragments in its grammar so that he grammar really is target language independent. With target language, the engine architecture but also programming language is meant.
There are a lot of grammar examples, and maybe there is one for xquery which would be very similar as AQL But writing the grammar is one thing, the next thing is, what to do with the generated Antlr4 visitor or tree classes ( in any programming language) Writing a query engine could be too much asked for an open source community. So better would be to use the grammar to come to an AQL to XQuery translation. When that is ready, we could use AQL on XML databases, which, are good performing when one is ready to pay a good price. Maybe there is an object oriented query language which engine can be used for OO databases, but I am afraid, that is not my metier. So, when the purpose is to use AQL on XML databases (I remember the old discussion we had about XML databases, but maybe we can agree to disagree on that), and translation to xquery is all right, then I can offer some help. I want to do that because I think a congruent set of standard techniques based on the same paradigm (ADL paths) wil offer enormous possibilities. So, please let me know if it is necessary to help. Maybe others are very experienced in Antlr4 and it could be that it is more efficient when they do it.
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