I wrote a library that takes a language specification (BNF, ABNF, etc) into a custom AST in Python. Then you can define a set of visitors for the custom AST to transform the tree into whatever you want. I implemented some checks integrated into the Python type system to type-check the visitors and I've used the library to do some non-trivial manipulations before.
It's mostly a pet project and I just wanted to share since it could maybe at least inspire something.