The only languages I know that can do prolog-like-constructs as-a-library are lisps, or at least langs that have reasonable symbol constructs. Usability is way way worse if you can’t talk about variables as first class objects.

I was talking to Bob Harper about this specific issue (context was why macro systems are important to me) and his answer was “you can just write a separate programming language”. Which I get.

But all of this is just to say that doing relational-programming-as-a-library has a ton of issues unless your language supports certain things.