If I understand constraint propagation and backtracking, why not just use those concepts when it fits the problem rather than an entirely new language meant specifically for that narrow range of problems?
https://old.reddit.com/r/ProgrammingLanguages/comments/9kb9z...
I have worked on and with Mercury for about 10 years. I had very little logic programming experience before this, while others tend to learn Prolog first.
I like Mercury, but not for the logic programming features, in practice they aren't used very often, there's not many parts of a program that require backtracking. So on reflection, I don't think it's something I'd include in a new language, it's not worth the implementation and maintenance costs. Not for most programs most of the time anyway.
On learning logic programming: It might sound like I'm discouraging anyone interested in this, far from it. Logic programming is a great paradigm to learn, another conceptual tool in your toolbox etc.
Another amusing property of Prolog is that a typical Prolog program can be run forward... and backward, i.e. it is possible to «ask» a question and receive an «answer», or if the «answer» is known, the program can arrive at a set of all possible «questions». Hence heavy Prolog use to build expert systems and alike.
Back in the days, we used to giggle that if the 42 answer was fed into a Prolog problem solver to arrive at a full and exhaustive set of questions about the meaning of life, the universe and stuff, that would instantly create a singularity at the point in time and space where the solver was run and henceforth result in a collapse of the universe unto itself.