Does anyone know if there's a datalog-like embeddable language that is suitable for simple integer constraints (the sum of a, b, c, and d must be e; e=10.,b>2.)?

For prolog, there's apparently https://github.com/triska/clpz/

But I really just want some simple constraints-oriented form validation (you have 100 points, please feel free to by x's for 1 point each, y's for a cost of 5 points. You don't have to spend all points - with feedback of how many points are left as more x and y are bought. But for more variables, where there are clear rules that can be modeled naturally as constraints rather than with state and callbacks on change).

Ed: specifically for forms, there's https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/HTML... - but it feels like it gets messy with interdependent fields.

https://github.com/Z3Prover/z3

It has bindings for other languages and is able to make some rather sophisticated arithmetical and logical deductions in an often pretty efficient way. I've used it successfully for parts of puzzle competitions and CTFs (and I'm really only beginning to scratch the surface of what it can do).

You can read more about the options in that area at

https://en.wikipedia.org/wiki/Satisfiability_modulo_theories

which also links to the related tools in

https://en.wikipedia.org/wiki/Answer_set_programming

https://en.wikipedia.org/wiki/Constraint_programming

These systems can feel pretty magical and are getting more powerful all the time. (Of course there are cases where there is a better algorithm that you could find, perhaps a much more efficient one, and using these solvers will be asymptotically or worst-case slower than it had to be given more human insight.)

Some of these could be overkill for your application, and Z3 doesn't have a Javascript binding, so I guess it's not as helpful for web forms.