D has a `pure` attribute for functions, and yes it is enforced. It's enforced even to the point where it becomes a PITA, but when you manage to make functions pure, you know it's solid.
One pain point with pure functions is you can't insert debug logging statements. D has a special case for that - purity for a statement isn't checked if it is prefixed with the `debug` keyword:
pure int square(int x) {
debug printf("called square\n");
return x * x;
}