How can I add a function (e.g., hammingweight) and use it in expressions occuring in the right-hand side is some (is)/2 goal?
Could something like goal_expansion or term_expansion help here?
I acknowledge that this is not a big feature, but it could increase readability of some of my Prolog programs.
Writing a custom (is)/2 predicate (implementing a custom expression evaluator) is do-able, but I would like to keep runtime overhead low, as I don't want to sacrifice readability for runtime overhead in this case.
There is no such provision in ISO Prolog, neither to extend (is)/2 nor to rely on goal expansion. And, looking at the various implementation specific features that are offered by some implementations to this end, there is no generally agreed upon way to do this. So implementing your own (my_is)/2 seems to be the best way to go.
Also note, that this would not only affect (is)/2 but also all other built-ins that use evaluable functors. In particular, all arithmetic comparison built-ins (8.7 Arithmetic comparison) (see this overview) would be affected.