I was referring to the use case, not the implementation details. ML-style functors may not be zero-cost abstractions, but they are a way to implement polymorph

I was referring to the use case, not the implementation details. ML-style functors may not be zero-cost abstractions, but they are a way to implement polymorphism, just like C++ templates. You may be able to achieve that with a PPX rewriter, but I'd rather use a functor.