A module functor is nothing more than a higher order module...one that takes as a parameter some data structure that conforms to an interface (signature). Rust may not have constructs called module functors, but it definitely has constructs that are capably equivalent, as well as some that are even more powerful (a claim that applies to Scala as well)

For me, type inference, strong typing, and ADTs with pattern matching puts a language into the ML family quite easily.

All right, I concede.:) Thanks for sharing your views.

Module functors were an SML feature - not really in the original 'ML' language. I would put Haskell in the ML family, and it doesn't have module functors. Rust has type inference, and a core type syntax and semantics that is heavily inspired by it.

I mean, Haskell doesn't have module functors, and I'd certainly claim Haskell is in the ML family. If Haskell isn't, what would be?

I think ML-family is fine. Scala is much closer to ML than what people sort into the Algol-family or C-family groups.