I think about this a lot: what is the best way we have to build software components? And what constructs are we missing? Because I really think such a construct

I think about this a lot: what is the best way we have to build software components? And what constructs are we missing? Because I really think such a construct should be first-class in a language to denote its’ significance. I suspect it is something close to ML modules. But I have a hard time seeing them as much better than the equivalent OOP-style of encapsulation. I figure I must be missing something but nothing seems to jump out at me upon repeated investigations.