>> I received email accusing me of “selling out to the OO crowd” and people whose experience and opinion I respect insisted that x.f(y) finding f(x,y) would seriously compromise their ability to design stable interfaces.

I think the concern is that a class "owns" its methods either directly or through inheritance. Therefor, the interface is expressed by this namespace and it can't be "invaded" or modified. From that perspective, having f(x,y) being callable as x.f(y) would be a violation of your interface.

But... the dot syntax does not make an interface, and neither does it make "object oriented programming". It implies that "f" is a specialized function for somehow applying "y" to "x".

If you have a generic "f" that can be compiled to a specialized "f" for your type... you are just adding composability to your language. That's a good thing! Calling "f" as x.f(y) in this case makes a lot of sense. It's just another specialized function for applying "y" to "x".