In most OO languages, message passing is just a fancy term for function calling. It’s really not fundamentally different from functional application on a closure value in FP.
OOP is defined by encapsulation and subtyping (polymorphism and inheritance). In fact, the one thing that doesn’t exist in standard FP is inheritance. (Encapsulation sort-of exists with closures, and polymorphism exists with function values.)
> In most OO languages, message passing is just a fancy term for function calling.
Having objects does not make a language oriented to those objects. By your definition C++ would be considered OO, but we know it is not. Kay quite explicitly stated that C++ is not OO. I expect you're thinking of Object-based programming.
If you look at the actual definition of OO, it basically is just a laundry list of Smalltalk features. It is probably not usable outside of Smalltalk for that reason, despite Joe Armstrong's playful insistence that Erlang is the only OO language in existence.
You might be able to include Ruby and Objective-C (and, iff you're really stretching, Swift with @objc enabled), but that's about the extent of it.
I don’t really care much about what Kay says. His view of OO isn’t what caught on. C++ as a descendant of Simula, which is considered the first object-oriented programming language, certainly supports OO. (C++ is really a multi-paradigm language.)
Object-based means OO without inheritance or subtyping.
Implying that the inventors of Simula had a time machine?
I am inclined to think the more logical explanation is that they didn't actually take OO back in time and that Simula isn't an OO language at all (perhaps confused by the first OO language being created with Simula?), but admittedly I'm rooting for the time machine! Look forward to you telling us more about it.
OOP is defined by encapsulation and subtyping (polymorphism and inheritance). In fact, the one thing that doesn’t exist in standard FP is inheritance. (Encapsulation sort-of exists with closures, and polymorphism exists with function values.)