Are FP and OO orthogonal?

Question

I have heard this time and again, and I am trying to understand and validate the idea that FP and OO are orthogonal.

First of all, what does it mean for 2 concepts t

Answer 1

Like all classifications, the division of programming languages into functional, object-oriented, procedural, etc. is fictional. But we do need classifications, and in programming languages we classify by a set of language features and the philosophical approach of those who use the language (where the later is influenced by the former).

So sometimes "object-oriented" languages can have success adopting the features and philosophies of "functional" programming languages and vice-versa. But certainly not all programming language features and philosophies are compatible.

For example, a functional language like OCaml accomplishes encapsulation through lexical scoping and closures, whereas a object-oriented languages use public/private access modifiers. These are not incompatible mechanisms per-se, but they are redundant, and a language like F# (a mostly functional language which seeks to live in harmony with the decidedly object-oriented .NET library and language stack) has to go to lengths to bridge the gap.

As another example, OCaml uses a structural type system for object-orientation, whereas most object-oriented languages use a nominal type system. These are pretty-much incompatible, and interestingly represent incompatibility within the realm of object-oriented languages.