AFAICT the core of FP is referential transparency. The "meat" of FP are the tools available which allow to (comfortably) increase the portion of the program tha

AFAICT the core of FP is referential transparency. The "meat" of FP are the tools available which allow to (comfortably) increase the portion of the program that is referentially transparent. Immutability, first class functions, laziness are all such tools. And which tools are used can vary, just like the tools that help "doing OO" can vary. But I think that FP does have that core which has a glass clear definition. I'm not sure which is the core of OO. Maybe it's the idea of "encapsulating" mutation (basically inverting the core of FP). Maybe it's the principle of virtual methods / messaging. It's not so clear I think.