Let's assume that we can question everything that is assumed to be a fact (or ask stupid questions - your take).

OOP was about message passing

Apparently, the language that is most close to the original idea of OOP, is ... Erlang! According to Alan Kay (mentioned this on different occassions).

Closures & Functional Programming

Here we are cheating a bit, just to communicate better. We can blame things after.

const (
    factor = 10
)

func add10(n int) int      { return n + factor }
func multiply10(n int) int { return n * factor }

Here, only functions are used. The factor constant, is a closure, accessed inside add10 and multiply10 functions. Very Functional so far.

Factor Out The Shared Closure

An idea: let's factor out the shared closure and put it inside something (we will call it an object later).

type factor struct {
    value int
}

func newFactor() factor {
    return factor{value: 10}
}

func (f factor) add10(n int) int      { return n + f.value }
func (f factor) multiply10(n int) int { return n * f.value }

Wow! An Object is very much like a bunch of functions with a shared closure!

Last Bit, Mutation

type factor struct {
    value int
}

func newFactor() factor {
    return factor{value: 10}
}

func (f factor) add10(n int) int                           { return n + f.value }
func (f factor) multiply10(n int) int                      { return n * f.value }
func (f *factor) makeOtherMethodsMeaningless(newFactor int) { f.value = newFactor }

Now we have a mutable closure, shared between a bunch of functions (now called methods).

OOP and Functional Duality

OOP and Functional Programming "can" be seen as a dual. They represent the same thing, to a great extent and they try to provide tools to solve the same problem: composition.

What about message passing?

We simply forgot about that - in time. OOP - in its original form - was about message-passing and behavior. And was trying to organize state and transformation through composing behaviors. While the OOP that we have tries to solve the composition problem, through data structures/type - same as Functional Programming.

Behavior has been forgotten.

This was a quick sketch for drafting out these ideas and get them written. Maybe it can be improved, maybe not.