One thing I don’t see mentioned in this thread is agency: a GenServer (or rather its process) is not a passive entity that is acted upon like in “traditional” OO languages, but an active one that can act entirely on its own.

This is very useful when working in domains where you have lots of objects that act independently of each other. Trying to manually juggle tons of different passive objects to simulate them being active is not an easy task.

I think that this is something that must definitely be stressed. There is no way that processes and modules are related or have some inherent connection. Any process can use any module and any module can be used by any process. So as someone later pointed out I can have functions in mu GenServer module which can be called by other modules or used by other GenServers or processes. This is not something you should be very careful if you do it but there is nothing stopping from doing it.

And remember GenServers are just normal processes, there is nothing special about them. It is the same with modules.

Actually Erlang/Elixir is a very egalitarian system, all processes are created equal and all modules are equal.

EDIT: I can for example kill any process in the system I want. The fail safe and unstoppable way of always crashing the system is:

iex(1)> init = Process.whereis(:init)
#PID<0.0.0>
iex(2)> Process.exit(init, :kill)
System process <0.0.0> terminated: killed

Crash dump is being written to: erl_crash.dump…done

And no, there is no way of prohibiting this!

Erlang might be the only object oriented language because the 3 tenets of object oriented programming are that it’s based on message passing, that you have isolation between objects and have polymorphism.

Joe Armstrong, creator of Erlang

Although you can make such analogy to get you going, it is still better to understand what an Erlang process / GenServer is on it’s own to move up to the next level. If you latch onto the GenServer-is-Object understanding much longer, eventually you will shoot yourself in the foot. A GenServer is much heavier weight than an Object in a typical OO language; so if you try to literately translate an OO program into Elixir, you will be disappointed by the performance.

Not wrong, only slightly inaccurate. The state that a process maintains via the process runloop is never mutated. The runloop is a recursive function, returning a new value into itself each time around the receive “loop”.

Also note that we don’t actually have loops in Elixir, Erlang or BEAM languages since valid bytecode is not allowed to “jump backwards” and therefore it cannot get “stuck” without yielding in a potentially infinite loop inlike almost all other languages I am aware of. The BEAM bytecode can only call another function and that is also how preemption is meticulously enforced through precise “reduction accounting” and why the BEAM provides such low latency. Even bad code cannot block work and processes can always be killed without cooperating and ruining a global “OO state graph” and leaving locks everywhere.

The important thing is that both state and execution context is bound together with real encapsulation. That is not the case in “OO languages”, as Joe’s Armstrong famously said in the book Coders at work:

I think the lack of reusability comes in object-oriented languages, not functional languages. Because the problem with object-oriented languages is they’ve got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle.

Yes you are correct. BEAM languages like Erlang and Elixir are the epitome of object oriented.

Alan Kay the father of the term object oriented said that Erlang is the most object oriented language.

The fact is that object oriented languages are not object oriented according to Alan Kaye’s definition.

OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things.

Alan said he imagined objects as biological cells communicating only via messages like computers on a network.

Well that describes an Elxir or Erlang process cluster communicating using only messages. Don’t you think?

Joe Armstrong thought so…

Erlang has got all these things. It’s got isolation, it’s got polymorphism and it’s got pure messaging. From that point of view, we might say it’s the only object oriented language.

I think perhaps there could be some room for object-splaining in order, however…

… it might be confusing, confronting and potentially create congnitive dissonance for many to say Elixir is a true object oriented language to those who have been conditioned to what they believe OO is.