[pdimitar]

> But I'll never understand that people like Elixir for being FP.

To me the answer is: using mutable state is opt-in. I disagree that "mutable state is rampant".

By opting in to the mutable state constructs you are basically saying "I know what I am doing, let me do my work" which is IMO quite fine because "pure" FP languages like Haskell can be a huge hassle when you actually need to deal with the real world.

To me Elixir is a very nice compromise.

[senderista]

Mutable state is essential to the actor model. Local arguments to a tail-recursive message loop which change based on the last received message and the previous arguments, and determine behavior (i.e. messages sent and side effects), are equivalent to local mutable state. State machines are a lot better than unstructured, freely mutable global variables, but they are still mutable state.

[pdimitar]

Sure. That's absolutely unavoidable in any FP language. All their compilers invisibly produce a lower-level code that's intrinsically using the mutable paradigm. It's how our hardware works currently.

[eru]

No, in an actor based model the mutable parts that the comment you replied to mentioned are surfaced to the user.

It's not about how the Erlang VM is implemented. You could implement it in pure Haskell and compile it so some hypothetical pure CPU. Wouldn't change that part of how you interact with it that is stateful as described.

(And I liked programming in Erlang.)

[pdimitar]

And I still disagree with that comment that "mutable state is rampant". There are realities which we all have to take into account. To give an extreme example, should we get rid of all mutable databases and use append-only journals? That might help eliminate another class of bugs but there are a number of (practical and political) limiting factors in commercial projects.

But maybe we'll talk the same language if you give a few examples. I was under the impression that my parent poster made the point of "but your FP code gets compiled to imperative mutable code so FP is not good" or something. If I was mistaken in my interpretation then we're talking past each other.

[senderista]

My point was that the actor model has the semantics of local mutable state. The fact that an actor model language is compiled for a von Neumann machine is irrelevant to my point.

[pdimitar]

Okay, that's true, but especially in Erlang's BEAM VM the mutable state's access is serialized / centralized. An actor's state is not a globally modifiable volatile variable a la the C/C++ ones.

[eru]

As far as I can tell, that's not what the great-grand parent comment was about.

Have you ever programmed in Erlang?

About your first paragraph: not all databases should be append only, but it would be a good idea to make append-only the default and only deviate for a good reason. (Instead of making mutable the default, and only weirdos go for append-only.)

[pdimitar]

I work almost exclusively with Elixir for 3.5 years and love it. But I am not seeing that poster's point still.