[Aside: Why do I have the Whiley (http://whiley.org/about/overview/) link marked seen?]

I was mildly curious why Graydon didn't mention my current, mildly passionate affair, Pony (https://www.ponylang.org/), and its use of capabilities (and actors, and per-actor garbage collection, etc.). Then, I saw,

"I had some extended notes here about "less-mainstream paradigms" and/or "things I wouldn't even recommend pursuing", but on reflection, I think it's kinda a bummer to draw too much attention to them. So I'll just leave it at a short list: actors, software transactional memory, lazy evaluation, backtracking, memoizing, "graphical" and/or two-dimensional languages, and user-extensible syntax."

Which is mildly upsetting, given that Graydon is one of my spirit animals for programming languages.

On the other hand, his bit on ESC/dependent typing/verification tech. covers all my bases: "If you want to play in this space, you ought to study at least Sage, Stardust, Whiley, Frama-C, SPARK-2014, Dafny, F∗, ATS, Xanadu, Idris, Zombie-Trellys, Dependent Haskell, and Liquid Haskell."

So I'm mostly as happy as a pig in a blanket. (Specifically, take a look at Dafny (https://github.com/Microsoft/dafny) (probably the poster child for the verification approach) and Idris (https://www.idris-lang.org/) (voted most likely to be generally usable of the dependently typed languages).

I don't know why actors are on his list, but speaking from my own experience, I think actors are one of those cases where some languages (Erlang in particular) got some stuff accidentally correct because they implemented actor-like stuff, but it wasn't necessarily the actors that did the good stuff. Go's runtime is theoretically somewhat similar here, and despite years of work in Erlang, I almost immediately found myself preferring to work in a relatively conventional imperative language where concurrency was cheap, rather than "an actor-based language".

Perhaps the core problem is that no matter how you slice it, the nature of being an actor-based system means that cross-actor calls are simply more runtime and cognitively expensive than a conventional function call, and across the sum total of the system, that ends up adding a lot of drag. It may be the case that "cheap concurrency" added value, and actors actually net removed value, but the sum total was positive in the end. (I'm spitballing a bit here, to be clear.)

To really see the cheap concurrency in play, Haskell is a great place to play. It has a ton of mechanisms for concurrency and ways of composing them together, which kind of makes it an interesting cage match environment for what concurrency primitives are useful. Actors would be really easy in Haskell, and they basically go unused. I suspect this means something.

Would you call Erlang/Elixir actor-based or just having actor-like features?

Actors in Erlang seem more emergent to me than fundamental. To first order, Go and Erlang use similar concurrency strategies with cheap green threads and channels for communication. The main difference being that Go has channels as separates objects that can be passed around (leaving the goroutine anonymous), while Erlang fuses the channels into the process. In this respect, they both have similar levels of "actor-ness" in my mind. The biggest upside that I can see to channels being separate is that a goroutine can have multiple channels, they can be sent around (though so can PIDs), etc. This matters a lot because the channels in Go are statically typed, while Erlang/Elixir's dynamic typing makes this less meaningful since you can send anything on the same channel.

Of course, I supposed if one defines an actor as a sequential threaded process with a built in (dynamically-typed) channel, then Erlang is actor-based, so maybe I contradicted myself. In Erlang/OTP, the actor part is important to the supervision tree error handling strategy, which I think is the biggest upside, but it's not obvious to me that you need the channel and the process totally fused together to handle that.

Specifically in response to your "cheap concurrency good, actors bad" thought, my hypothesis is that actors are at least the more natural strategy in a dynamically typed language, which is why they seem to work well in Erlange/Elixir, but don't see much use in languages like Haskell (or Scala, it seems, where I at least though the Akka actors were kind of awful). Meanwhile, channels seem to fit better with static typing, though I can't quite put my finger on why.

"Would you call Erlang/Elixir actor-based or just having actor-like features?"

First, fair question and I the nature of your follow-on discussion and musings.

IMHO, one of the lessons of Go for other languages is just how important the culture of a language can be. I say that because on a technical level, Go does basically nothing at all to "solve" concurrency. When you get down to it, it's just another threaded language, with all the concurrency issues thereto. An Erlanger is justified in looking at Go's claim to be "good at concurrency" and wondering "Uh... yeah... how?"

And the answer turns out to be the culture that Go was booted up with moreso than the technicals. When you have a culture of writing components to share via communication rather than sharing memory, and even the bits that share memory to try to isolate those into very small elements rather than have these huge conglomerations of locks for which half-a-dozen must be taken very carefully to do anything, you end up with a mostly-sane concurrency experience rather than a nightmare. Technically you could have done that with C++ in the 90s, it's just that nobody did, and none of the libraries would have helped you out.

That did not directly bear on your question. I mention that because I think that while you are correct that Erlang is technically not necessarily actor-oriented, the culture is. OTP pushes you pretty heavily in the direction of actors. Where in Go a default technique of composing two bits of code is to use OO composition, in Erlang your bring them both up as actors using gen_* and wire them together.

"my hypothesis is that actors are at least the more natural strategy in a dynamically typed language, which is why they seem to work well in Erlange/Elixir, but don't see much use in languages like Haskell"

I can pretty much prove that they don't: https://github.com/thejerf/suture It's the process that needs monitoring, and that process may have 0-n ways to communicate. But that's not a criticism of Erlang, as I think that's actually what it does and it just happens to have a fused message box per process.

"my hypothesis is that actors are at least the more natural strategy in a dynamically typed language, which is why they seem to work well in Erlange/Elixir, but don't see much use in languages like Haskell"

An intriguing hypothesis I'll have to consider. Thank you.

> I can pretty much prove that they don't: https://github.com/thejerf/suture It's the process that needs monitoring, and that process may have 0-n ways to communicate. But that's not a criticism of Erlang, as I think that's actually what it does and it just happens to have a fused message box per process.

One, very neat library. Two, while I agree this proves the point that the actor model is not needed in the language to build a process supervisor, I think that your Go Supervisor looks a lot like an actor, at least in the way Erlang/Elixir uses them. From what I can see, the Supervisor itself works by looping over channel receives and acts on it. The behavior of the Supervisor lives in a separate goroutine, and you pass around an object that can send messages to this inner behavior loop via some held channels. So basically the object methods provide a client API and the inner goroutine plays the role of a server, in the same separate of responsibilities that gen_* uses.

If we squint a little bit, actors actually look a lot like regular objects with a couple of specific restrictions: all method calls are automatically synchronized at the call/return boundaries (in Go, this is handled explicitly by the channel boundaries instead), no shared memory is allowed, and data fields are always private. I'm sure this wouldn't pass a formal description, but this seems like a pragmatically useful form.

I agree that Go is less actor-oriented than Erlang/Elixir, but given how often I've seen that pattern you used in the Supervisor (and it's one I have also naturally used when writing Go) I'd argue that "Actor" is a major Go design pattern, even if it doesn't go by that name. The difference then, is the degree to how often one pulls out the design pattern. I think the FP aspect pushes Erlang/Elixir in that direction more, as this "Actor" pattern has a second function there -- providing mutable state -- that Go allows more freely.

This discussion has really made me think, thanks. I think you're right that actor-like features are valuable and that the Actor Model in the everything-is-an-actor is not itself the value (or even a positive).