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Comment by spinningslate | Hacker News Reader

> those messages go into the actor’s mailbox, and then the actor processes them synchronously

Not trying to be pedantic, but that's technically not true of the Actor formalism [0]. Quoting wikipedia:

> an actor can designate the behavior to be used to process the next message, and then in fact begin processing another message M2 before it has finished processing M1.

Available implementations (such as Erlang, Akka, Pony) do not support that pipelining so are single threaded per actor as you state though. As an aside, the behaviour you describe is part of the CSP formalism [1] - a related but different approach to concurrent systems.

[0]: https://en.wikipedia.org/wiki/Actor_model#Inherently_concurr...

[1]: https://en.wikipedia.org/wiki/Communicating_sequential_proce...

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Just to be a bit pedantic (but important), Erlang is not an actor system. It's a system designed for fault tolerance and definable failure domains. Concurrency fell out of those requirements and it just happens to vaguely look like an actor system if you squint hard enough. I don't believe theories of actor systems played into the design of Erlang.

Perhaps some of the blame belongs on the creators of Erlang for kind of jumping on the "actor" (and later, "OOP") bandwagons as marketing? to try to make Erlang less scary.

Fair point: Erlang wasn't designed as an Actor system. But you don't have to squint that hard to see the similarities. Again quoting wikipedia [0]:

> An actor is a computational entity that, in response to a message it receives, can concurrently:

    send a finite number of messages to other actors;
    create a finite number of new actors;
    designate the behavior to be used for the next message it receives.

> There is no assumed sequence to the above actions and they could be carried out in parallel.

All above are true of Erlang except intra-actor concurrency. Even the last bullet on designating behaviour: An Erlang process (actor equivalent) can decide to use a different function when receiving the next message. It just can't pipeline handling messages.

So Erlang isn't that far removed from the Actor model in its behaviour, even if it wasn't designed as an Actor system in the first place. One might say actors are an (approximate) emergent property of Erlang rather than an intentionally designed one.

Slightly off-topic but the only system I've used that was designed & built from the outset as an actor system is Rosette [1]. It was a really interesting project, and does support pipelining, but has been dormant for more than a decade.

[0]: https://en.wikipedia.org/wiki/Actor_model#Fundamental_concep...

[1]: https://github.com/leithaus/Rosette

EDIT: clarified that Rosette is the only system I've used that was explicitly based on the Actor model from the outset.

No but also key to the definition of the actor system is that those are the only things they are allowed to do. Also, there's strange stuff like naming processes for service discovery, and iirc something is wrong with the way that Erlang does selective receives that disqualifies it from being an actor system, and ultimately more modern Erlang things like ets tables and sharing memory with nifs.

Point is all of these deviations from actor system were choices made by the Erlang team in the name of pragmatism. They all exist because there was a use case and the first teams using Erlang needed them for something real; that Erlang is not an actor system is important, because it's a highly pragmatic system -- not one that is based in theory.

> Also, there's strange stuff like naming processes for service discovery, and iirc something is wrong with the way that Erlang does selective receives that disqualifies it from being an actor system, and ultimately more modern Erlang things like ets tables and sharing memory with nifs.

Ets tables are observationally equivalent to an Erlang process per table and sending it a message for each function call. It's not actually implemented that way, but I don't think that is grounds to disqualify it. I don't remember enough details about the naming process, but I think that might be similar; you could send a message to a naming process to set and lookup the names (although you'd have a bit of trouble finding out what the process id of the naming process is, wouldn't you?), it's just not very pragmatic.

Nifs certainly have the potential to break the model of course. I'd think selective receive should be fine too, it's equivalent to reading (or peeking) and saving messages until a matching message is received, processing that message, then processing future messages from the saved queue. It's just implemented in a more pragmatic way.

If immutability is important, then the process dictionary means Erlang doesn't qualify, but again, it's pragmatic.

I'd rather have a pragmatic almost actor system than a dogmatic Actor system that's hard to use.

> designate the behavior to be used for the next message it receives.

I think that is the key insight of actor-systems.

Actors don't have state. But they can calculate their replacement based on their immutable data. Thus the way an actor at a given address evolves is described by the functions that calculate the successor actors.

Thus you get Pure Functional Programming implemented on top of a fabric of distributed evolving entities. You can understand the behavior and evolution of such a system as a composition of function-calls, where functions always produce the same result for the same arguments.

I mean, if it doesn't count as an implementation of an actor system, then I'm not sure what does, regardless of whether it was incidental to the design goals of Erlang as a language. It certainly walks and quacks like an actor system in my opinion, and I don't think you have to squint very hard at all to see it.

Perhaps we have different ideas of what an "actor system" is though.

Dr. Hewitt typically chimes in on these threads to emphasize some of the differences between his model and Erlang. I don’t recall his username to locate some of those.

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