Actor frameworks with message passing, just like direct mutation, don't compose.
On Haskell, yes, you can just work off a thread and write shared buffers. But you are better off using STM or Async (these are a bit like futures), which compose, and you can write pipelines out of them.
Actor frameworks don't compose, because actors are an atomic compute unit. You can't really take a horse actor and a bird actor and turn them into a pegasus actor. There are non-trivial interactions between message handling logic that would prevent such a thing with some fairly trivial-to-generate breaking cases.
Maybe composition for compositions' sake isn't that important?
> Maybe composition for compositions' sake isn't that important?
Definitely. I was about to ask "but when did you actually need to compose actors and how does that even make sense?" -- and while I am sure there are people who would find a scenario I feel that would still be tarrying on minutiae.
Obviously the actor model is not a panacea. But for my commercial work Elixir -- and thus Erlang's OTP -- has been a true blessing. There aren't many commercial scenarios where OTP is a very poor fit. They do exist but I'd dare saying they are no more than 10-15% of everything you can stumble upon out there.
This is definitely true, I am just not sure how well it applies to actors. For all data-modelling scenarios and OOP/FP ways of doing polymorphism, composability is a life-saver though.
