"I do not know where Clojure or Scala have a generic cross-process messaging ability, and I'm not saying it's impossible by any means, but it's much easier if you start from scratch with that in mind."
I don't understand how. The only difference I see is whether the interprocess communication parts are coded into the language itself or whether they're coded into a library. It could theoretically make the problem more difficult to put it in the language itself. Instead of only affecting the programs that use that library, now your concurrency features could potentially affect any program written in your language.
Could you give me an example of a concurrency feature that really, truly benefits from being in the language and not the standard library? I simply can't think of any examples.
For the real answer to this question, attempt to implement a generic serialization syntax for Haskell that requires no work by the user to use. Not even declaring typeclasses. Not even requiring Typeable to be implemented. Just feed it a datatype, and even if the other end has a different version of the software installed (which can happen in a distributed system, after all), it'll all Just Work to some degree.
Erlang has that, because it has its datatypes and a defined serialization for them and there is nothing in the language that is not those datatypes. It also doesn't permit you to layer any type-level assertions about those types into a user type. In fact, Erlang basically has no concept of user types. (Records are syntax sugar around the built-in tuple.) Since the Erlang data types are so weak, an automated serialization can be built that requires no work to use. But that doesn't come without cost
If you don't start with that, you have to use some sort of introspection to examine data types, and you probably don't have a good story for what to do if two ends have totally different ideas about those datatypes, or how to upgrade running processes where you literally want to change the datatype without shutting the process down. The stronger your type system, the harder that gets. The easier it is, the weaker your type system must be. Erlang's capabilities don't come for free, they exist because they wrote their (non-)type system so that their data structures never have any guarantees in them that don't trivially travel across the network. This has its own issues; Erlang has just about the weakest types you can have without actually having arbitrary casts, and that has consequences too.
There are a handful of characteristics in a language, like its type system, that have radical impacts throughout the language and no amount of library jiggery-pokery can completely paper over. Another example not entirely relevant to cross-process concurrency (but not entirely irrelevant) is immutability; no amount of library work can turn a mutable language into an immutable language.
This comment resonates with me especially well today.
I've been attempting to work with GWT and the serialization issues are entertaining (did you implement IsSerializable? is there a no-arg constructor? Do all of the classes your class depend on have the same? Did you remember to recompile the GWT/JS code to update the serialization white list? etc.).
It makes me appreciate more playing with Erlang a while back and shooting data around being so simple.
> Could you give me an example of a concurrency feature that really, truly benefits from being in the language and not the standard library?
Isn't that a bit of a loaded question? In some academic sense, an ideal programming system might have a kernel language that is probably rather small and certainly clean, flexible and extensible in its feature set, and then almost everything else built on top of that kernel in libraries. In practical programming systems, the perfect kernel has proven rather elusive, though.
It's rather like the question of whether a standard library should be small, clean and extensible or comprehensive with a good-enough version of everything. Many of us might be inclined toward the former approach from an academic/theoretical/intuitive point of view, but in practice, just about every widely used programming language from the past two decades has come with a batteries-included library. While those libraries are often criticised for various technical weaknesses, and many of those weaknesses are never addressed because it's too much work, these systems are still good enough for most users out of the box.
I don't understand how. The only difference I see is whether the interprocess communication parts are coded into the language itself or whether they're coded into a library. It could theoretically make the problem more difficult to put it in the language itself. Instead of only affecting the programs that use that library, now your concurrency features could potentially affect any program written in your language.
Could you give me an example of a concurrency feature that really, truly benefits from being in the language and not the standard library? I simply can't think of any examples.