[Groxx]

I meant it more in that "first-class" tends to mean "this is a thing that is represented in the language / type system".

Go has first-class functions, because you can make a `var fn func() string` field/variable/argument/etc that holds a reference to a func that returns a string.

Go does not have first-class types, because you can't reference or store a type directly. You can use reflection to pass a reflected thing representing a type, but not the type itself. Generics muddies this somewhat, but I'll argue that falls under "generics", not "first-class types". In contrast, Java has both generics and first-class types, because you can pass `SomeClass` itself as an argument.

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Node.js arguably has first-class concurrency. It has async/await: you do not have concurrency without those keywords. If they exist, you have potential concurrency. If they do not, you do not. (there may be exceptions here for true thread use, and JS runtimes vary, but you get the idea)

Rust has async/await now, and also has Send/Sync, which gives it a very strong claim to "first-class concurrency".

Go's concurrency constructs have no representation in the type system. They're totally invisible. Channels and select are mostly used with concurrency, but they do not define concurrency, and can be (and are) used synchronously as well.

`go` is a keyword, but I don't see how that's any different than `new Thread(fn)`... except that the Thread has a better claim to first-class-ness, because it returns a value that represents the concurrently-executing thread. If you have a thread reference, you know that concurrency exists. The reverse is not true though.

[lmm]

I would say the notion of something being first-class is that you can manipulate it in the same way as a regular value in the language; in particular, you can use an expression that evaluates to such a thing in all the same ways that you can use a built-in version of that thing. Certainly Java does not have first-class types: you can pass a value that is sort of a representation a flattened form of a type, but you can't use that kind of value in a "new" expression or as a function return type.

AIUI Rust's async/await still has quite a lot of special case support. IMO concurrency is only really first-class in languages like Haskell where you can manipulate async actions the same way as a user-defined type and implement concurrency-related operations in plain old code.

[throwaway894345]

I mean, Go clearly has goroutines as a “first class” concept for some value of “first class”, and (as they are a sort of thread) goroutines are concurrency. This is to say, I don’t think your claim about “must have async/await in order to have first class concurrency” is correct in any formal sense (maybe you’re defining “first class concurrency” as requiring async/await rather than asserting that this is what first-class concurrency means to programmers generally?). I agree though that goroutines have no representation in the type system, but that’s because they aren’t values, so one wouldn’t expect them to have a type or a type system representation. Yet they are very much part of the Go runtime and not a library or a syscall or similar.

[Groxx]

By that description though, Go also has first class types. And that kind of makes the distinction meaningless because essentially every programming language has types.

There might be room to claim first-class support for green threads? But if so it's a very weak "first class" since all you can do is start them.

[throwaway894345]

Yes, I think in the general sense of “first-class X”, “first-class types” would refer to any language with a concept of “type” (except perhaps certain dynamic languages where types are built from language primitives, but I’m not so sure about that case). I’m inclined to say that “first-class types” overrides that general formulation to mean “reified types” specifically. It’s also possible that I’m mistaken and the general formulation of “first-class X” always means exactly “reified X” (although I think reflection is a form of reification, and thus Go would have “first-class types” despite your above distinction between first-class types and reflected types), in which case Go doesn’t have “first-class goroutines”.