[takikawa]

In Racket, any custom data type can be turned into a function by using the `prop:procedure` structure type property. It's usually not done with things like maps and sets because this kind of "punning" is not idiomatic. Being a function is not actually a necessary condition for being "first-class" though.

[wonderzombie]

I agree that it's not necessary. But it's a very strong signal, and thus illustrative.

AFAICT there is not a well-understood definition of first-class outside of "first-class functions." If I had to nail it down, I'd probably include the notion of first-class as applied to functions, plus:

* literal representations (where applicable) * idiomatic, as demonstrated/enabled by the standard library * well-integrated/interop with other primitives

This is just off the top of my head. And sometimes this is relative to other constructs in the language, or fuzzier, like Haskell lists vs. maps vs. sets.

[takikawa]

> AFAICT there is not a well-understood definition of first-class outside of "first-class functions."

Why not just the same notion of "first-class" as functions? In other words, that the feature is actually represented by a run-time value that can be passed around freely and stored. This is the usual definition of "first-class" that I hear most people use in the programming languages world. Examples include first class control (continuations), first class references (boxes, mutable cons cells, etc.), first class environments, first class modules (see OCaml, units, etc.), first class labels, and so on.