What is a “First Class” type?

Question

What does it mean for a type T to be a \"First Class\" type?

Answer 1

The use of "T" make it sound like maybe someone was speaking about the state of generics in Java (they get erased, meaning that while you can check if something is a List at runtime, you can't check if it's a List of Integer).

However, there are also "first class types," meaning that types themselves (not just instances of them) can show up anywhere, like as the value of an expression. For instance, code snippets like

someType s = new someType();
new typeOf(s); // makes a new instance of someType

But you don't see that in the wild much, since if your types depend on a value, type-checking requires more computation, and if you allow types to depend on any value, then checking becomes undecidable.