Unit companion object is not allowed in source

Question
1

The Scastie interface gives the following error.

`Unit` companion object is not allowed in source; instead, use `()` for the unit value

Is that a thing now that using Unit as a value is bad form? Is () really better?

2

Yes. () has always been the idiomatic form of the value (as opposed to the type Unit), and it sounds like that’s now being enforced.

3

Rather, using Unit was never supported, and worked by accident.

You can get the functionality back by using any other object.

4

I think I first used Unit because it was the notation in the Pierce book, Types and Programming Languages.

5

It works for the same reason

def foo: Unit = 7

Works: when a value of type Unit is expected, and the last expression is not of type Unit, the compiler helpfully inserts () so you don’t have to.

Object Unit is not of type Unit, but it’s own unrelated type (to wit: Unit.type), and the compiler helpfully inserts ().

Using Unit where you mean () can actually trip up inference (e.g. in List(Unit, ()) so it’s now a compiler warning.