What does () mean in Scala?

Question

I've found a Scala code snippet which declares a method <init> and puts () right below the invocation.

I have a question about line number 5. What does () mean here?

(() => {
  final class $anon extends MutableProjection {
    def <init>() = {
      super.<init>();
      ()
    };
    ...
  };
  new $anon()
})

Here is a code with full example.

Answer 1

Every Scala function has a return type. By convention (and highly encouraged by some language features), functions that don't need to return anything have a return type called Unit , which has a singleton value written as () .

The last expression in a function body is its return value. That author made this be () to cause the compiler to infer that the return type should be Unit . But it would have been more clear to just do that with a type annotation. If a function's return type is Unit , Scala will implicitly return () from the function no matter what the last statement in the body is. So this

def <init>() = {
  super.<init>()
  ()
}

could be written equivalently as

def <init>(): Unit = super.<init>()

Answer 2

() can meean a few things, depending on context.

As a value, it is an empty tuple, or the singleton type. It's type is Unit .

It can denote a function or method that takes no parameters, such as:

def foo() = "Hello world"

Note that when writing an anonymous function, the () is by itself but still means a function with no parameters.

val x = () => 2

The type of x is () => Int , a function taking no parameters and returning an int.

As a source of infinite confusion, you can get examples like this:

val y = () => ()

The type of y here is () => Unit , a function of no parameters returning Unit , not Unit => Unit , which would be writen as val z = (x:Unit) => () , and called like z(())

The unit vs empty parameter distinction has been awkward for me in the past so hopefully that demystifies some of it.