What does `() => Unit` mean?

Question

Like the question says.

This was found in List[() => Unit] which was used to store callback functions.

I understand List[type] and that Unit is a return type of a function that doesnt return anything.

Answer 1

(T1,...,Tn) => T is the type of functions that take parameters of types T1 through Tn and return type T. So () => Unit is the type of functions that take no parameters and have return type Unit. Consequently List[() => Unit] is the type of lists containing such functions.

Answer 2

First off, you have a list of functions. Each function takes no parameters (this is what the open-close parentheses () mean) and returns Unit, which is a value for no result, similar to void.

In a pure functional world, a function of type ()=>Unit is useless because it takes nothing and returns nothing. However, Scala is not a purely functional language; it has side effects. To be useful, the functions in the list will surely have side-effects. Since they are callback functions, they also have an idea about when they fire.