Scala method types and methods as parameters

Question

In the following code example, I do not understand why the function fun can be passed as an argument to the method addAction. The method fun is of

Answer 1

There is a difference between methods and functions. In your case actions is a list of functions. When the compiler knows that a function is required (like in the case of addAction) it can automatically convert a method fun into a function. Now :: is also a method, therefore the compiler also knows that it takes functions as parameters. But the problem is the syntactic sugar of the right-associative operator ::. If you were to call it like a method: actions.::(fun) it will compile (although I can't test it at the moment). When writing fun :: actions the compiler thinks that fun is an expression and therefore evaluates it and since it "returns" a Unit you get your compiler error.

EDIT

Since I now have the possibility to test my hypothesis (which was wrong) here are your options:

// Usual syntax
actions.::[() => Unit](fun)
actions.::(fun: () => Unit)
actions.::(fun _)
// Operator syntax
(fun: () => Unit) :: actions
(fun _) :: actions