Function Variables

Going back to this example from the previous section:

val doubledInts = ints.map((i: Int) => i * 2)

We noted that this part of the expression is an anonymous function:

(i: Int) => i * 2

The reason it’s called anonymous is because it’s not assigned to a variable, and therefore doesn’t have a name.

However, an anonymous function—also known as a function literal—can be assigned to a variable to create a function variable:

val double = (i: Int) => i * 2

This creates a function variable named double. In this expression, the original function literal is on the right side of the = symbol:

val double = (i: Int) => i * 2
             -----------------

the new variable name is on the left side:

val double = (i: Int) => i * 2
    ------

and the function’s parameter list is underlined here:

val double = (i: Int) => i * 2
             --------

Like the parameter list for a method, this means that the double function takes one parameter, an Int named i. You can see in the REPL that double has the type Int => Int, meaning that it takes a single Int parameter and returns an Int:

scala> val double = (i: Int) => i * 2
val double: Int => Int = ...

Invoking the function

Now you can call the double function like this:

val x = double(2)   // 4

You can also pass double into a map call:

List(1, 2, 3).map(double)   // List(2, 4, 6)

Furthermore, when you have other functions of the Int => Int type:

val triple = (i: Int) => i * 3

you can store them in a List or Map:

val functionList = List(double, triple)

val functionMap = Map(
  "2x" -> double,
  "3x" -> triple
)

If you paste those expressions into the REPL, you’ll see that they have these types:

// a List that contains functions of the type `Int => Int`
functionList: List[Int => Int]

// a Map whose keys have the type `String`, and whose
// values have the type `Int => Int`
functionMap: Map[String, Int => Int]

Key points

The important parts here are:

• To create a function variable, just assign a variable name to a function literal
• Once you have a function, you can treat it like any other variable, i.e., like a String or Int variable

And thanks to the improved Eta Expansion functionality in Scala 3, you can treat methods in the same way.