Scala Cookbook

9.2. Using Functions as Variables

Problem

You want to pass a function around like a variable, just like you pass String, Int, and other variables around in an object-oriented programming language.

Solution

Use the syntax shown in Recipe 9.1 to define a function literal, and then assign that literal to a variable.

The following code defines a function literal that takes an Int parameter and returns a value that is twice the amount of the Int that is passed in:

(i: Int) => { i * 2 }

As mentioned in Recipe 9.1, you can think of the => symbol as a transformer. In this case, the function transforms the Int value i to an Int value that is twice the value of i.

You can now assign that function literal to a variable:

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

The variable double is an instance, just like an instance of a String, Int, or other type, but in this case, it’s an instance of a function, known as a function value. You can now invoke double just like you’d call a method:

double(2)   // 4
double(3)   // 6

Beyond just invoking double like this, you can also pass it to any method (or function) that takes a function parameter with its signature. For instance, because the map method of a sequence is a generic method that takes an input parameter of type A and returns a type B, you can pass the double method into the map method of an Int sequence:

scala> val list = List.range(1, 5)
list: List[Int] = List(1, 2, 3, 4)

scala> list.map(double)
res0: List[Int] = List(2, 4, 6, 8)

Welcome to the world of functional ...