Scala Functions as Values

Before you start

Scala basics.

Define a method and pass it to a higher-order function.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

def square(n: Int): Int = n * n defines a method whose body is a single expression. In Scala functions are values: a function of type Int => Int can be stored in a variable or passed as an argument.

twice takes a function f and a value x and applies f two times. Passing the method square where a function is expected works because Scala converts the method into a function automatically. twice(square, 3) is therefore square(square(3)) = 81.

Code example

Scala

object Main {
  def square(n: Int): Int = n * n

  def main(args: Array[String]): Unit = {
    val twice = (f: Int => Int, x: Int) => f(f(x))
    println(square(5))
    println(twice(square, 3))
  }
}

Output

25
81

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1object Main {

2 def square(n: Int): Int = n * n

4 def main(args: Array[String]): Unit = {

5 val twice = (f: Int => Int, x: Int) => f(f(x))

6 println(square(5))

7 println(twice(square, 3))

8 }

9}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
A method with an expression body.	`—`	—
A function value that applies another function two times.	`—`	—
Call the method directly.	`—`	25
square(3) is 9, then square(9) is 81.	`inner = 9, result = 81`	25 81

Behind the scenes

How Scala processes this code

Scala compiles for the JVM, which invokes the main entry point.

Cost and common mistakes

O(1) time and space.

Int overflow wraps silently, so squaring large numbers gives wrong results.
The last expression is the return value; adding a stray statement after it changes the result.

Try it yourself

Predict twice(square, 2).

Reveal answer and reasoning

square(2) is 4 and square(4) is 16.