Higher-Order Functions in Scala

In this blog, I’m going to explain higher-order functions.

A higher-order function takes other functions as a parameter or returns a function as a result.

This is possible because functions are a first-class value in Scala. What does that mean?

It means that functions can be passed as arguments to other functions, and functions can return other functions.

The map function is a classic example of a higher order function.

val list = List(1,2,3)

Let’s define a function that doubles each value that is given to it.

def doubleValue = (x: Int) => x * x
doubleValue: Int => Int

val doubledList = list.map(x => doubleValue(x))

This example will call List(1,2,3).map(x => doubleValue(x)) and gives us a list with values (1, 4, 9). The function map will be called on each element of the list that will be passed to doubleValue.

We can also give it an anonymous function.

List(1,2,3) map (x => x + 1)

Amap is a function that takes another function(x => x+1) and a list as its arguments and applies the given function to all the elements of the list.

Create Our Own Higher-Order Function:

Example 1

def addition(f: (Int, Int) => Int,a: Int, b:Int): Int = f(a,b)

This addition takes a higher-order function as an input, which, in turn, takes two integers as an input and returns an integer.

val squareSum = (x: Int, y: Int) => (x*x + y*y)
val cubeSum = (x: Int, y: Int) => (x*x*x + y*y*y)
val intSum = (x: Int, y: Int) => (x + y)

val squaredSum = addition(squareSum, 1, 2)
val cubedSum = addition(cubeSum, 1, 2)
val normalSum = addition(intSum, 1, 2)

Note that:

• addition(squareSum, 1, 2) will call squareSum(1,2).
• addition(cubeSum, 1, 2) will call cubeSum(1,2).
• addition(intSum, 1, 2) will call intSum(1,2).

Example 2

def applyPatternToText(text:String,f:String => String): String = {
f(text)
}

The applyPatternToText function takes another function as a parameter.

def appendTag(data:String): String => String = {
_ : String => s"$data"
}

The appendTag function returns a function itself.

val message = "scala"
println( applyPatternToText(message, appendTag(message)))

This will call applyPatternToText (“scala”,appendTag(“scala”)).

f:String => String will be replaced by appendTag(“scala”).

 appendTag(“scala”) will print scala.

Example 3

Let’s take another example to find the sum of a list and product of a list using the higher-order function:

def operateList(list: List[Int], f: (Int, Int) => Int, operation: String): Int = {
def inner(list: List[Int], result: Int): Int = {
list match {
case head :: tail => inner(tail, f(head, result))
case Nil => result
}
}

operation.toLowerCase match {
case "product" => inner(list, 1)
case "sum" => inner(list, 0)
}
}

The operateList (List(1,2,3),(a, b) => a + b, “sum”) will call the inner(List(1,2,3),0).

After match condition, the next call will be inner(List(2,3),f(1,0)), which will give inner(List(2,3),1).

The next call would be inner(List(3),f(2,1)), which will give inner(List(3),3). Then, the next call would be inner(Nil,f(3,3)), which will give inner(Nil,6).

The final result would be 6!