Steps

1. How to define a higher order function which takes another function as parameter

Let's review the function totalCostWithDiscountFunctionParameter() which we used in the Higher Order Function tutorial.

println("\nStep 1: How to define a higher order function which takes another function as parameter")
def totalCostWithDiscountFunctionParameter(donutType: String)(quantity: Int)(f: Double => Double): Double = {
  println(s"Calculating total cost for $quantity $donutType")
  val totalCost = 2.50 * quantity
  f(totalCost)
}

NOTE:

• The function has a by-name parameter which is expected to be a function that has a parameter of type Double and will also return a type of Double.
• The by-name parameter function will apply some discount to the totalCost value.

2. How to define and pass a def function to a higher order function

Let's also review how we defined a discount function using the def keyword and passed it to the Higher Order Function totalCostWithDiscountFunctionParameter()

println("\nStep 2: How to define and pass a def function to a higher order function")
def applyDiscount(totalCost: Double): Double = {
  val discount = 2 // assume you fetch discount from database
  totalCost - discount
}
println(s"Total cost of 5 Glazed Donuts with discount def function = ${totalCostWithDiscountFunctionParameter("Glazed Donut")(5)(applyDiscount(_))}")

You should see the following output when you run your Scala application in IntelliJ:

Step 2: How to define and pass a def function to a higher order function
Calculating total cost for 5 Glazed Donut
Total cost of 5 Glazed Donuts with discount def function = 10.5

3. How to define and pass a val function to a higher order function

If you've also reviewed the Scala Features Tutorial, you should already be familiar with the fact that functions are first class citizens in Scala.

As a result, Scala allows you to define value function using the val keyword as shown below:

println("\nStep 3: How to define and pass a val function to a higher order function")
val applyDiscountValueFunction = (totalCost: Double) => {
  val discount = 2 // assume you fetch discount from database
  totalCost - discount
}
println(s"Total cost of 5 Glazed Donuts with discount val function = ${totalCostWithDiscountFunctionParameter("Glazed Donut")(5)(applyDiscountValueFunction)}")

You should see the following output when you run your Scala application in IntelliJ:

Step 3: How to define and pass a val function to a higher order function
Calculating total cost for 5 Glazed Donut
Total cost of 5 Glazed Donuts with discount val function = 10.5

NOTE:

• The syntax for defining value function is slightly different to functions defined with def keyword.
• In val applyDiscountValueFunction = (totalCost: Double) => { ... } we did not specify the return type of the function and are making use of Scala Type Inference.
• If you want to add the return type, the syntax for the value function would look as follows: val applyDiscountValueFunction: Double => Double = totalCost => { ... }
• When passing the value function to the Higher Order Function, you no longer need to explicitly make use of the wildcard operator _