Steps

1. Assume a pre-calculated total cost amount

Let's start with a simple totalCost value which represents the total cost in dollar figure for a particular customer buying donuts from your store.

println("Step 1: Assume a pre-calculated total cost amount")
val totalCost: Double = 10

2. How to define a val function to apply discount to total cost

Similar to the example from the previous tutorial on defining function with val keyword, let's define a val function which will apply some discount dollar value from the total cost figure.

println("\nStep 2: How to define a val function to apply discount to total cost")
val applyDiscountValFunction = (amount: Double) => {
  println("Apply discount function")
  val discount = 2 // fetch discount from database
  amount - discount
}

3. How to call a val function
Calling the val function applyDiscountValFunction from Step 2 is very straight-forward. You simply need to pass it the totalCost value from Step 1.

println("\nStep 3: How to call a val function")
println(s"Total cost of 5 donuts with discount = ${applyDiscountValFunction(totalCost)}")

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

Step 3: How to call a val function
Apply discount function
Total cost of 5 donuts with discount = 8.0

4. How to define a val function to apply tax to total cost

Let's go ahead and define another val function which should apply some tax amount to the totalCost value.

println("\nStep 4: How to define a val function to apply tax to total cost")
val applyTaxValFunction = (amount: Double) => {
  println("Apply tax function")
  val tax = 1 // fetch tax from database
  amount + tax
}

5. How to call andThen on a val function

As we've seen from the previous tutorial on defining function with val keyword, val function inherits an andThen function.

Calling andThen will take the result from the first function and pass it as input parameter to the second function. Let's use the andThen semantics to apply discount andThen apply tax to the totalCost figure as shown below.

println("\nStep 5: How to call andThen on a val function")
println(s"Total cost of 5 donuts = ${ (applyDiscountValFunction andThen applyTaxValFunction)(totalCost) }")

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

Step 5: How to call andThen on a val function
Apply discount function
Apply tax function
Total cost of 5 donuts = 9.0

NOTE:

• The apply discount function was called first andThen the apply tax function was called.
• The output from the first apply discount function was also passed through as input parameter to the second apply tax function