Steps

1. How to create and instantiate a class

Let's start with creating a class named DonutCostCalculator and hard-coding a value for the total cost as 100. I'm sure in practice you will not be hard-coding the total cost value in your program :)

class DonutCostCalculator {

// We are hard-coding the totalCost value for simplicity.
val totalCost = 100
}

Next, let's add a function named minusDiscount() which will return the totalCost - some discount parameter.

def minusDiscount(discount: Double): Double = {
  totalCost - discount
}

Next, let's create an instance of the DonutCostCalculator as follows:

println("Step 1: How to create and instantiate a class")
val donutCostCalculator = new DonutCostCalculator()

2. How to call a function from an instantiated class

If you have used Java or .NET in the past, calling a function on an instantiated class should feel very familiar.

To call the minusDiscount() function, you simply need to call the function on the donutCostCalculator value.

println("\nStep 2: How to call a function from an instantiated class")
println(s"Calling minusDiscount() function = ${donutCostCalculator.minusDiscount(10.5)}")

NOTE:

• Since the minusDiscount() function accepts a Double as its parameter, we are passing some random double of 10.5.

3. How to define function whose name is just the symbol minus  

Defining a function whose name is a symbol is essentially identical to defining any other functions. Instead of having a name, the function will be defined with some symbol.

// Step 3: How to define function whose name is just the symbol minus -
def -(discount: Double): Double = {
  totalCost - discount
}

NOTE:

• We've defined a function called - where the name of function is simply the symbol - itself.

4. How to call function whose name is just a symbol

Calling a function whose name is just a symbol like the - function from Step 3 above is no different than how you would call any other functions.

println("\nStep 4: How to call function whose name is just the symbol -")
println(s"Calling function - = ${donutCostCalculator.-(10.5)}")

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

Step 4: How to call function whose name is just the symbol -
Calling function - = 89.5

5. How to call a function using the operator style notation

Scala allows you to call functions using the operator style notation as shown below:

println("\nStep 5: How to call a function using the operator style notation")
println(s"Calling function - with operator style notation = ${donutCostCalculator - 10.5}")

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

Step 5: How to call a function using the operator style notation
Calling function - with operator style notation = 89.5

NOTE:

• Calling a function using the operator style is not much different than calling a function except that you do not need to specify the .
• Using operator style is more clear when calling functions whose names are just symbols.

6. How to define function whose name is just the symbols +++

Now that you know how to define functions whose names are just symbols, let us create another function named +++.

// Step 6: How to define function whose name is just the symbols +++
def +++(taxAmount: Double): Double = {
  totalCost + taxAmount
}

NOTE:

• This is where it starts getting a bit trickier when defining functions with symbols.
• Is +++ clear that we are adding some taxes to the totalCost? Probably not!
• While using functions which are defined as symbols can be very powerful, try not to abuse this feature as shown here.