Welcome, learners! In this unit, we're diving into function return values in Scala programming. Function return values, often simply referred to as "returns," are results given back by a function after performing its assigned task. Let's dive in!

Let's create a function that returns a result. Suppose we want our function to perform a calculation — for instance, adding two numbers in a calculator program. Here's how we can write a function that adds two numbers and returns the sum:

def addNumbers(num1: Int, num2: Int): Int =
    // This function adds num1 and num2 and returns the sum.
    num1 + num2

@main def run: Unit =
    // Call addNumbers with 5 and 7 as arguments and print the returned value
    println(addNumbers(5, 7)) // Output: 12

This function definition starts with the def keyword followed by the function name (addNumbers), a parameter list in parentheses (num1: Int, num2: Int), and the return type (: Int). The function body follows the = symbol. The value of the last expression in the function (in this case, num1 + num2) is the returned result.

Returns are not always simple numbers as in the example above. A function can return any type of data from the range Scala supports, including Int, String, Double, Boolean, Char, and Long. Let's explore this further using the calculation of a circle's area as an example:

def circleArea(radius: Double) = 
    // This function calculates the circle's area and returns it.
    3.14 * radius * radius

@main def run: Unit =
    println(circleArea(5)) // Output: 78.5

In this example, the function circleArea takes a Double parameter named radius and returns a Double representing the area of the circle. The function uses the standard formula for the area of a circle (πr²) with 3.14 as the approximation for π. The return type in this case is not explicitly specified; Scala can infer that the result is a Double based on the expression. However, it can be a good practice to specify the return type explicitly:

def circleArea(radius: Double): Double = 
    3.14 * radius * radius

@main def run: Unit =
    println(circleArea(5)) // Output: 78.5

Here, the return type is explicitly specified after the colon :. The body of the function calculates the area and returns it as the result.