Functions are first-class citizens

In Scala, functions are first-class citizens because we can do the following things with them:

• We can use functions as values or like normal variables; we can replace a variable or value with a function
• We can assign a function literal to a variable
• We can pass one or more functions as another function's parameters
• We can return a function from another function

We will discuss the last two points in the Scala Higher-Order Functions section. Let's discuss the first two points here.

We can use a function like a normal value or variable, as shown here:

scala> def doubleIt(x: Int) = x * x 
doubleIt: (x: Int)Int 
 
scala> def addOne(x: Int) = x + 1 
addOne: (x: Int)Int 
 
scala> val result = 10 + doubleIt(20) + addOne(49) 
result: Int = 460 

We can assign an anonymous function to a variable, as shown in the following code snippet...