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 and use that variable where we have that function literal requirement:

val addFunction = (a: Int, b: Int) => a + b 

Let us experiment this with Scala REPL:

scala> val addFunction = (a: Int, b: Int) => a + b 
addFunction: (Int, Int) => Int = <function2>
 
scala> addFunction(10,20) 
res10: Int = 30 
 
scala> def display(x: Int) = println("x value = " + x) 
display: (x: Int)Unit 
 
scala> display(addFunction(10,20)) 
x value = 30 

As shown in the preceding example, we can use a functional literal or anonymous function as a function parameter.