Function types

In Kotlin, every entity is represented as an object, including functions. Therefore, like all other objects, functions also have a specific type. The function type describes the Quantity and Type of parameters, which assumes a function, as well as the Type of value it returns, without mentioning the function name. Function types allow functions to be used as values ​​and stored in variables, passed as arguments to other functions or used as return values.

The function type in Kotlin is represented as follows:

(Parameter types) -> Return type 

First, let's consider a simple function that has no parameters (takes no arguments) and returns no return value.

fun greeting() {
    println("Hello World!")
}

This function has type Unit.

() -> Unit

The empty pair of parentheses () indicates that there are no parameters. The arrow -> separates the parameter list from the return type. The key word Unit indicates that the function does not return a value, but only performs an action. The guy Unit is a special type in Kotlin that is used as the return type of functions that do not have a return.

And now let's look at several functions that have predefined return values. Based on this information, we try to derive their function types.

fun add(a: Int, b: Int): Int {
    return a + b
}
// Funktionstyp: (Int, Int) -> Int

fun greeting(name: String): String {
    return "Hello, $name!"
}
// Funktionstyp: (String) -> String

fun isTrue(value: Boolean): String {
    return if (value) "The value is true." else "The value is false."
}
// Funktionstyp: (Boolean) -> String

Knowing the type of a function tells us about the expected input parameters and the returned value of a function. This helps in the correct use of functions, allows for better code readability, and reduces the likelihood of errors.