Types of Functions in Kotlin

inline,infix,operator

Introduction

A Function is a block of code that performs a specific task. It can be called from anywhere in a program. Kotlin functions are declared using the keyword fun.

fun double(x: Int): Int {
    return 2 * x
}

Method are functions used to perform certain action.

Calling a function

val result = double(2)
print(result)
output
4

What is a function parameter? this is a variable defined during function declaration or definition. It’s data passed into a function, it is specified after the function name inside the parentheses. It is defined using pascal notation name:type.
Parameters are separated by commas.

fun powerOf(number: Int, exponent: Int): Int { /*...*/ }

Functional parameters can have default values. It is set by appending = to the type.

fun calc(
    age: Int = 0,
    len: Int = b.size,
) { /*...*/ }

Note: Overriding methods always use the base method’s default parameter. When ovveriding a method that has default parameter values, the default parameter values must be ommitted from the signature.

open class A {
    open fun foo(i: Int = 10) { /*...*/ }
}

 class B : A() {
    override fun foo(i: Int) { /*...*/ }  // No default value is allowed.
}

The open keyword with the class means the class is open for the extension meaning that we can create a subclass of that open class.

Return Values
To return a value from a function we used the keyword return and specify the return type after the functionL parenthesis.

fun myFunction(x: Int): Int {
  return (x + 5)
}

fun main() {
  var result = myFunction(3)
  println(result)
}

You can use the = operator instead of return without specifying a type

fun myFunction(x: Int, y: Int) = x + y

fun main() {
  var result = myFunction(3, 5)
  println(result)
}

Inline Function

It instructs the compiler to insert the complete body of the function wherever that function gets used in the code.
Adv: Function overhead does not occur.
When to use:

• When the function code is very small

When not to use inline

• when the function code is large and called from so many places. It will be a bad idea since the code will be repeated again and again.

fun guide(){
  print("guide start")
teach()
print("guide end")
}

inline fun teach(){
print("teach")
}

Infix Function

This function is called without any dot or parenthesis .

What makes an infix function

• Member function
• Has single parameter
• Is marked with infix keyword

Example

Or method

Operator Function

This is a function which is declared with a special keyword operator and are not different from regular functions when called via function calls.

It allows instance of a class to be called as if they were functions.

The invoke function is invoked when you use the parenthesis () syntax as the object of the class . Reference