How to Create Infix Function in Kotlin: Simple Guide

In Kotlin, you create an infix function by marking a function with the infix keyword and defining it as a member or extension function with a single parameter. This allows you to call the function using infix notation without dots or parentheses, making code more readable.

Syntax

An infix function must be a member function or an extension function with exactly one parameter. It is marked with the infix keyword before the function name.

• infix: keyword to enable infix call style
• fun: declares a function
• functionName: name of the function
• parameter: single parameter the function takes

Example syntax:

kotlin

infix fun ClassName.functionName(parameterType) { /* function body */ }

Example

This example shows how to create an infix function shout for the String class that repeats the string a given number of times. It demonstrates calling the function using infix notation for cleaner code.

kotlin

infix fun String.shout(times: Int): String {
    return this.repeat(times).uppercase()
}

fun main() {
    val result = "hello" shout 3
    println(result)
}

Output

HELLOHELLOHELLO

Common Pitfalls

Common mistakes when creating infix functions include:

• Defining the function with zero or more than one parameter (infix functions must have exactly one parameter).
• Not marking the function with the infix keyword.
• Trying to use infix notation on functions that are not infix.

Example of a wrong and right way:

kotlin

// Wrong: no infix keyword
fun Int.add(x: Int): Int = this + x

// Right: with infix keyword
infix fun Int.add(x: Int): Int = this + x

fun main() {
    // This will not compile: 1 add 2
    // Correct usage:
    val sum = 1 add 2
    println(sum)
}

Output

3

Quick Reference

Summary tips for infix functions:

• Use infix keyword before function name.
• Function must have exactly one parameter.
• Can be member or extension function.
• Call using object functionName argument syntax without dots or parentheses.

Key Takeaways

Mark functions with the infix keyword to enable infix call style.

Infix functions must have exactly one parameter.

You can define infix functions as member or extension functions.

Call infix functions without dots or parentheses for cleaner syntax.

Avoid missing the infix keyword or using wrong parameter counts.