Infix functions in Kotlin

Kotlin is known for being expressive and concise. Recently, I reviewed Kotlin again and one of the cool features I encountered was the infix functions. This feature helps our code look more like natural language.

In this blog, let's explore together what infix functions are, how to create them, and most importantly why they work the way they do.

📚 1. What is an infix function?

To understand the feature easier, let's start with the normal way to call function that we almost do every day:

/**
 * An extension function of String.
 * Repeat the string N times
 */
fun String.repeatN(times: Int): String {
    require(times >= 0)
    return this.repeat(times)
}

fun main() {
    val kotlin = "Kotlin."
    print(kotlin.repeatN(3)) // Output: Kotlin.Kotlin.Kotlin.
}

With this normal way, we will need to use the dot . and the parentheses () when calling it. But did you know that you can call this kind of function without the dot and the parentheses? The answer is Yes! In Kotlin you can write the code like this:

// The only difference: The function signature starts with the "infix" keyword
infix fun String.repeatN(times: Int): String {
    require(times >= 0)
    return this.repeat(times)
}

fun main() {
    val kotlin = "Kotlin."
    // Normal way
    println(kotlin.repeatN(3)) // Output: Kotlin.Kotlin.Kotlin.

    // Use infix syntax
    println(kotlin repeatN 3) // Output: Kotlin.Kotlin.Kotlin.
}

It looks more natural! Here is the syntax to call the infix function:

receiver infixFunctionName parameter
   |             |             |      
 kotlin       repeatN          3

So if someone asks you "What is an infix function in Kotlin?", you may answer:

But it will be better if we can give more clarification. The fact is we can not use the infixkeyword for all functions. Did you notice that I previously highlighted the phrase "this kind of function"? That means a function can only be an infix function if it satisfies some rules. Let's move on and see the rules immediately.

📚 2. The Rules of Using Infix Functions

Rule #1: They must be member functions or extension functions.

Rule #2: They must have a single parameter and must not accept variable number of arguments (vararg)

Rule #3: The infix functions must explicitly use infix keyword

Let's div into each rule and understand why they exist.

Rule #1: They must be member functions or extension functions

This rule means an infix function either:

• Be defined inside a class (member function)

OR

• Extend and existing type (extension function)

Let's look at the example we created above:

fun main() {
    val kotlin = "Kotlin."
    // Normal way
    println(kotlin.repeatN(3)) // Output: Kotlin.Kotlin.Kotlin.

    // Use infix syntax
    println(kotlin repeatN 3) // Output: Kotlin.Kotlin.Kotlin.
}

Here in this example:

• The variable kotlin is a receiver

• The repeatN is the infix function

• The number 3 is the sing parameter passed to the function

That's why we can not use infix for a top-level function (not inside a class). If you do that, you defintely get an error:

Rule #2: They must have a single parameter and must not accept variable number of arguments (vararg)

Kotlin requires exactly one argment when calling an infix function (number 3 in our example). Similarly vararg is not allowed because with vararg we can pass multiple arguments to the function.

Rule #3: The infix functions must explicitly use infix keyword

This is quite obvious, because if we don't declare the function with infix Kotlin will treat it as a normal function and we can not call it using infix syntax.

🔥 3. Common built-in infix functions in Kotlin

🚫 4. When not to use infix functions

Although infix function is cool and help the code looks more natural (if used wisely), overusing it can make the code less readable and hard to use. For example:

user update data // ← What exactly is happening here?

data apply 5 // ← What is "apply" doing here?

Conclusion

Infix functions are Kotlin’s way of letting your code speak like a sentence. Used wisely, they can boost readability and charm. Used carelessly, they can make code look like riddles. When your function acts like a relationship between two things, infix style might just be the best fit.

Happy coding! See you in another blog!