Infix Extension Functions

Writing an Infix Extension

Combining infix with fun ReceiverType.name(param: Type) adds an operator-like call syntax to an existing type, usable as receiver name argument.

Example: Writing an Infix Extension

infix fun Int.plusPercent(percent: Int): Int = this + (this * percent / 100)

fun main() {
    val price = 100
    println("Price with 20% added: ${price plusPercent 20}")
}

Building Readable Domain Logic

Infix extensions are often used to give domain-specific operations a natural, sentence-like reading, which can make specialized code much clearer than a plain method call.

Example: Building Readable Domain Logic

infix fun Int.daysAfter(startDay: Int): Int = (startDay + this) % 7

fun main() {
    val monday = 1
    println("3 days after Monday: ${3 daysAfter monday}")
}

Infix Extensions on Custom Types

Infix extensions work on your own classes too, letting you define natural-reading operations between instances of types you control.

Example: Infix Extensions on Custom Types

data class Vector(val x: Int, val y: Int)

infix fun Vector.dot(other: Vector): Int = x * other.x + y * other.y

fun main() {
    val v1 = Vector(2, 3)
    val v2 = Vector(4, 5)
    println("Dot product: ${v1 dot v2}")
}

When to Avoid Infix Style

Infix notation reads best for operations that resemble a natural verb or relationship between two values; for anything else, a regular method call with a clear name is more appropriate.

Note: Prefer a regular method name like combineWith() over infix style if the relationship being expressed isn't intuitive from the words alone.

Example: When to Avoid Infix Style

infix fun String.repeatedTwice(separator: String): String = "$this$separator$this"

fun main() {
    println("Hi" repeatedTwice ", ")
}