Infix Functions

An infix function is a function that appears in between two variables. Kotlin makes it easy to create functions like this.

Key points

Infix functions are declared similarly to extension functions
They are declared by writing infix fun at the start of the function definition
When applied like 1 plus 2, the function belongs to the variable on the left (1) and the variable on the right (2) is the function parameter
Can be used to create DSLs

Imagine that the + operator doesn’t exist and you want a function to add two integers, like this:

val x = 1 plus 1

Just write an infix function like this:

infix fun Int.plus(that: Int) = this + that

Here’s how it looks in the Kotlin REPL:

> 1 plus 1
2

Note the infix keyword:

infix fun Int.plus(that: Int) = this + that
-----

Notice that it’s an extension function for the Int type:

infix fun Int.plus(that: Int) = this + that
          ---

By changing the function:

infix fun Int.plus(that: Int): Int {
    println("this: ${this}, that: ${that}")
    return this + that
}

> 1 plus 2
this: 1, that: 2
3

You can see: