If you add infix to a qualifying function declaration, you can use it via infix notation, where you skip the parentheses for the parameter to the call and skip the dot to indicate the receiver of the call. Instead, the function name is simply bracketed by spaces, causing it to look like an operator.

To qualify, the function:

• Must have exactly one parameter (including no varargs and no default values)

• Must be on an ordinary object, directly or via an extension function (e.g., no top-level functions)

• the documentation
• Elements of Kotlin