Infix notation in function

Another Kotlin treasure that I have not known is the ability to write infix notation in function. We use infix notation in most arithmetic operation such as a + b. The infix notation is where the operator (+) is used between the operands (a) and (b). In infix notation, only functions defined using the infix keyword can be called this way.

There are 3 things to make this infix notation.

1. They must be member functions or extension functions.
2. They must have a single parameter.
3. The parameter must not accept variable number of arguments and must have no default value.

class MyStringCollection {
    infix fun add(s: String) { /*...*/ }

    fun build() {
        this add "abc"   // Correct
        add("abc")       // Correct
        //add "abc"        // Incorrect: the receiver must be specified
    }
}