fold on a List

object Raindrops {
    fun convert(n: Int) =
            listOf(3 to "Pling", 5 to "Plang", 7 to "Plong")
                    .fold("") { output, pair ->
                        output + if (n % pair.first == 0) pair.second else ""
                    }
                    .ifEmpty { n.toString() }
}

An object declaration is used to define Raindrops as essentially a singleton object instantiation of the class. This is sufficient, since there is no object state that needs to change with each call of the convert method.

The convert function is implemented with a call to a chain of functions which returns a single value. Although the convert function has multiple lines, it consists only of the one expression resulting from the function chain, so it is defined using single-expression function syntax, with the curly braces omitted from the convert function call and the return type inferred.

The first function is the listOf method to create a List of Pair values, using the to keyword to associate a number with its sound.

The fold method is called on the List. Its accumulating value is initialized with an empty String. The lambda of fold takes the accumulating String value as well as the Pair of each List element being iterated.

If the input number is evenly divisible by the number in the current Pair being iterated, then the sound from the Pair is concatenated to the accumulating String, otherwise, an empty string is concatenated to it. The accumulating String is returned to the next iteration of fold.

When fold has iterated through all of the List elements, the StringBuilder is the result. The convert function returns the result of calling the ifEmpty method on the StringBuilder. If the StringBuilder content is empty, then it outputs the input number converterd to a String. Otherwise, it returns the content of the StringBuilder as a String.