How to make a reference to a sorting function from...

# getting-started

[David Kubecka]

How to make a reference to a sorting function from stdlib? My use case is as follows:

fun List<String>.sort(order: String) {
  val sortFun = if (order == "asc") List<String>::sortedBy else List<String>::sortedByDescending
}

I can't figure out the right typing for sortFun

[CLOVIS]

Something like this?

fun List<String>.sort(order: String) =
	if (order == "asc") sorted() else sortedDescending()

[CLOVIS]

If you want to know the type of a variable, you can use CTRL SHIFT P in IntelliJ—but I don't understand why you would need this in your example

[Sam]

The reason the example doesn't work is because sortedBy and sortedByDescending have two type parameters, one for the list items and another for the sort key. The first can be inferred from context, but the second would need to be specified.

[Sam]

The type should be ((List<T>, selector: (T) -> R?), where T is the list items type and R is some Comparable to act as the sort key

[Sam]

So for example, this works:

fun <T, R: Comparable<R>> List<T>.sort(order: String) {
  val sortFun: ((List<T>, selector: (T) -> R?) -> List<T>) =
    if (order == "asc") List<T>::sortedBy else List<T>::sortedByDescending
}

[David Kubecka]

Thanks, Sam. The type is quite complicated so in this case a better option would probably be to duplicate the selector function, i.e.

if (order == "asc") {
  sortedBy { it.sortingField }
} else {
  sortedByDescending { it.sortingField }
}

Sometimes deduplication efforts are counterproductive 🙂