Implicitly map to set in Kotlin

Question

After I map through one Set, I have to explicitly cast it back to Set. For example:

fun getNewSet(oldSet: Set<String>): Set<String> { return oldSet         .map{ getNewStringFromOld(it) }         .toSet() } 

What is the proper way to map through the set without explicitly casting it back to Set?

Answer 1

Looking at the library code, the only other way to do this would be to call mapTo which takes a destination collection:

oldSet.mapTo(HashSet<String>()) { getNewStringFromOld(id) } 

I'm not sure the default is wrong. The problem is that map might be used in a way that results in a list of non-unique values. If there was a special version of Set.map() that returned a set, you couldn't use map in that way - any non-unique value would replace the value in the result. I could certainly see the case being made that map should always result in a collection that is the same size as the source collection, and if map created a Set, this wouldn't always be the case.

If you do this often, perhaps just create your own extension function:

public inline fun <T, R> Iterable<T>.mapToSet(transform: (T) -> R): Set<R> {   return mapTo(HashSet<R>(), transform) }