Kotlin short circuit map{}.firstOrNull{}

Question

I need to map a list and retrieve the first non null element, and I need the map operation to be short circuited like it should be in Java 8 streams API. Is there a ready way to do this in Kotlin, without Java 8 streams?

I created my own extension method to do this:

fun <T, R> Iterable<T>.firstNonNullMapping(transform: (T) -> R?): R? {
    for (element in this) {
        val result = transform(element)
        if (result != null) {
            return result
        }
    }
    return null
}

A test proves that this works

val firstNonNullMapping = listOf(null, 'a', 'b')
        .firstNonNullMapping {
            assertNotEquals(it, 'b') // Mapping should be stopped before reaching 'b'
            it
        }
assertEquals(firstNonNullMapping, 'a')

IntelliJ, however, suggest that I replace my for loop with the much neater

return this
        .map { transform(it) }
        .firstOrNull { it != null }

Problem is that this will map all elements of the iterable, and it is essential to my use case that is stops at the first non null element.

Answer 1

Kotlin has lazily evaluated sequences that correspond to Java 8 streams, instead of invoking stream() on a collection, you invoke asSequence():

return this
        .asSequence()
        .mapNotNull { transform(it) }
        .firstOrNull()