singleOrNull

public fun <T> List<T>.singleOrNull(): T? {
    return if (size == 1) this[0] else null
}


val list = listOf(1, 2, 3, 4, 5)
val singleList = listOf(1)
val emptyList = listOf<Int>()

println(list.singleOrNull())  // null
println(singleList.singleOrNull())  // 1
println(emptyList.singleOrNull())  // null

public inline fun <T> Iterable<T>.singleOrNull(predicate: (T) -> Boolean): T? {
    var single: T? = null
    var found = false
    for (element in this) {
        if (predicate(element)) {
            if (found) return null
            single = element
            found = true
        }
    }
    if (!found) return null
    return single
}


val list = listOf(1, 2, 3, 4, 5)
println(list.singleOrNull{ it > 3 })  // null
println(list.singleOrNull{ it > 4 })  // 5
println(list.singleOrNull{ it > 5 })  // null