[JAVA] I want to use Clojure's convenient functions in Kotlin

Unique to functional languages? I investigated & created how to use the list operation function of Kotlin.

Building a list


(def list1 (range 1 6))
(def list2 [6 6 7 8 7 7])


val list1 = IntRange(1, 5)
val list2 = listOf(6, 6, 7, 8, 7, 7)

Kotlin is a Closed Range, isn't it?

List concatenation


(conj list2 10)
(concat list1 list2)


list2 + 10
list1 + list2

Infinite list


(->> (repeat 123) (take 3))
(->> (iterate #(* 4 %) 3) (take 3))


generateSequence { 123 }.take(3)
generateSequence(3) { it * 4 }.take(3)

List operation (basic)


filter, remove


(filter even? list1)
(remove even? list1)


list1.filter { it % 2 == 0 }
list1.filterNot { it % 2 == 0 }



(some #(when (even? %) %) list1)


list1.find { it % 2 == 0 }
list1.first { it % 2 == 0 }

If none match the condition, find returns null and first raises an exception.



(map #(* % 2) list1)
(keep #(when (> % 3) (* % 2)) list1)
(map-indexed #(* %1 %2) list1)
(keep-indexed #(when (> % 3) (* % %2)) list1)
(mapcat #(list % (* % %)) list1)


list1.map { it * 2 }
list1.mapNotNull { if (it > 3) it * 2 else null }
list1.mapIndexed { i, v -> i * v }
list1.mapIndexedNotNull { i, v -> if (i > 3) i * v else null }
list1.flatMap { listOf(it, it * it) }



(reduce #(* %1 %2) input)
(reduce #(* %1 %2) 10 input)
(reduce #(if (> % 20) (reduced %) (* % %2)) input)


list1.reduce { acc, v -> acc * v }
list1.fold(10) { acc, v -> acc * v },
list1.reduce { acc, v -> if (acc > 20) [email protected] acc else acc * v }

If you want to stop reduce in the middle, you can use labeled return.

List operation (application)

The number of things that must be made will gradually increase.

Delete duplicates, delete consecutive duplicates


(distinct list2)
(dedupe list2)


fun <T> Iterable<T>.dedupe(): List<T> {
    if (count() == 0) return toList()
    return fold(listOf(first())) { acc, v -> if (v != acc.last()) acc + v else acc }




(shuffle list2)


import java.util.Collections

fun <T> Iterable<T>.shuffle(): List<T> = toMutableList().apply { Collections.shuffle(this) }


apply is convenient, isn't it?

Construction of grouping map and appearance count map


(group-by #(mod % 2) list2)
(frequencies list2)


fun <T> Iterable<T>.frequencies(): Map<T, Int> = groupingBy { it }.eachCount()

list2.groupBy { it % 2 }

Value scissors


(interleave list1 list2)
(interpose 999 list1)


fun <T> Iterable<T>.interleave(list: Iterable<T>): List<T> = zip(list).flatMap { it.toList() }

fun <T> Iterable<T>.interpose(sep: T): List<T> =
        zip(Collections.nCopies(count(), sep)).flatMap { it.toList() }.dropLast(1)


Cumulative calculation


(reductions * list1)
(reductions * 10 list1)


fun <T> Iterable<T>.reductions(function: (T, T) -> T): List<T> {
    if (count() <= 1) return toList()
    return drop(1).reductions(first(), function)

fun <T, R> Iterable<T>.reductions(init: R, function: (R, T) -> R): List<R> {
    if (count() == 0) return listOf(init)
    return fold(listOf(init)) { acc, v -> acc + function(acc.last(), v) }

list1.reductions { i, j -> i * j }
list1.reductions(10, Int::times)

Split the list every n


(partition 3 list1)
(partition-all 3 list1)
(partition 4 2 list1)
(partition-all 4 2 list1)
(partition 4 2 [:a :b] list1)


fun <T> Iterable<T>.partition(n: Int, step: Int = n, pad: List<T> = listOf<T>()): List<List<T>> {
    var tmp = toList()
    return mutableListOf<List<T>>().apply {
        while (tmp.count() >= n) {
            tmp = tmp.drop(step)
        if (!pad.isEmpty() && !tmp.isEmpty())
            add((tmp + pad).take(n))

fun <T> Iterable<T>.partitionAll(n: Int, step: Int = n): List<List<T>> {
    var tmp = toList()
    return mutableListOf<List<T>>().apply {
        while (!tmp.isEmpty()) {
            tmp = tmp.drop(step)


The partition function originally in Kotlin returns Pair <List <T>, List <T >>, so the usage is different.

Split the list according to conditions


(partition-by identity list2)
(partition-by #(> % 7) list2)


fun <T, R> Iterable<T>.partitionBy(function: (T) -> R): List<List<T>> {
    if (count() == 0) return listOf()

    return drop(1).fold(listOf(listOf(first()))) { acc, v ->
        if (function(acc.last().last()) != function(v))
            acc + listOf(listOf(v))
            acc.dropLast(1) + listOf(acc.last() + v)

list2.partitionBy { it }
list2.partitionBy { it > 7 }

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