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NonEmptyList is a data type used in Λrrow to model ordered lists that have at least one value. NonEmptyList is available in the arrow-core-data module under the import arrow.core.NonEmptyList

// gradle
compile "io.arrow-kt:arrow-core-data:$arrow_version"
// namespace
import arrow.core.*


A NonEmptyList guarantees the list always has at least 1 element.

nonEmptyListOf(1, 2, 3, 4, 5) // NonEmptyList<Int>
nonEmptyListOf(1, 2) // NonEmptyList<Int>
//nonEmptyListOf() // does not compile

Unlike List#[0], NonEmptyList#head is a safe operation that guarantees no exception throwing.

nonEmptyListOf(1, 2, 3, 4, 5).head


When we fold over a NonEmptyList, we turn a NonEmptyList< A > into B by providing a seed value and a function that carries the state on each iteration over the elements of the list. The first argument is a function that addresses the seed value, this can be any object of any type which will then become the resulting typed value. The second argument is a function that takes the current state and element in the iteration and returns the new state after transformations have been applied.

fun sumNel(nel: NonEmptyList<Int>): Int =
  nel.foldLeft(0) { acc, n -> acc + n }

sumNel(nonEmptyListOf(1, 1, 1, 1))


map allows us to transform A into B in NonEmptyList< A >

nonEmptyListOf(1, 1, 1, 1).map { it + 1 }


flatMap allows us to compute over the contents of multiple NonEmptyList< * > values

val nelOne: NonEmptyList<Int> = nonEmptyListOf(1)
val nelTwo: NonEmptyList<Int> = nonEmptyListOf(2)

nelOne.flatMap { one -> { two ->
    one + two

Applicative Builder

Λrrow contains methods that allow you to preserve type information when computing over different NonEmptyList typed values.

import arrow.core.*
import java.util.*

data class Person(val id: UUID, val name: String, val year: Int)

// Note each NonEmptyList is of a different type
val nelId: NonEmptyList<UUID> = nonEmptyListOf(UUID.randomUUID(), UUID.randomUUID())
val nelName: NonEmptyList<String> = nonEmptyListOf("William Alvin Howard", "Haskell Curry")
val nelYear: NonEmptyList<Int> = nonEmptyListOf(1926, 1900), nelYear) { id, name, year ->
  Person(id, name, year)


  • NonEmptyList is used to model lists that guarantee at least one element
  • We can easily construct values of NonEmptyList with nonEmptyListOf
  • foldLeft, map, flatMap, and others are used to compute over the internal contents of a NonEmptyList value.
  • NonEmptyList.mapN(..) { ... } can be used to compute over multiple NonEmptyList values preserving type information and abstracting over arity with map

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