//arrow-core/arrow.core/NonEmptyList

NonEmptyList

common class NonEmptyList<out A>(val head: A, val tail: List<A>) : AbstractList<A>

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

nonEmptyListOf

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

import arrow.core.nonEmptyListOf

val value =
//sampleStart
 // nonEmptyListOf() // does not compile
 nonEmptyListOf(1, 2, 3, 4, 5) // NonEmptyList<Int>
//sampleEnd
fun main() {
 println(value)
}

head

Unlike List[0], NonEmptyList.head it’s a safe operation that guarantees no exception throwing.

import arrow.core.nonEmptyListOf

val value =
//sampleStart
 nonEmptyListOf(1, 2, 3, 4, 5).head
//sampleEnd
fun main() {
 println(value)
}

foldLeft

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.

import arrow.core.NonEmptyList
import arrow.core.nonEmptyListOf

//sampleStart
fun sumNel(nel: NonEmptyList<Int>): Int =
 nel.foldLeft(0) { acc, n -> acc + n }
val value = sumNel(nonEmptyListOf(1, 1, 1, 1))
//sampleEnd
fun main() {
 println("value = $value")
}

map

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

import arrow.core.nonEmptyListOf

val value =
//sampleStart
 nonEmptyListOf(1, 1, 1, 1).map { it + 1 }
//sampleEnd
fun main() {
 println(value)
}

Combining NonEmptyLists

flatMap

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

import arrow.core.NonEmptyList
import arrow.core.nonEmptyListOf

//sampleStart
val nelOne: NonEmptyList<Int> = nonEmptyListOf(1, 2, 3)
val nelTwo: NonEmptyList<Int> = nonEmptyListOf(4, 5)

val value = nelOne.flatMap { one ->
 nelTwo.map { two ->
   one + two
 }
}
//sampleEnd
fun main() {
 println("value = $value")
}

zip

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

import arrow.core.NonEmptyList
import arrow.core.nonEmptyListOf
import arrow.core.zip
import java.util.UUID

//sampleStart
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)

val value = nelId.zip(nelName, nelYear) { id, name, year ->
 Person(id, name, year)
}
//sampleEnd
fun main() {
 println("value = $value")
}

Summary

• 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.
• a.zip(b, c) { ... } can be used to compute over multiple NonEmptyList values preserving type information and abstracting over arity with zip

Constructors

Types

Functions

Properties

Extensions