arrow-core-data / arrow.core / NonEmptyList / <init>
NonEmptyList(head: A, tail:
List
<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-data
module under the import arrow.core.NonEmptyList
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)
}
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)
}
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
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)
}
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")
}
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")
}
NonEmptyList
is used to model lists that guarantee at least one elementNonEmptyList
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
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