## NonEmptyList

`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.*
``````

## of

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
``````

## 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.

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

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

## map

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

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

## flatMap

`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 ->
nelTwo.map { 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)

nelId.zip(nelName, nelYear) { id, name, year ->
Person(id, name, year)
}
``````

### 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.
• `NonEmptyList.mapN(..) { ... }` can be used to compute over multiple `NonEmptyList` values preserving type information and abstracting over arity with `map`

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