Λrrow Optics

Getter

A Getter is an optic that can focus into a structure and get its focus. It can be seen as a wrapper of a get function (S) -> A that can be composed with other optics.

Creating a Getter can be done by referencing a property of a data classes or by providing a function.

import arrow.optics.*
import arrow.*

data class Player(val health: Int)

val healthGetter = Getter(Player::health)
val player = Player(75)
healthGetter.get(player)
// 75

import arrow.core.*

fun <T> nonEmptyListHead() = Getter<NonEmptyList<T>, T> {
    it.head
}

nonEmptyListHead<Int>().get(NonEmptyList.of(1, 2, 3, 4))
// 1

Or, from any of the optics defined in arrow-optics that allow getting its focus safely.

import arrow.core.*
import arrow.optics.extensions.*

val headGetter: Getter<NonEmptyList<String>, String> = NonEmptyList.head<String>().asGetter()
val tupleGetter: Getter<Tuple2<String, Int>, String> = Tuple2.first<String, Int>().asGetter()

Composition

Unlike a regular get function, a Getter composes. Similar to a Lens, we can compose Getters to create telescopes and zoom into nested structures.

val firstBar: Getter<NonEmptyList<Player>, Int> = NonEmptyList.head<Player>() compose healthGetter
firstBar.get(Player(5).nel())
// 5

Getter can be composed with Getter, Iso, Lens, and Fold, and the composition results in the following optics:

	Iso	Lens	Prism	Optional	Getter	Setter	Fold	Traversal
Getter	Getter	Getter	X	X	Getter	X	Fold	X