Getter

beginner

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 Foo(val bar: Int)

val barGetter = Getter(Foo::bar)

barGetter.get(Foo(5))
// 5
import arrow.data.*

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 to safely getting its focus.

import arrow.core.*
import arrow.optics.instances.*

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<Foo>, Int> = NonEmptyList.head<Foo>() compose barGetter
firstBar.get(Foo(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