Every combines the powers of Traversal and Fold. It can focus into a structure S to see all its foci A.


Every can easily be created given a Traversal and Fold instance.

import arrow.optics.*
import arrow.optics.typeclasses.*

val every: Every<List<Int>, Int> = Every.from(Traversal.list<Int>(), Fold.list<Int>())

every.lastOrNull(listOf(1, 2, 3))
// 3

Creating your own Every instances

Arrow provides Every instances for some common datatypes in Arrow and Kotlin Std. You can find them on the companion object of Every.

You may create instances of Every for your own datatypes, which you will be able to use as demonstrated in the example above. This can be done by implementing both foldMap and modify yourself.

import arrow.typeclasses.Monoid

fun <A> PEvery.Companion.list(): Every<List<A>, A> = object : Every<List<A>, A> {
  override fun <R> foldMap(M: Monoid<R>, s: List<A>, map: (A) -> R): R =
    M.run { s.fold(empty()) { acc, a -> acc.combine(map(a)) } }

  override fun modify(s: List<A>, map: (focus: A) -> A): List<A> =

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