A Setter is an optic that can see into a structure and set or modify its focus.

It is a generalisation of Functor#map. Given a Functor<F> we can apply a function (A) -> B to Kind<F, A> and get Kind<F, B>. We can think of Kind<F, A> as a structure S that has a focus A. So given a PSetter<S, T, A, B> we can apply a function (A) -> B to S and get T.

  • Kind<F, A>, f: (A) -> B) -> Kind<F, B>
  • PSetter.modify(s: S, f: (A) -> B): T

You can get a Setter for any existing Functor.

import arrow.*
import arrow.optics.*

val setter: Setter<ListKOf<Int>, Int> = Setter.fromFunctor()
setter.set(listOf(1, 2, 3, 4).k(), 5)
// ListK(list=[5, 5, 5, 5])
setter.modify(listOf(1, 2, 3, 4).k()) { int -> int + 1 }
// ListK(list=[2, 3, 4, 5])

To create your own Setter you need to define how to apply (A) -> B to S.

A Setter<Foo, String> can set and modify the value of Foo. So we need to define how to apply a function (String) -> String to Foo.

data class Foo(val value: String)

val fooSetter: Setter<Foo, String> = Setter { f: (String) -> String ->
    { foo: Foo ->
        val fValue = f(foo.value)
        foo.copy(value = fValue)
val uppercase: (String) -> String = String::toUpperCase
fooSetter.modify(Foo("foo"), uppercase)
// Foo(value=FOO)
val lift = fooSetter.lift(uppercase)
// Foo(value=FOO)


Unlike a regular set function a Setter composes. Similar to a Lens we can compose Setters to focus into nested structures and set or modify a value.

data class Bar(val foo: Foo)

val barSetter: Setter<Bar, Foo> = Setter { modifyFoo ->
    { bar ->
        val modifiedFoo = modifyFoo(
        bar.copy(foo = modifiedFoo)

(barSetter compose fooSetter).modify(Bar(Foo("some value")), String::toUpperCase)
// Bar(foo=Foo(value=SOME VALUE))

Setter can be composed with all optics but Getter and Fold. It results in the following optics.

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

Polymorphic setter

When dealing with polymorphic types we can also have polymorphic setters that allow us to morph the type of the focus. Previously when we used a Setter<ListKOf<Int>, Int> it was able to morph the Int values in the constructed type ListK<Int>. With a PSetter<ListKOf<Int>, ListKOf<String>, Int, String> we can morph an Int value to a String value and thus also morph the type from ListK<Int> to ListK<String>.

val pSetter: PSetter<ListKOf<Int>, ListKOf<String>, Int, String> = PSetter.fromFunctor()
pSetter.set(listOf(1, 2, 3, 4).k(), "Constant")
// ListK(list=[Constant, Constant, Constant, Constant])
pSetter.modify(listOf(1, 2, 3, 4).k()) {
    "Value at $it"
// ListK(list=[Value at 1, Value at 2, Value at 3, Value at 4])


Arrow provides SetterLaws in the form of test cases for internal verification of lawful instances and third party apps creating their own setters.