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The Functor typeclass abstracts the ability to map over the computational context of a type constructor. Examples of type constructors that can implement instances of the Functor typeclass include Option, NonEmptyList, List and many other datatypes that include a map function with the shape fun F<A>.map(f: (A) -> B): F<B> where F refers to Option, List or any other type constructor whose contents can be transformed.


Oftentimes we find ourselves in situations where we need to transform the contents of some datatype. Functor#map allows us to safely compute over values under the assumption that they’ll be there returning the transformation encapsulated in the same context.

Consider both Option and Try:

Option<A> allows us to model absence and has two possible states, Some(a: A) if the value is not absent and None to represent an empty case.

In a similar fashion Try<A> may have two possible cases Success(a: A) for computations that succeed and Failure(e: Throwable) if they fail with an exception.

Both Try and Option are example datatypes that can be computed over transforming their inner results.

import arrow.*
import arrow.core.*

Try { "1".toInt() }.map { it * 2 }
Option(1).map { it * 2 }
// Some(2)

Both Try and Option include ready to use Functor instances:

val optionFunctor = Option.functor()
val tryFunctor = Try.functor()

Mapping over the empty/failed cases is always safe since the map operation in both Try and Option operate under the bias of those containing success values

Try { "x".toInt() }.map { it * 2 }
none<Int>().map { it * 2 }
// None

Main Combinators

Kind<F, A>#map

Transforms the inner contents

fun <A, B> Kind<F, A>.map(f: (A) -> B): Kind<F, B> { Option(1).map { it + 1 } }
// Some(2)


Lift a function to the Functor context so it can be applied over values of the implementing datatype

fun <A, B> lift(f: (A) -> B): (Kind<F, A>) -> Kind<F, B>

val lifted = optionFunctor.lift({ n: Int -> n + 1 })
// Some(2)

Other combinators

For a full list of other useful combinators available in Functor see the Source


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

Creating your own Functor instances

Arrow already provides Functor instances for most common datatypes both in Arrow and the Kotlin stdlib. Oftentimes you may find the need to provide your own for unsupported datatypes.

You may create or automatically derive instances of functor for your own datatypes which you will be able to use in the context of abstract polymorphic code as demonstrated in the example above.

See Deriving and creating custom typeclass

Data Types

The following datatypes in Arrow provide instances that adhere to the Functor typeclass.

Additionally all instances of Applicative, Monad and their MTL variants implement the Functor typeclass directly since they are all subtypes of Functor