Option

sealed class Option<out A>

If you have worked with Java at all in the past, it is very likely that you have come across a NullPointerException at some time (other languages will throw similarly named errors in such a case). Usually this happens because some method returns null when you weren’t expecting it and, thus, isn’t dealing with that possibility in your client code. A value of null is often abused to represent an absent optional value. Kotlin tries to solve the problem by getting rid of null values altogether, and providing its own special syntax Null-safety machinery based on ?.

Arrow models the absence of values through the Option datatype similar to how Scala, Haskell, and other FP languages handle optional values.

Option<A> is a container for an optional value of type A. If the value of type A is present, the Option<A> is an instance of Some<A>, containing the present value of type A. If the value is absent, the Option<A> is the object None.

import arrow.core.Option
import arrow.core.Some
import arrow.core.none

//sampleStart
val someValue: Option<String> = Some("I am wrapped in something")
val emptyValue: Option<String> = none()
//sampleEnd
fun main() {
 println("value = $someValue")
 println("emptyValue = $emptyValue")
}

Let’s write a function that may or may not give us a string, thus returning Option<String>:

import arrow.core.None
import arrow.core.Option
import arrow.core.Some

//sampleStart
fun maybeItWillReturnSomething(flag: Boolean): Option<String> =
 if (flag) Some("Found value") else None
//sampleEnd

Using getOrElse, we can provide a default value "No value" when the optional argument None does not exist:

import arrow.core.None
import arrow.core.Option
import arrow.core.Some
import arrow.core.getOrElse

fun maybeItWillReturnSomething(flag: Boolean): Option<String> =
 if (flag) Some("Found value") else None

val value1 =
//sampleStart
 maybeItWillReturnSomething(true)
    .getOrElse { "No value" }
//sampleEnd
fun main() {
 println(value1)
}

import arrow.core.None
import arrow.core.Option
import arrow.core.Some
import arrow.core.getOrElse

fun maybeItWillReturnSomething(flag: Boolean): Option<String> =
 if (flag) Some("Found value") else None

val value2 =
//sampleStart
 maybeItWillReturnSomething(false)
  .getOrElse { "No value" }
//sampleEnd
fun main() {
 println(value2)
}

Checking whether option has value:

import arrow.core.None
import arrow.core.Option
import arrow.core.Some

fun maybeItWillReturnSomething(flag: Boolean): Option<String> =
 if (flag) Some("Found value") else None

 //sampleStart
val valueSome = maybeItWillReturnSomething(true) is None
val valueNone = maybeItWillReturnSomething(false) is None
//sampleEnd
fun main() {
 println("valueSome = $valueSome")
 println("valueNone = $valueNone")
}

Creating a Option<T> of a T?. Useful for working with values that can be nullable:

import arrow.core.Option

//sampleStart
val myString: String? = "Nullable string"
val option: Option<String> = Option.fromNullable(myString)
//sampleEnd
fun main () {
 println("option = $option")
}

Option can also be used with when statements:

import arrow.core.None
import arrow.core.Option
import arrow.core.Some

//sampleStart
val someValue: Option<Double> = Some(20.0)
val value = when(someValue) {
 is Some -> someValue.value
 is None -> 0.0
}
//sampleEnd
fun main () {
 println("value = $value")
}

import arrow.core.None
import arrow.core.Option
import arrow.core.Some

//sampleStart
val noValue: Option<Double> = None
val value = when(noValue) {
 is Some -> noValue.value
 is None -> 0.0
}
//sampleEnd
fun main () {
 println("value = $value")
}

An alternative for pattern matching is folding. This is possible because an option could be looked at as a collection or foldable structure with either one or zero elements.

One of these operations is map. This operation allows us to map the inner value to a different type while preserving the option:

import arrow.core.None
import arrow.core.Option
import arrow.core.Some

//sampleStart
val number: Option<Int> = Some(3)
val noNumber: Option<Int> = None
val mappedResult1 = number.map { it * 1.5 }
val mappedResult2 = noNumber.map { it * 1.5 }
//sampleEnd
fun main () {
 println("number = $number")
 println("noNumber = $noNumber")
 println("mappedResult1 = $mappedResult1")
 println("mappedResult2 = $mappedResult2")
}

Another operation is fold. This operation will extract the value from the option, or provide a default if the value is None

import arrow.core.Option
import arrow.core.Some

val fold =
//sampleStart
 Some(3).fold({ 1 }, { it * 3 })
//sampleEnd
fun main () {
 println(fold)
}

import arrow.core.Option
import arrow.core.none

val fold =
//sampleStart
 none<Int>().fold({ 1 }, { it * 3 })
//sampleEnd
fun main () {
 println(fold)
}

Arrow also adds syntax to all datatypes so you can easily lift them into the context of Option where needed.

import arrow.core.some

//sampleStart
 val some = 1.some()
 val none = none<String>()
//sampleEnd
fun main () {
 println("some = $some")
 println("none = $none")
}

import arrow.core.toOption

//sampleStart
val nullString: String? = null
val valueFromNull = nullString.toOption()

val helloString: String? = "Hello"
val valueFromStr = helloString.toOption()
//sampleEnd
fun main () {
 println("valueFromNull = $valueFromNull")
 println("valueFromStr = $valueFromStr")
}

You can easily convert between A? and Option<A> by using the toOption() extension or Option.fromNullable constructor.

import arrow.core.firstOrNone
import arrow.core.toOption

//sampleStart
val foxMap = mapOf(1 to "The", 2 to "Quick", 3 to "Brown", 4 to "Fox")

val empty = foxMap.entries.firstOrNull { it.key == 5 }?.value.let { it?.toCharArray() }.toOption()
val filled = Option.fromNullable(foxMap.entries.firstOrNull { it.key == 5 }?.value.let { it?.toCharArray() })

//sampleEnd
fun main() {
 println("empty = $empty")
 println("filled = $filled")
}

Transforming the inner contents

import arrow.core.Some

fun main() {
val value =
 //sampleStart
   Some(1).map { it + 1 }
 //sampleEnd
 println(value)
}

Computing over independent values

import arrow.core.Some

 val value =
//sampleStart
 Some(1).zip(Some("Hello"), Some(20.0), ::Triple)
//sampleEnd
fun main() {
 println(value)
}

Computing over dependent values ignoring absence

import arrow.core.computations.option
import arrow.core.Some
import arrow.core.Option

suspend fun value(): Option<Int> =
//sampleStart
 option {
   val a = Some(1).bind()
   val b = Some(1 + a).bind()
   val c = Some(1 + b).bind()
   a + b + c
}
//sampleEnd
suspend fun main() {
 println(value())
}

import arrow.core.computations.option
import arrow.core.Some
import arrow.core.none
import arrow.core.Option

suspend fun value(): Option<Int> =
//sampleStart
 option {
   val x = none<Int>().bind()
   val y = Some(1 + x).bind()
   val z = Some(1 + y).bind()
   x + y + z
 }
//sampleEnd
suspend fun main() {
 println(value())
}

Credits

Contents partially adapted from Scala Exercises Option Tutorial Originally based on the Scala Koans.

Functions

align	`fun <B> align(b:` `Option<B>):` `Option<Ior<A, B>>` `fun <B, C> align(b:` `Option<B>, f: (Ior<A, B>) -> C):` `Option<C>`
all	Returns true if this option is empty ‘'’or’’’ the predicate $predicate returns true when applied to this $option’s value.`fun all(predicate: (A) ->` `Boolean):` `Boolean`
and	`infix fun <X> and(value:` `Option<X>):` `Option<X>`
ap	`fun <B> ~~ap~~(ff:` `Option<(A) -> B>):` `Option<B>`
crosswalk	`fun <B> crosswalk(f: (A) ->` `Option<B>):` `Option<Option<B>>`
crosswalkMap	`fun <K, V> crosswalkMap(f: (A) ->` `Map<K, V>):` `Map<K,` `Option<V>>`
crosswalkNull	`fun <B> crosswalkNull(f: (A) -> B?):` `Option<B>?`
exists	Returns true if this option is nonempty ‘'’and’’’ the predicate $p returns true when applied to this $option’s value. Otherwise, returns false.`fun exists(predicate: (A) ->` `Boolean):` `Boolean`
filter	Returns this $option if it is nonempty ‘'’and’’’ applying the predicate $p to this $option’s value returns true. Otherwise, return $none.`fun filter(predicate: (A) ->` `Boolean):` `Option<A>`
filterNot	Returns this $option if it is nonempty ‘'’and’’’ applying the predicate $p to this $option’s value returns false. Otherwise, return $none.`fun filterNot(predicate: (A) ->` `Boolean):` `Option<A>`
findOrNull	Returns the $option’s value if this option is nonempty ‘'’and’’’ the predicate $p returns true when applied to this $option’s value. Otherwise, returns null.`fun findOrNull(predicate: (A) ->` `Boolean): A?`
flatMap	Returns the result of applying $f to this $option’s value if this $option is nonempty. Returns $none if this $option is empty. Slightly different from `map` in that $f is expected to return an $option (which could be $none).`fun <B> flatMap(f: (A) ->` `Option<B>):` `Option<B>`
fold	`fun <R> fold(ifEmpty: () -> R, ifSome: (A) -> R): R`
foldLeft	`fun <B> foldLeft(initial: B, operation: (B, A) -> B): B`
foldMap	`fun <B> foldMap(MB:` `Monoid<B>, f: (A) -> B): B`
foldRight	`fun <B> ~~foldRight~~(initial:` `Eval<B>, operation: (A,` `Eval<B>) ->` `Eval<B>):` `Eval<B>`
isDefined	Returns true if the option is an instance of Some, false otherwise.`fun isDefined():` `Boolean`
isEmpty	Returns true if the option is None, false otherwise.`abstract fun isEmpty():` `Boolean`
isNotEmpty	`fun isNotEmpty():` `Boolean`
map	Returns a Some$B containing the result of applying $f to this $option’s value if this $option is nonempty. Otherwise return $none.`fun <B> map(f: (A) -> B):` `Option<B>`
mapNotNull	Returns $none if the result of applying $f to this $option’s value is null. Otherwise returns the result.`fun <B> mapNotNull(f: (A) -> B?):` `Option<B>`
nonEmpty	alias for isDefined`fun nonEmpty():` `Boolean`
orNull	`fun orNull(): A?`
padZip	`fun <B> padZip(other:` `Option<B>):` `Option<Pair<A?, B?>>` `fun <B, C> padZip(other:` `Option<B>, f: (A?, B?) -> C):` `Option<C>`
reduceOrNull	`fun <B> reduceOrNull(initial: (A) -> B, operation: (acc: B, A) -> B): B?`
reduceRightEvalOrNull	`fun <B> reduceRightEvalOrNull(initial: (A) -> B, operation: (A, acc:` `Eval<B>) ->` `Eval<B>):` `Eval<B?>`
replicate	`fun replicate(n:` `Int):` `Option<List<A>>`
toEither	`fun <L> toEither(ifEmpty: () -> L):` `Either<L, A>`
toList	`fun toList():` `List<A>`
toString	`open fun toString():` `String`
traverse	`fun <B> traverse(fa: (A) ->` `Iterable<B>):` `List<Option<B>>`
traverseEither	`fun <AA, B> traverseEither(fa: (A) ->` `Either<AA, B>):` `Either<AA,` `Option<B>>`
traverseValidated	`fun <AA, B> traverseValidated(fa: (A) ->` `Validated<AA, B>):` `Validated<AA,` `Option<B>>`
void	`fun void():` `Option<Unit>`
zip	`fun <B> zip(other:` `Option<B>):` `Option<Pair<A, B>>` `fun <B, C> zip(b:` `Option<B>, map: (A, B) -> C):` `Option<C>` `fun <B, C, D> zip(b:` `Option<B>, c:` `Option<C>, map: (A, B, C) -> D):` `Option<D>` `fun <B, C, D, E> zip(b:` `Option<B>, c:` `Option<C>, d:` `Option<D>, map: (A, B, C, D) -> E):` `Option<E>` `fun <B, C, D, E, F> zip(b:` `Option<B>, c:` `Option<C>, d:` `Option<D>, e:` `Option<E>, map: (A, B, C, D, E) -> F):` `Option<F>` `fun <B, C, D, E, F, G> zip(b:` `Option<B>, c:` `Option<C>, d:` `Option<D>, e:` `Option<E>, f:` `Option<F>, map: (A, B, C, D, E, F) -> G):` `Option<G>` `fun <B, C, D, E, F, G, H, I> zip(b:` `Option<B>, c:` `Option<C>, d:` `Option<D>, e:` `Option<E>, f:` `Option<F>, g:` `Option<G>, map: (A, B, C, D, E, F, G) -> H):` `Option<H>` `fun <B, C, D, E, F, G, H, I> zip(b:` `Option<B>, c:` `Option<C>, d:` `Option<D>, e:` `Option<E>, f:` `Option<F>, g:` `Option<G>, h:` `Option<H>, map: (A, B, C, D, E, F, G, H) -> I):` `Option<I>` `fun <B, C, D, E, F, G, H, I, J> zip(b:` `Option<B>, c:` `Option<C>, d:` `Option<D>, e:` `Option<E>, f:` `Option<F>, g:` `Option<G>, h:` `Option<H>, i:` `Option<I>, map: (A, B, C, D, E, F, G, H, I) -> J):` `Option<J>` `fun <B, C, D, E, F, G, H, I, J, K> zip(b:` `Option<B>, c:` `Option<C>, d:` `Option<D>, e:` `Option<E>, f:` `Option<F>, g:` `Option<G>, h:` `Option<H>, i:` `Option<I>, j:` `Option<J>, map: (A, B, C, D, E, F, G, H, I, J) -> K):` `Option<K>`

Companion Object Functions

catch	`fun <A> catch(recover: (Throwable) ->` `Unit, f: () -> A):` `Option<A>`
fromNullable	`fun <A> fromNullable(a: A?):` `Option<A>`
invoke	`operator fun <A> invoke(a: A):` `Option<A>`
lift	`fun <A, B> lift(f: (A) -> B): (Option<A>) ->` `Option<B>`

Extension Functions

combine	`fun <A>` `Option<A>.combine(SGA:` `Semigroup<A>, b:` `Option<A>):` `Option<A>`
combineAll	`fun <A>` `Option<A>.combineAll(MA:` `Monoid<A>): A`
compareTo	`operator fun <A :` `Comparable<A>>` `Option<A>.compareTo(other:` `Option<A>):` `Int`
ensure	`fun <A>` `Option<A>.ensure(error: () ->` `Unit, predicate: (A) ->` `Boolean):` `Option<A>`
filterIsInstance	Returns an Option containing all elements that are instances of specified type parameter R.`fun <B>` `Option<*>.filterIsInstance():` `Option<B>`
flatten	`fun <A>` `Option<Option<A>>.flatten():` `Option<A>`
getOrElse	Returns the option’s value if the option is nonempty, otherwise return the result of evaluating `default`.`fun <T>` `Option<T>.getOrElse(default: () -> T): T`
handleError	`fun <A>` `Option<A>.handleError(f: (Unit) -> A):` `Option<A>`
handleErrorWith	`fun <A>` `Option<A>.handleErrorWith(f: (Unit) ->` `Option<A>):` `Option<A>`
or	`infix fun <T>` `Option<T>.or(value:` `Option<T>):` `Option<T>`
orElse	Returns this option’s if the option is nonempty, otherwise returns another option provided lazily by `default`.`fun <A>` `Option<A>.orElse(alternative: () ->` `Option<A>):` `Option<A>`
redeem	`fun <A, B>` `Option<A>.redeem(fe: (Unit) -> B, fb: (A) -> B):` `Option<B>`
redeemWith	`fun <A, B>` `Option<A>.redeemWith(fe: (Unit) ->` `Option<B>, fb: (A) ->` `Option<B>):` `Option<B>`
replicate	`fun <A>` `Option<A>.replicate(n:` `Int, MA:` `Monoid<A>):` `Option<A>`
rethrow	`fun <A>` `Option<Either<Unit, A>>.rethrow():` `Option<A>`
salign	`fun <A>` `Option<A>.salign(SA:` `Semigroup<A>, b:` `Option<A>):` `Option<A>`
separateEither	Separate the inner Either value into the Either.Left and Either.Right.`fun <A, B>` `Option<Either<A, B>>.separateEither():` `Pair<Option<A>,` `Option<B>>`
separateValidated	Separate the inner Validated value into the Validated.Invalid and Validated.Valid.`fun <A, B>` `Option<Validated<A, B>>.separateValidated():` `Pair<Option<A>,` `Option<B>>`
sequence	`fun <A>` `Option<Iterable<A>>.sequence():` `List<Option<A>>`
sequenceEither	`fun <A, B>` `Option<Either<A, B>>.sequenceEither():` `Either<A,` `Option<B>>`
sequenceValidated	`fun <A, B>` `Option<Validated<A, B>>.sequenceValidated():` `Validated<A,` `Option<B>>`
unalign	`fun <A, B>` `Option<Ior<A, B>>.unalign():` `Pair<Option<A>,` `Option<B>>` `fun <A, B, C>` `Option<C>.unalign(f: (C) ->` `Ior<A, B>):` `Pair<Option<A>,` `Option<B>>`
unite	`fun <A>` `Option<Iterable<A>>.unite(MA:` `Monoid<A>):` `Option<A>`
uniteEither	`fun <A, B>` `Option<Either<A, B>>.uniteEither():` `Option<B>`
uniteValidated	`fun <A, B>` `Option<Validated<A, B>>.uniteValidated():` `Option<B>`
unzip	`fun <A, B>` `Option<Pair<A, B>>.unzip():` `Pair<Option<A>,` `Option<B>>` `fun <A, B, C>` `Option<C>.unzip(f: (C) ->` `Pair<A, B>):` `Pair<Option<A>,` `Option<B>>`
widen	Given A is a sub type of B, re-type this value from Option to Option`fun <B, A : B>` `Option<A>.widen():` `Option<B>`

Inheritors

None	`object None :` `Option<Nothing>`
Some	`data class Some<out T> :` `Option<T>`