//arrow-core/arrow.core/Option

Option

common 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 = 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:

{: data-executable='true'}

```kotlin
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 = 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:

{: data-executable='true'}

```kotlin
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().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.

{: data-executable='true'}

```kotlin
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.

{: data-executable='true'}

```kotlin
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 = //sampleStart option { val x = none().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.

Types

Functions

Inheritors

Extensions