Option
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
//sampleEndUsing 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
import arrow.core.none
//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
import arrow.core.Option
//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.
Types
Functions
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.
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).
Inheritors
Extensions
Separate the inner Either value into the Either.Left and Either.Right.
Separate the inner Validated value into the Validated.Invalid and Validated.Valid.