Optional/Maybe type in Kotlin

Maybe.kt

package me.ramseyboy.function

import java.util.*

/**
 * A container object which may or may not contain a non-null value. If a value is defined, `isdefined()` will return `true` and `get()` will return the value.

 *
 * Additional methods that depend on the presence or absence of a contained value are provided,
 * such as [orElse()][.orElse] (return a default value if value not defined)
 * and [ifdefined()][.ifdefined] (execute a block of code if
 * the value is defined).
 
 */
interface Maybe<T> {

    /**
     * If non-null, the value; if null, indicates no value is defined
     */
    val value: T?

    /**
     * If a value is defined in this `Maybe`, returns the value, otherwise throws `NoSuchElementException`.

     * @return the non-null value held by this `Maybe`
     * *
     * @throws NoSuchElementException if there is no value defined
     * *
     * @see Maybe.isDefined
     */
    public fun get(): T? {
        if (value == null) {
            throw NoSuchElementException("No value defined")
        }
        return value
    }

    /**
     * Return `true` if there is a value defined, otherwise `false`.

     * @return `true` if there is a value defined, otherwise `false`
     */
    public fun isDefined(): Boolean {
        return value != null
    }

    /**
     * If a value is defined, invoke the specified consumer with the value, otherwise do nothing.

     * @param consumer block to be executed if a value is defined
     * *
     * @throws NullPointerException if value is defined and `consumer` is null
     */
    public fun ifDefined(consumer: Consumer<in T?>) {
        if (value != null)
            consumer.accept(value)
    }

    /**
     * If a value is defined, and the value matches the given predicate, return an `Maybe`
     * describing the value, otherwise return an empty `Maybe`.

     * @param predicate a predicate to apply to the value, if defined
     * *
     * @return an `Maybe` describing the value of this `Maybe` if a value is
     * * defined and the value matches the given predicate, otherwise an empty `Maybe`
     * *
     * @throws NullPointerException if the predicate is null
     */
    public fun filter(predicate: Predicate<in T>?): Maybe<T> {
        if (predicate == null) {
            throw NullPointerException()
        }

        if (!isDefined())
            return this
        else
            return if (predicate.test(value!!)) this else empty<T>()
    }

    /**
     * If a value is defined, apply the provided mapping function to it, and if the result is
     * non-null, return an `Maybe` describing the result.  Otherwise return an empty `Maybe`.

     * @param     The type of the result of the mapping function
     * *
     * @param mapper a mapping function to apply to the value, if defined
     * *
     * @return an `Maybe` describing the result of applying a mapping function to the value
     * * of this `Maybe`, if a value is defined, otherwise an empty `Maybe`
     * *
     * @throws NullPointerException if the mapping function is null
     * *
     * @apiNote This method supports post-processing on Maybe values, without the need to
     * * explicitly check for a return status.  For example, the following code traverses a stream of
     * * file names, selects one that has not yet been processed, and then opens that file, returning
     * * an `Maybe&lt;FileInputStream&gt;`:
     * *
     * * `Maybe&lt;FileInputStream&gt; fis =
     * names.stream().filter(name -&gt; !isProcessedYet(name))
     * .findFirst()
     * .map(name -&gt; new FileInputStream(name));
    ` *
     * *
     * * Here, `findFirst` returns an `Maybe&lt;String&gt;`, and then `map` returns an
     * * `Maybe&lt;FileInputStream&gt;` for the desired file if one exists.
     */
    public fun <U> map(mapper: Function<in T, out U>?): Maybe<U> {
        if (mapper == null) {
            throw NullPointerException()
        }

        if (!isDefined())
            return empty()
        else {
            return Maybe.ofNullable(mapper.apply(value!!))
        }
    }

    /**
     * If a value is defined, apply the provided `Maybe`-bearing mapping function to it,
     * return that result, otherwise return an empty `Maybe`.  This method is similar to
     * [.map], but the provided mapper is one whose result is already an `Maybe`, and if invoked, `flatMap` does not wrap it with an additional `Maybe`.

     * @param     The type parameter to the `Maybe` returned by
     * *
     * @param mapper a mapping function to apply to the value, if defined the mapping function
     * *
     * @return the result of applying an `Maybe`-bearing mapping function to the value of
     * * this `Maybe`, if a value is defined, otherwise an empty `Maybe`
     * *
     * @throws NullPointerException if the mapping function is null or returns a null result
     */
    public fun <U> flatMap(mapper: Function<in T, Maybe<U>>?): Maybe<U> {
        if (mapper == null) {
            throw NullPointerException()
        }

        if (!isDefined())
            return empty()
        else {
            return mapper.apply(value!!)
        }
    }

    /**
     * Return the value if defined, otherwise return `other`.

     * @param other the value to be returned if there is no value defined, may be null
     * *
     * @return the value, if defined, otherwise `other`
     */
    public fun orElse(other: T): T {
        return value ?: other
    }

    /**
     * Return the value if defined, otherwise invoke `other` and return the result of that
     * invocation.

     * @param other a `Supplier` whose result is returned if no value is defined
     * *
     * @return the value if defined otherwise the result of `other.get()`
     * *
     * @throws NullPointerException if value is not defined and `other` is null
     */
    public fun orElseGet(other: Supplier<out T>): T {
        return value ?: other.get()
    }

    /**
     * Return the contained value, if defined, otherwise throw an exception to be created by the
     * provided supplier.

     * @param                Type of the exception to be thrown
     * *
     * @param exceptionSupplier The supplier which will return the exception to be thrown
     * *
     * @return the defined value
     * *
     * @throws X                    if there is no value defined
     * *
     * @throws NullPointerException if no value is defined and `exceptionSupplier` is null
     * *
     * @apiNote A method reference to the exception constructor with an empty argument list can be
     * * used as the supplier. For example, `IllegalStateException::new`
     */
    @Throws(Throwable::class)
    public fun <X : Throwable> orElseThrow(exceptionSupplier: Supplier<out X>): T? {
        if (value != null) {
            return value
        } else {
            throw exceptionSupplier.get()
        }
    }

    /**
     * Indicates whether some other object is "equal to" this Maybe. The other object is
     * considered equal if:   * it is also an `Maybe` and;  * both instances have no
     * value defined or;  * the defined values are "equal to" each other via `equals()`.
     *

     * @param obj an object to be tested for equality
     * *
     * @return {code true} if the other object is "equal to" this object otherwise `false`
     */
    override fun equals(obj: Any?): Boolean {
        if (obj == null) {
            throw NullPointerException()
        }

        if (this === obj) {
            return true
        }

        if (obj !is Maybe<*>) {
            return false
        }

        return equals(value, obj.value)
    }

    /**
     * Returns the hash code value of the defined value, if any, or 0 (zero) if no value is
     * defined.

     * @return hash code value of the defined value or 0 if no value is defined
     */
    override fun hashCode(): Int {
        return hashCode(value)
    }

    /**
     * Returns a non-empty string redefinedation of this Maybe suitable for debugging. The exact
     * definedation format is unspecified and may vary between implementations and versions.

     * @return the string redefinedation of this instance
     * *
     * @implSpec If a value is defined the result must include its string redefinedation in the
     * * result. Empty and defined Maybes must be unambiguously differentiable.
     */
    override fun toString(): String {
        return if (value != null){
            "Maybe[$value]"
        } else {
            "Maybe.empty"
        }
    }

    companion object {
        /**
         * Common instance for `empty()`.
         */
        private val EMPTY = None<Any>()

        /**
         * Returns an empty `Maybe` instance.  No value is defined for this Maybe.

         * @param  Type of the non-existent value
         * *
         * @return an empty `Maybe`
         * *
         * @apiNote Though it may be tempting to do so, avoid testing if an object is empty by comparing
         * * with `==` against instances returned by `Option.empty()`. There is no guarantee
         * * that it is a singleton. Instead, use [.isdefined].
         */
        public fun <T> empty(): Maybe<T> {
            val t = EMPTY as Maybe<T>
            return t
        }

        /**
         * Returns an `Maybe` with the specified defined non-null value.

         * @param    the class of the value
         * *
         * @param value the value to be defined, which must be non-null
         * *
         * @return an `Maybe` with the value defined
         * *
         * @throws NullPointerException if value is null
         */
        public fun <T> of(value: T): Maybe<T> {
            return Some(value)
        }

        /**
         * Returns an `Maybe` describing the specified value, if non-null, otherwise returns an
         * empty `Maybe`.

         * @param    the class of the value
         * *
         * @param value the possibly-null value to describe
         * *
         * @return an `Maybe` with a defined value if the specified value is non-null,
         * * otherwise an empty `Maybe`
         */
        public fun <T> ofNullable(value: T?): Maybe<T> {
            return if (value == null) empty<T>() else of(value)
        }

        /**
         * Null-safe equivalent of `a.equals(b)`.
         */
        public fun equals(a: Any?, b: Any?): Boolean {
            return if ((a == null)) (b == null) else a == b
        }

        /**
         * Returns 0 for null or `o.hashCode()`.
         */
        public fun hashCode(o: Any?): Int {
            return if ((o == null)) 0 else o.hashCode()
        }
    }
}

final class Some<T> : Maybe<T> {

    override val value: T

    /**
     * Constructs an instance with the value defined.

     * @param value the non-null value to be defined
     * *
     * @throws NullPointerException if value is null
     */
    constructor(value: T) {
        if (value == null) {
            throw NullPointerException()
        }
        this.value = value
    }
}

final class None<T> : Maybe<T> {

    override val value: T?

    /**
     * Constructs an empty instance.

     * @implNote Generally only one empty instance, [Maybe.EMPTY], should exist per VM.
     */
    constructor() {
        this.value = null
    }
}

fun main(args: Array<String>) {
    val maybe: Maybe<String> = Some("defined")
    val unwrapped = when(maybe) {
        is Some -> maybe.get()
        is None -> "default value"
        else -> throw RuntimeException("neva gonna happen")
    }
    println(unwrapped)
}