Koptional — Minimalistic Optional type for Kotlin

Disclaimer:

We don't think that Kotlin itself needs Optional because it has strong null-safe type system that effectively eliminates need in such a wrapper. However there are APIs and libraries like RxJava 2 which don't accept null values.

We also think that in many cases you can use sealed classes to express absent values, however in simple cases like passing String? through Rx stream Optional is a more convenient solution.

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The goal of this implementation is to be convenient to use and fit Kotlin's null-safe type system, which resulted in:

• Only two functions (mimics Kotlin std toInt()/toBoolean()/etc):
  • fun <T : Any> T?.toOptional(): Optional<T>
  • fun Optional.toNullable(): T?
• Some and None are declared as top level types — no need to write Optional.Some or Optional.None
• No functions like map(), getOrElse(), filter(), etc — apply toNullable() and use Kotlin sdt functions like let(), takeIf() and so on.

Usage

Create Some

val some = Some(value)

Use None

val none = None // It's an object!

Convert T? to Optional<T>

// If something is null — you'll get None, otherwise you'll get Some(something).
val o = something.toOptional()

Convert Optional<T> to T?

// If Optional is None — you'll get null, otherwise you'll get not null T value.
val t = optional.toNullable()

Get value or fall back to something else if None (getOrElse())

val f = optional.toNullable() ?: "fallback"

Check if Optional is Some or None

when (optional) {
    is Some -> {}
    is None -> {}
}

Filter only Some values emitted by RxJava 2 or Project Reactor

val values: Observable<String> = Observable
    .just(Some("a"), None, Some("b"))
    .filterSome()
    .map { value: String -> } // filterSome() unwraps value for you.

Filter only None values emitted by RxJava 2 or Project Reactor

val noneSignals: Observable<Unit> = Observable
    .just(Some("a"), None, Some("b"))
    .filterNone()
    .map { none: Unit -> } // filterNone() maps None to Unit.

Download

Koptional is available on jcenter.

Optional type:

compile 'com.gojuno.koptional:koptional:put-some-version'

RxJava 2 extensions:

compile 'com.gojuno.koptional:koptional-rxjava2-extensions:put-some-version'

All the releases and changelogs can be found on Releases Page.

How to build

Dependencies: you only need docker and bash installed on your machine.

bash ci/build.sh

License

Copyright 2017 Juno, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.