rybak / either-jvm

Type Either for JVM languages

Either-JVM

This is a collection of several libraries for JVM, implemented in several JVM languages. All variants of same library implement the Either data type in form of a JVM class.

Motivation

Experience of people using functional programming, with Haskell in particular, has shown that Either is a very useful data type. There are several implementations of it for JVM and for Kotlin Multiplatform:

• https://www.javadoc.io/doc/io.vavr/vavr/latest/io/vavr/control/Either.html
• https://github.com/functionaljava/functionaljava/blob/series/5.x/core/src/main/java/fj/data/Either.java
• https://github.com/meoyawn/kotlin-either
• https://github.com/aSoft-Ltd/either
• https://github.com/JavierSegoviaCordoba/network-either-kmp

However, each of them has some combination of the following properties, which make their usage inconvenient or cumbersome:

• don't include convenient structural pattern matching facilities for their Either type
• don't use functional interfaces from java.util.function directly (actual only for Java libraries)
• are included as part of very big unwieldy functional programming library
• don't provide APIs with conventional naming structure
• don't utilize wildcard type parameters, making it hard to use functional variables and method references with inexact type
• are not abstract enough, with specialization for something specific, like passing an Either over the network

Either-JVM is also a personal exercise in library writing and programming language learning.

Features

1. Convenient structural pattern matching.
2. For Java libraries – interoperation with java.util.function.* and java.util.Stream APIs.

either-java8

Compatible with Java 8 and any later versions.

either-java17

Compatible with Java 17 and any later versions. Utilizes the sealed classes feature which was introduced to Java in JEP 409.

Technically, it can be used together with pattern matching in switch statements and expressions, which was introduced as a preview feature in JEP 406, but I don't recommend it – pattern matching in switch has changed a lot. Latest preview in Java 20 is described in JEP 433. JEP 441: Pattern Matching for switch will be released in Java 21 in September of 2023.

either-kotlin

TBD

I haven't figured out yet how versioning of Kotlin language works yet, I'll add something here when I do.

Usage

So far, only snapshots versions are available through Sonatype Nexus repository. You can browse the published libraries:

• via Nexus web GUI
• via index

Gradle

repositories {
	maven {
		name = "Sonatype Nexus Snapshots"
		url = uri("https://s01.oss.sonatype.org/content/repositories/snapshots/")
	}
}
dependencies {
	implementation(group = "dev.andrybak.either-jvm", name = "either-java8", version = "0.2-SNAPSHOT")
	implementation(group = "dev.andrybak.either-jvm", name = "either-java17", version = "0.2-SNAPSHOT")
	implementation(group = "dev.andrybak.either-jvm", name = "either-kotlin", version = "0.2-SNAPSHOT")
}