Java tagged unions

https://github.com/wehjin/varnum

Varnum is an annotation processor that generates tagged-union classes, also known as variant-enums or sum-types, for Kotlin, Java, and Android.

An annotated interface specifies the name of the tagged-union along with the names and data-types of its members. The interface begets a tagged-union class, construction functions for each member of the union, and a match function that identifies a member and unwraps its data.

Varnum is 100% generated code with no reflection or byte-code manipulation. You can see the generated classes yourself by viewing the auto-generated tagged-union class after a build.

Varnum works by building a class for each interface annotated with @TaggedUnion, e.g.

@TaggedUnion("Message")
interface MessageSpec {
    void Reset();
    void SetSize(Integer size);
    void Multi(Integer first, String second);
}

This example generates a tagged-union class named Message which has code to create an instance of each of its members: Reset, SetSize, and Multi. Additional code matches an instance to a member and unwraps the member's data.

Construction takes place like this:

final Message message = Message.SetSize(5);  // Construct a message

Later, you can match a specific message and unwrap its data with Message.match

// Match and unwrap message
Message.match(message)
    .isSetSize(new MatchAction1<Integer>() {
        @Override
        public void call(Integer value) {
            Log.d("Matched SetSize: " + value);
        }
    })
    .isReset(new MatchAction1() {
        @Override
        public void call() {
            Log.d("Matched Reset");
        }
    })
    .orElse(new MatchAction0() {
        @Override
        public void call() {
            Log.d("Matched wildcard");
        }
    });

Tagged unions are particularly useful for declaring the actions that an actor or thread can perform. To schedule an action with the actor, one need only create an instance of the actor's input type (the tagged union) and add the instance to the actor's message queue or stream. The actor then has access to the action type and parameters packed together in one neat object.

repositories {
    maven { url 'https://jitpack.io' }
}
dependencies {
    compile 'com.github.wehjin.varnum:varnum:v1.1'
    kapt 'com.github.wehjin.varnum:varnum-compiler:v1.1'
}

repositories {
    maven { url 'https://jitpack.io' }
}
dependencies {
    compile 'com.github.wehjin.varnum:varnum:v1.1'
    apt 'com.github.wehjin.varnum:varnum-compiler:v1.1'
}

Copyright 2016 Jeffrey Yu.

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

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