Sum Type is an annotation processor that makes it simple to emulate sum types in Java via visitors.
See https://en.wikipedia.org/wiki/Tagged_union for an in depth explanation of sum types, read on, or check out the samples.
Java lacks support for sum types which can be useful. We can get close with a composite value object and a visitor, but that implementation is cumbersome to write and maintain.
Assuming you had three types (Loading, Error, Results) you wanted to capture in a sum type, create an interface
with those types and annoate it with @SumType
@SumType
public interface Query {
Loading loading();
Results results();
Error error();
}
This will generate several classes for you to use, most important being QuerySumType and QuerySumTypeVisitor.
To build a new QuerySumType, the value holder, use one of the generated static factory methods:
QuerySumType query = QuerySumType.ofLoading(new Loading());
To ensure your code handles all of the cases a sum type can be in, pass in an implementation of a generated visitor interface.
query.accept(new QuerySumTypeVisitor() {
@Override
public void visitLoading(Loading loading) {
System.out.println(loading.toString());
}
@Override
public void visitResults(Results results) {
System.out.println(results.toString());
}
@Override
public void visitError(Error error) {
System.out.println(error.toString());
}
});
Sum types are exceptionally useful when dealing with event based architectures and we would like to minimize the friction required to make use of them in a language where they are not a first class citizen.
buildscript {
repositories {
mavenCentral()
}
dependencies {
// Or latest versions
classpath 'com.android.tools.build:gradle:1.1.2'
classpath 'com.neenbedankt.gradle.plugins:android-apt:1.4' // Or your preferred apt plugin if not using Android (see samples)
}
}
apply plugin: 'com.android.application'
apply plugin: 'android-apt' // Or your preferred apt plugin if not using Android (see samples)
dependencies {
apt 'com.madebyatomicrobot:sumtype-compiler:{latest-version}'
compile 'com.madebyatomicrobot:sumtype-annotations:{latest-version}'
}| Artifact | Latest Version |
|---|---|
| sumtype-compiler |  |
| sumtype-annotations |  |
Snapshots of the development version are available in Sonatype’s snapshots repository.
Copyright 2015 Atomic Robot LLC
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.
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