DataEnum

DataEnum allows you to work with algebraic data types in Java.

You can think of it as an enum where every individual value can have different data associated with it.

What problem does it solve?

The idea of algebraic data types is not new and already exists in many other programming languages, for example:

• Sealed classes in Kotlin
• Enumerations with associated values in Swift
• Case classes in Scala
• Data types in Haskell

It is possible to represent such algebraic data types using subclasses: the parent class is the "enumeration" type, and each child class represents a case of the enumeration with it's associated parameters. This will however either require you to spread out your business logic in all the subclasses, or to cast the child class manually to access the parameters and be very careful to only cast if you know for sure that the class is of the right type.

The goal of DataEnum is to help you generate all these classes and give you a fluent API for easily accessing their data in a type-safe manner.

The primary use-case we had when designing DataEnum was to execute different business logic depending on an incoming message. And as mentioned above, we wanted to keep all that business logic in one place, and not spread it out in different classes. With plain Java, you’d have to write something like this:

if (message instanceof Login) {
    Login login = (Login) message;
    // login logic here
} else if (message instanceof Logout) {
    Logout logout = (Logout) message;
    // logout logic here
}

There are a number of things here that developers tend to not like: repeated if-else statements, manual instanceof checks and safe-but-noisy typecasting. On top of that it doesn't look very idiomatic and there's a high risk that mistakes get introduced over time. If you use DataEnum, you can instead write the same expression like this:

message.match(
   login -> { /* login logic; the 'login' parameter is 'message' but cast to the type Login. */ },
   logout -> { /* logout logic; the 'logout' parameter is 'message' but cast to the type Logout. */ }
);

In this example only one of the two lambdas will be executed depending on the message type, just like with the if-statements. match is just a method that takes functions as arguments, but if you write expressions with linebreaks like in the example above it looks quite similar to a switch-statement, a match-expression in Scala, or a when-expression in Kotlin. DataEnum makes use of this similarity to make match-statements look and feel like a language construct.

There are many compelling use-cases for using an algebraic data type to represent values. To name a few:

• Create a vocabulary of possible actions. List all the actions that can be performed in a certain part of your application, for example on a login/logout page. Each action can have different data associated with it, for example the login action would have a username and password, while a logout action doesn't have any data.

• Representing states of a state machine. This allows you to only keep the data that actually is available in each state, making it impossible to even reference data that isn't available in a particular state.

• Rich, type-safe error handling Instead of having just an error code as a result of a network request, you can have different types for different errors, each with relevant information attached: ConnectivityLost, NoRouteToHost(String host), TooManyRetries(int retryCount).

• Metadata in RxJava streams. It is often useful to wrap data in RxJava in order to provide metadata about what's happening. One common example is to represent different kinds of success and failure: InProgress(T placeholder), Success(T data), Error(String reason).

Status

DataEnum is in Beta status, meaning it is used in production in Spotify Android applications, but we may keep making changes relatively quickly.

It is currently built for Java 7 (because Android doesn't support Java 8 well yet), hence the duplication of some concepts defined in java.util.function (Consumer, Function, Supplier).

Using it in your project

The latest version of DataEnum is available through Maven Central (LATEST_RELEASE below is ):

Gradle

implementation 'com.spotify.dataenum:dataenum:LATEST_RELEASE'                
annotationProcessor 'com.spotify.dataenum:dataenum-processor:LATEST_RELEASE' 

Maven

<dependencies>
  <dependency>
    <groupId>com.spotify.dataenum</groupId>
    <artifactId>dataenum</artifactId>
    <version>LATEST_RELEASE</version>
  </dependency>
  <dependency>
    <groupId>com.spotify.dataenum</groupId>
    <artifactId>dataenum-processor</artifactId>
    <version>LATEST_RELEASE</version>
    <scope>provided</scope>
  </dependency>
</dependencies>

It may be an option to use the annotationProcessorPaths configuration option of the maven-compiler-plugin rather than an optional dependency.

How do I create a DataEnum type?

First, you define all the cases and their parameters in an interface like this:

@DataEnum
interface MyMessages_dataenum {
    dataenum_case Login(String userName, String password);
    dataenum_case Logout();
    dataenum_case ResetPassword(String userName);
}

Then, you apply the dataenum-processor annotation processor to that code, and your DataEnum case classes will be generated for you.

Some things to note:

• We use a Java interface for the specification. The rationale is that it allows the IDE to help you find and import types correctly. We deliberately made it look weird, so nobody would think it’s a normal class. This is abusing Java a bit, but we’re OK with that.

• The interface will never be used for anything other than code generation, so you should normally make the interface package-private. The one exception is when one _dataenum spec needs to reference another as described below.

• The interface name has to end with _dataenum. This is to make the interface stick out and make it easier to filter out from artifacts and exclude from static analysis.

• The methods in the interface have to be declared as returning a dataenum_case. Each method corresponds to one of the possible cases of the enum, and the parameters of the method become the member fields of that case. Note that the method names from the interface will be used as class names for the cases, so you'll want to name them using CamelCase as in the example above. The methods  in the _dataenum interface will never be implemented, and there is no way to create a dataenum_case instance. The type is only used as a marker.

• The prefix of the @DataEnum annotated interface will be used as the name of a generated super-class (MyMessages in the example above). This class will have factory methods for all the cases.

• For each method in the interface, an inner class will be generated (in this example MyMessages.Login, MyMessages.Logout and MyMessages.ResetPassword). These classes will extend the outer class MyMessages.

Using the generated DataEnum class

Some usage examples, based on the @DataEnum specification above:

// Instantiate by passing in the required parameters. 
// You’ll get something that is of the super type - this is to help Java’s 
// not-always-great type inference do the right thing in many common cases.
MyMessages message = MyMessages.login("petter", "s3cr3t");

// If you actually needed the subtype you can easily cast it using the as-methods.
Logout logout = MyMessages.logout().asLogout();

// For every as-method there is also an is-method to check the type of the message.
assertThat(message.isLogin(), is(true));

// Apply different business logic to different message types. Note how getters are generated (but not
// setters, DataEnum case types should be considered immutable).
message.match(
    login -> Logger.debug("got a login request from user: {}", login.userName()),
    logout -> Logger.debug("user logged out"),
    resetPassword -> Logger.debug("password reset requested for user: {}", resetPassword.userName())
);

// So far we've been looking at 'match', but there is also the very useful 'map' which is used to
// transform values. When using 'map' you define how the message should be transformed in each case.
int passwordLength = message.map(
    login -> login.password().length(),
    logout -> 0,
    resetPassword -> -1);
}

// There are some utility methods provided that allow you to deal with unimplemented or illegal cases:
int passwordLength = message.map(
    login -> login.password().length(),
    logout -> Cases.illegal("logout message does not contain a password"), // throws IllegalStateException
    resetPassword -> Cases.todo()); // throws UnsupportedOperationException
}

// Sometimes, only a minority of cases are handled differently, in which cas