This repository contains a simple implementation of Rust's result type and question mark like operator for java. Since ? is already symbol for defining types in java and no other way of calling it is syntactically valid we will use a method tryGet() to achieve the same effect. This plugin works by replacing the bytecode of result.tryGet() with the bytecode equivalent to below code

if (!result instanceof Ok) {
    return Err(ErrorType.from(result.error())
}

Warning: This plugin is a project for fun and not tested in production and may have bugs. Please use it at your own risk.

The module test-module contains sample code that shows how to use the result type and the tryGet() method. The module result contains the implementation of the result type and the tryGet() method.

• Clone this repository and run mvn clean install. This step is required because these artifacts are not available in any maven repository.
• Add the following dependency to your pom.xml

<dependency>
    <groupId>org.example</groupId>
    <artifactId>result-type-api</artifactId>
    <version>1.0-SNAPSHOT</version>
</dependency>

• Add the result-type-plugin to your pom.xml

<build>
    <plugins>
        <plugin>
            <groupId>org.example</groupId>
            <artifactId>result-type-plugin</artifactId>
            <version>1.0-SNAPSHOT</version>
            <executions>
                <execution>
                    <id>return-result</id>
                    <phase>prepare-package</phase>
                    <goals>
                        <goal>process</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

• Start using the result type and the tryGet() method. ensure that any function that uses the tryGet() method returns a Result type.

public Result<String, StdError> method1() {
    Result<String, StdError> res = getRes();
    System.out.println(res.errorContext("Failed to get hello").tryGet());
    return res;
}

• Any method that uses tryGet() must return a Result type.
• The Error part of the Returned Result type must not be a generic type like T, U or an unbounded type like ?.
• You can define your own error type by implementing the IError interface. but ensure that the type implements a static method from that takes IError as input and returns the error type. look at the StdError class for an example.
  • The static method is chosen over constructor because
    1. You cannot call constructors in enums, interfaces or abstract classes
    2. This is much closer to Rust's api.
    3. Constructors take 4 instructions (new, dup, current error, ) and 2 new entries in the stack
• Sometimes you may get error that StackMapFrames are inconsistent. This is because I don't fully understand it and somehow made it work. Incase you face this issue the workaround is to comment the aa.visitFrame(F_NEW, locals.size(), locals.toArray(), stack.size(), stack.toArray()); line in ResultTypeMethodVisitor class and run your java application with -noverify flag
  • This is not a good solution and if someone knows how to fix this please raise a PR.