Result

A Java implementation of a Result monad inspired by Rust's Result struct

Important: This package is a mirror of https://github.com/jasongoodwin/better-java-monads, however that package is not maintained any more. This way we can maintain the code ourselves. Furthermore, the dependency as pulled in from maven-central was also unable to properly show javadoc comments for unknown reason. Big chunks of the following text are taken straight from the original source code.

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About

This library adds a Result monad to Java. While Optional exists to express nulls in types, there is no way to express a success/failure of a callable lambda expression. Result fills this gap.

The Result type is very similar to the Result in Scala's standard lib. Some inspiration is also taken from the Result monad in the Rust language, and subsequently from @sapphire/result, the TypeScript implementation of the former.

Lastly, this version of the Result monad also addresses all open issues in the original project, as well as including the code of all open pull requests.

Installation

Maven

<dependency>
    <groupId>tech.favware</groupId>
    <artifactId>result</artifactId>
    <version>1.2.0</version>
</dependency>

Gradle

With Gradle Groovy

dependencies {
    implementation 'tech.favware:result:1.2.0'
}

With Kotlin DSL

dependencies {
    implementation("tech.favware:result:1.2.0")
}

Usage

The Result monad was attributed to Twitter and placed into the Scala standard library. While both Scala and Haskell have a monad Either which has a left and a right type, a Result is specifically of a type T on success or an exception on failure.

The Result api is meant to be similar to the Optional type so has the same functions.

• get() returns the held value or throws the thrown exception
• getUnchecked() returns the held value or throws the thrown exception wrapped in a RuntimeException instance
• map(x) maps the success value x to a new value and type or otherwise passes the Failure forward.
• flatMap((x) -> f(x)) maps the success value x to a new Result of f(x).
• recover((t) -> x) will return the success value of the Result in the success case or the value x in the failure case. Exposes the exception.
• recoverWith((t) -> f(x)) will return the success value of the Result in the success case or a new Result of f(x) in the failure case. Exposes the exception.
• filter((x) -> isTrue(x)) - If Success, returns the same Success if the predicate succeeds, otherwise, returns a Failure with type NoSuchElementException.
• onSuccess((x) -> f(x)) execute some code on success - takes a Consumer (e.g. requires no return value).
• onFailure((x) -> f(x)) execute some code on failure - takes a Consumer (e.g. requires no return value).
• raise(x) -> will throw an exception of type x when it happens.
• orElse(x) will return the success value of the Result in success case or the value x in failure case.
• orElseResult(f) will return the success value of the Result in success case or a new Result(f) in the failure case.
• orElseThrow(() -> throw new T) gets result or on failure will throw checked exception of type T
• toOptional() will return Optional of success value of Result (if not null), otherwise it will return an empty Optional
• mapErr(e) maps the failure value e to a new error type or otherwise passes the success forward.
• match((x, y) Takes the result of the Result and applies the first function to the success value or the second function to the failure value. Transforms the return type to the return type of the applied function.

Contributors

Please make sure to read the Contributing Guide before making a pull request.

Thank you to all the people who already contributed to Sapphire!