Feature: Add Result<T, E> which represents a computation that may either result in a value (success) or an exception (failure)

[jabrena]

Context:

In Java 8, the language added few functional programming Types like Optional<T> or CompletableFuture<T> (Promise) but not others like Either<L, R> which is included in this library.

Other modern programming language like Kotlin, Rust, Swift, Ocaml or F# add Result which was designed handle Error handling.

• Kotlin: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-result/
• Rust: https://doc.rust-lang.org/std/result/
• Swift: https://developer.apple.com/documentation/swift/result
• Ocaml: https://ocaml.org/manual/5.2/api/Result.html
• F#: https://fsharp.github.io/fsharp-core-docs/reference/fsharp-core-fsharpresult-2.html

Goal:

Add Result<T> in the library.

Examples:

//Error handling easily
Function<String, Result<URI>> toURI = address -> {
    return Result.runCatching(() -> {
        return new URI(address);
    });
};

class ResultExampleTest {

    @Test
    void should_work() {
        List<String> endpoints = Arrays.asList(
            "https://jsonplaceholder.typicode.com/posts/1",
            "https://jsonplaceholder.typicode.com/posts/2",
            "https://jsonplaceholder.typicode.com/posts/3"
        );

        // @formatter:off

        List<Result<String>> results = endpoints.stream()
            .map(ResultExampleTest::fetchData)
            .toList();

        List<String> successfulResults = results.stream()
            .filter(Result::isSuccess)
            .map(Result::getValue)
            .flatMap(Optional::stream)
            .toList();

        // @formatter:on

        successfulResults.forEach(System.out::println);

        assertThat(successfulResults.size()).isGreaterThan(0);
    }

    private static Result<String> fetchData(String endpoint) {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder().uri(URI.create(endpoint)).build();

        return Result.runCatching(() -> {
            HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
            if (response.statusCode() == 200) {
                return response.body();
            } else {
                throw new IOException("Failed to fetch data from " + endpoint);
            }
        });
    }
}

//Example using Railway-oriented programming
class ResultROPTest {

    Result<Integer> divide(int dividend, int divisor) {
        try {
            if (divisor == 0) {
                throw new IllegalArgumentException("Division by zero");
            }
            return Result.success(dividend / divisor);
        } catch (Exception e) {
            return Result.failure(e);
        }
    }

    Result<Integer> parseInteger(String input) {
        try {
            int parsedValue = Integer.parseInt(input);
            return Result.success(parsedValue);
        } catch (NumberFormatException e) {
            return Result.failure(e);
        }
    }

    // @formatter:off

    Result<Integer> calculate(String input1, String input2) {
        return parseInteger(input1)
                .flatMap(value1 -> parseInteger(input2)
                .flatMap(value2 -> divide(value1, value2)));
    }

    // @formatter:on

    @Test
    void should_work_ok() {
        Result<Integer> result = calculate("10", "2");
        int finalValue = result.getOrElse(() -> 0);

        assertThat(finalValue).isEqualTo(5);
    }

    @Test
    void should_work_ko() {
        Result<Integer> result = calculate("0", "2");
        int finalValue = result.getOrElse(() -> 0);

        assertThat(finalValue).isZero();
    }
}

Pending discussions:

• What design is better? Result<T> or Result<T, E>
• Add recommendations about when use Result and when use Either, Optional to minimize the usage of Java Exceptions
• Implement the solution

Note: Result<T, E> it is quite similar to Either<L, R>

Discussion branch:

• https://github.com/vavr-io/vavr/tree/result

Other links:

• https://arrow-kt.io/learn/typed-errors/wrappers/
• https://www.baeldung.com/kotlin/result-class
• https://github.com/jabrena/typed-errors

Juan Antonio

[Neodia]

Isn't that Try already ? The fact that you are creating the Result from inside try blocks further reinforces my thinking

[pivovarit]

I'm thinking this could be useful more like opinionated Either where you can use arbitrary types and not just exceptions

[jabrena]

Lets continue in Conversations about Result<T, E>. @pivovarit

[pivovarit]

Let me migrate the issue to discussions

[pivovarit]

Done