Result type

Short description: Concept in functional programming

In functional programming, a result type is a monadic type holding a returned value or an error code. They provide an elegant way of handling errors, without resorting to exception handling; when a function that may fail returns a result type, the programmer is forced to consider success or failure paths, before getting access to the expected result; this eliminates the possibility of an erroneous programmer assumption.

Examples

• In C++, it is defined by the standard library as std::expected<T, E>.
• In Elm, it is defined by the standard library as type Result e v = Ok v | Err e.
• In Haskell, by convention the Either type is used for this purpose, which is defined by the standard library as data Either a b = Left a | Right b, where a is the error type and b is the return type.
• In Java, it is not natively in the standard library, but is available from third party libraries. For example, result4j which includes an interface Result<R, E> similar to Rust Result<T, E>, and vavr includes an interface Either<L, R> similar to Haskell Either a b. Because Java and Kotlin are cross-compatible, Java can use the Result type from Kotlin.
• In Kotlin, it is defined by the standard library as value class Result<out T>.
• In OCaml, it is defined by the standard library as type ('a, 'b) result = Ok of 'a | Error of 'b type.
• In Python, it is not natively in the standard library, but is available from third party libraries such as returns and result.
• In Rust, it is defined by the standard library as enum Result<T, E> { Ok(T), Err(E) }.
• In Scala, the standard library also defines an Either type, however Scala also has more conventional exception handling.
• In Swift, it is defined by the standard library as @frozen enum Result<Success, Failure> where Failure : Error.
• In V, the result type is implemented natively using !T as the return type of a function. For example fn my_function() !string { ... }. Error Handling in V.

C++

The expected<T, E> class uses std::unexpected() to return the type E, and can return T directly.

import std;

using std::expected;
using std::ifstream;
using std::string;
using std::stringstream;
using std::unexpected;
using std::filesystem::path;

enum class FileError {
    MissingFile,
    NoPermission,
    // more errors here
};

expected<string, FileError> loadConfig(const path& p) noexcept {
    if (!std::filesystem::exists(p)) {
        return unexpected(FileError::MissingFile);
    }
    ifstream config{p};
    stringstream buffer;
    if (!config.is_open()) {
        return unexpected(FileError::NoPermission);
    }
    buffer << config.rdbuf();
    config.close();
    return buffer.str();
}

int main(int argc, char* argv[]) {
    path p{/* some path here */};
    if (const expected<String, FileError> s = loadConfig(p); s.has_value()) {
        std::println("Config contents: {}", s.value());
    } else if (s.error() == FileError::MissingFile) {
        std::println("Error: path {} not valid or missing!", p);
    } else if (s.error() == FileError::NoPermission) {
        std::println("Error: no permission to read file at path {}!", p);
    } else {
        std::unreachable();
    }
}

Rust

The result object has the methods is_ok() and is_err().

const CAT_FOUND: bool = true;

fn main() {
    let result: Result<(), String> = pet_cat();
    if result.is_ok() {
        println!("Great, we could pet the cat!");
    } else {
        let error: String = result.unwrap_err();
        println!("Oh no, we couldn't pet the cat: {}", error);
    }
}

fn pet_cat() -> Result<(), String> {
    if CAT_FOUND {
        Ok(())
    } else {
        Err(String::from("The cat is nowhere to be found!"))
    }
}

Vlang

The Error type is an interface for iError.

const cat_found = true

fn main() {
    cat_name := get_pet_cat_name() or { 
        println("Oh no, we couldn't pet the cat!")
        exit(1)
    }

    println('Great, we could pet the cat ' + cat_name)
}

fn get_pet_cat_name() !string {
    if cat_found { return 'Max' } 
    else { return error('the cat is nowhere to be found') }
}

See also

• Option type
• Exception handling
• Tagged union
• Return type