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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 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 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 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 C++, it is defined by the standard library as std::expected<T, E>.
• In Python, it is available from third party libraries such as returns and result.

Rust

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

const CAT_FOUND: bool = true;
fn main() {
    let result = pet_cat();
    if result.is_ok() {
        println!("Great, we could pet the cat!");
    } else {
        println!("Oh no, we couldn't pet the cat!");
    }
}
fn pet_cat() -> Result<(), String> {
    if CAT_FOUND {
        Ok(())
    } else {
        Err(String::from("the cat is nowhere to be found"))
    }
}

See also

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

References

1. "Result · An Introduction to Elm". guide.elm-lang.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
2. "Data.Either". hackage.haskell.org. 22 September 2023. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
3. "Result - Kotlin Programming Language". kotlinlang.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
4. "Error Handling · OCaml Tutorials". ocaml.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
5. "std::result - Rust". doc.rust-lang.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
6. "stdlib: Add result module · rust-lang/rust@c1092fb". github.com. 29 October 2011. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
7. "Scala Standard Library 2.13.12 - scala.util.Either". www.scala-lang.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
8. "Result | Apple Developer Documentation". developer.apple.com. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
9. "std::expected - cppreference.com". en.cppreference.com. 25 August 2023. Archived from the original on 9 October 2023. Retrieved 9 October 2023.

• Functional programming