Result type
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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
.[1] - In Haskell, by convention the
Either
type is used for this purpose, which is defined by the standard library asdata Either a b = Left a | Right b
.[2] - In OCaml, it is defined by the standard library as
type ('a, 'b) result = Ok of 'a | Error of 'b type
.[3] - In Rust, it is defined by the standard library as
enum Result<T, E> { Ok(T), Err(E) }
.[4] - In Scala, the standard library also defines an
Either
type,[5] 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
.[6]
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
References
- ^ "Result · An Introduction to Elm". guide.elm-lang.org.
- ^ "Data.Either". hackage.haskell.org.
- ^ "Error Handling – OCaml". ocaml.org.
- ^ "std::result - Rust". doc.rust-lang.org.
- ^ "Scala Standard Library 2.13.3 - scala.util.Either". www.scala-lang.org. Retrieved 9 October 2020.
- ^ "Apple Developer Documentation". developer.apple.com.