Result type

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>. ^[1]
• In Elm, it is defined by the standard library as typeResultev=Okv|Erre. ^[2]
• In Haskell, by convention the Either type is used for this purpose, which is defined by the standard library as dataEitherab=Lefta|Rightb, where a is the error type and b is the return type. ^[3]
• 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 valueclassResult<outT>. ^[4]
• In OCaml, it is defined by the standard library as type('a,'b)result=Okof'a|Errorof'btype. ^[5]
• 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 enumResult<T,E>{Ok(T),Err(E)}. ^{[6] [7]}
• In Scala, the standard library also defines an Either type, ^[8] however Scala also has more conventional exception handling.
• In Swift, it is defined by the standard library as @frozenenumResult<Success,Failure>whereFailure:Error. ^[9]
• 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.

importstd;usingFileInputStream=std::ifstream;usingString=std::string;usingStringStream=std::stringstream;usingPath=std::filesystem::path;enumclassFileError{MissingFile,NoPermission,// more errors here};std::expected<String,FileError>loadConfig(constPath&p)noexcept{if(!std::filesystem::exists(p)){returnstd::unexpected(FileError::MissingFile);}FileInputStreamconfig{p};StringStreambuffer;if(!config.is_open()){returnstd::unexpected(FileError::NoPermission);}buffer<<config.rdbuf();config.close();returnbuffer.str();}intmain(intargc,char*argv[]){Pathp{/* some path here */};if(conststd::expected<String,FileError>s=loadConfig(p);s.has_value()){std::println("Config contents: {}",s.value());}elseif(s.error()==FileError::MissingFile){std::println("Error: path {} not valid or missing!",p);}elseif(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().

constCAT_FOUND:bool=true;fnmain(){letresult:Result<(),String>=pet_cat();ifresult.is_ok(){println!("Great, we could pet the cat!");}else{leterror:String=result.unwrap_err();println!("Oh no, we couldn't pet the cat: {}",error);}}fnpet_cat()->Result<(),String>{ifCAT_FOUND{Ok(())}else{Err(String::from("The cat is nowhere to be found!"))}}

Vlang

The Error type is an interface for iError.

constcat_found=truefnmain(){cat_name:=get_pet_cat_name()or{println("Oh no, we couldn't pet the cat!")exit(1)}println('Great,wecouldpetthecat'+cat_name)}fnget_pet_cat_name()!string{ifcat_found{return'Max'}else{returnerror('thecatisnowheretobefound')}}

See also

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

References

1. "std::expected - cppreference.com". en.cppreference.com. 25 August 2023. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
2. "Result · An Introduction to Elm". guide.elm-lang.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
3. "Data.Either". hackage.haskell.org. 22 September 2023. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
4. "Result - Kotlin Programming Language". kotlinlang.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
5. "Error Handling · OCaml Tutorials". ocaml.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
6. "std::result - Rust". doc.rust-lang.org. Archived from the original on 9 October 2023. Retrieved 9 October 2023.
7. "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.
8. "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.
9. "Result | Apple Developer Documentation". developer.apple.com. Archived from the original on 9 October 2023. Retrieved 9 October 2023.