{{Short description|Concept in functional programming}} {{Use dmy dates|date=October 2023}} {{More citations needed|date=January 2021|bot=noref (GreenC bot)}} 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 {{code|2=cpp|1=std::expected<T, E>}}.<ref>{{cite web |date=2023-08-25 |title=std::expected - cppreference.com |url=https://en.cppreference.com/w/cpp/utility/expected |url-status=live |archive-url=https://web.archive.org/web/20231009033232/https://en.cppreference.com/w/cpp/utility/expected |archive-date=2023-10-09 |access-date=2023-10-09 |website=en.cppreference.com}}</ref> * In Elm, it is defined by the standard library as {{code|2=elm|1=type Result e v = Ok v {{!}} Err e}}.<ref>{{cite web |title=Result · An Introduction to Elm |url=https://guide.elm-lang.org/error_handling/result.html |url-status=live |archive-url=https://web.archive.org/web/20231009032650/https://guide.elm-lang.org/error_handling/result.html |archive-date=2023-10-09 |access-date=2023-10-09 |website=guide.elm-lang.org}}</ref> * In Haskell, by convention the {{code|Either|haskell}} type is used for this purpose, which is defined by the standard library as {{code|1=data Either a b = Left a {{!}} Right b|2=haskell}}, where {{code|a|haskell}} is the error type and {{code|b|haskell}} is the return type.<ref>{{cite web |date=2023-09-22 |title=Data.Either |url=https://hackage.haskell.org/package/base-4.18.1.0/docs/Data-Either.html |url-status=live |archive-url=https://web.archive.org/web/20231009031549/https://hackage.haskell.org/package/base-4.18.1.0/docs/Data-Either.html |archive-date=2023-10-09 |access-date=2023-10-09 |website=hackage.haskell.org}}</ref> * In Java, it is not natively in the standard library, but is available from third party libraries. For example, [https://github.com/sviperll/result4j result4j] which includes an interface <code>Result<R, E></code> similar to Rust <code>Result<T, E></code>, and [https://github.com/vavr-io/vavr vavr] includes an interface <code>Either<L, R></code> similar to Haskell <code>Either a b</code>. Because Java and Kotlin are cross-compatible, Java can use the <code>Result</code> type from Kotlin. * In Kotlin, it is defined by the standard library as {{code|2=kotlin|1=value class Result<out T>}}.<ref>{{cite web |title=Result - Kotlin Programming Language |url=https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-result/ |url-status=live |archive-url=https://web.archive.org/web/20231009032856/https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-result/ |archive-date=2023-10-09 |access-date=2023-10-09 |website=kotlinlang.org}}</ref> * In OCaml, it is defined by the standard library as {{code|2=ocaml|1=type ('a, 'b) result = Ok of 'a {{!}} Error of 'b type}}.<ref>{{cite web |title=Error Handling · OCaml Tutorials |url=https://ocaml.org/docs/error-handling#using-the-result-type-for-errors |url-status=live |archive-url=https://web.archive.org/web/20231009030828/https://ocaml.org/docs/error-handling#using-the-result-type-for-errors |archive-date=2023-10-09 |access-date=2023-10-09 |website=ocaml.org}}</ref> * In Python, it is not natively in the standard library, but is available from third party libraries such as [https://github.com/dry-python/returns returns] and [https://github.com/rustedpy/result result]. * In Rust, it is defined by the standard library as {{code|2=rust|1=enum Result<T, E> { Ok(T), Err(E) } }}.<ref>{{cite web |title=std::result - Rust |url=https://doc.rust-lang.org/std/result/index.html |url-status=live |archive-url=https://web.archive.org/web/20231009032955/https://doc.rust-lang.org/std/result/index.html |archive-date=2023-10-09 |access-date=2023-10-09 |website=doc.rust-lang.org}}</ref><ref>{{cite web |date=2011-10-29 |title=stdlib: Add result module · rust-lang/rust@c1092fb |url=https://github.com/rust-lang/rust/commit/c1092fb6d88efe51e42df3aae2a321cc669e12a0 |url-status=live |archive-url=https://web.archive.org/web/20231009033047/https://github.com/rust-lang/rust/commit/c1092fb6d88efe51e42df3aae2a321cc669e12a0 |archive-date=2023-10-09 |access-date=2023-10-09 |website=github.com}}</ref> * In Scala, the standard library also defines an {{code|Either|scala}} type,<ref>{{cite web |title=Scala Standard Library 2.13.12 - scala.util.Either |url=https://www.scala-lang.org/api/current/scala/util/Either.html |url-status=live |archive-url=https://web.archive.org/web/20231009032237/https://www.scala-lang.org/api/current/scala/util/Either.html |archive-date=2023-10-09 |accessdate=2023-10-09 |website=www.scala-lang.org}}</ref> however Scala also has more conventional exception handling. * In Swift, it is defined by the standard library as {{code|2=swift|1=@frozen enum Result<Success, Failure> where Failure : Error}}.<ref>{{cite web |title=Result {{!}} Apple Developer Documentation |url=https://developer.apple.com/documentation/swift/result |url-status=live |archive-url=https://web.archive.org/web/20231009032505/https://developer.apple.com/documentation/swift/result |archive-date=2023-10-09 |access-date=2023-10-09 |website=developer.apple.com}}</ref> * In V, the result type is implemented natively using <code>!T</code> as the return type of a function. For example <code>fn my_function() !string { ... }</code>. [https://v-lang.dev/book/en/error_handling.md Error Handling in V].
=== C++ === The <code>expected<T, E></code> class uses <code>std::unexpected()</code> to return the type <code>E</code>, and can return <code>T</code> directly. <syntaxhighlight lang="cpp"> import std;
using std::expected; using std::ifstream; using std::string; using std::stringstream; using std::unexpected; using std::filesystem::path;
enum class FileError { MISSING_FILE, NO_PERMISSION, // more errors here };
expected<string, FileError> loadConfig(const path& p) noexcept { if (!std::filesystem::exists(p)) { return unexpected(FileError::MISSING_FILE); } ifstream config{p}; stringstream buffer; if (!config.is_open()) { return unexpected(FileError::NO_PERMISSION); } buffer << config.rdbuf(); config.close(); return buffer.str(); }
int main(int argc, char* argv[]) { path p{"configs/my_config.txt"}; if (const expected<String, FileError> s = loadConfig(p); s.has_value()) { std::println("Config contents: {}", s.value()); } else { switch (s.error) { case FileError::MISSING_FILE: std::println("Error: path {} not valid or missing!", p); break; case FileError::NO_PERMISSION: std::println("Error: no permission to read file at path {}!", p); break; // additional cases... default: std::unreachable(); } } } </syntaxhighlight>
=== Rust === Enums in Rust are tagged unions, which can be unpacked with strong type checking through pattern matching. <syntaxhighlight lang="rust"> const CAT_FOUND: bool = true;
fn main() { let result: Result<(), String> = pet_cat(); match result { Ok(_) => println!("Great, we could pet the cat!"), Err(error) => 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!")) } } </syntaxhighlight>
=== Vlang === The <code>Error</code> type is an interface for <code>iError</code>. <syntaxhighlight lang="rust">
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') } } </syntaxhighlight>
== See also == * Option type * Exception handling * Tagged union * Return type
== References == {{Reflist}}
Category:Functional programming