Option type
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In programming languages (especially functional programming languages) and type theory, an option type or maybe type is a polymorphic type that represents encapsulation of an optional value; e.g., it is used as the return type of functions which may or may not return a meaningful value when they are applied. It consists of a constructor which either is empty (often named None
or Nothing
), or which encapsulates the original data type A
(often written Just A
or Some A
).
A distinct, but related concept outside of functional programming, which is popular in object-oriented programming, is called nullable types (often expressed as A?
). The core difference between option types and nullable types is that option types support nesting (e.g. Maybe (Maybe String)
≠ Maybe String
), while nullable types do not (e.g. String??
= String?
).
Theoretical aspects
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In type theory, it may be written as: . This expresses the fact that for a given set of values in , an option type adds exactly one additional value (the empty value) to the set of valid values for . This is reflected in programming by the fact that in languages having tagged unions, option types can be expressed as the tagged union of the encapsulated type plus a unit type.[1]
In the Curry–Howard correspondence, option types are related to the annihilation law for ∨: x∨1=1.[how?]
An option type can also be seen as a collection containing either one or zero elements.[original research?]
The option type is also a monad where:[2]
return = Just -- Wraps the value into a maybe Nothing >>= f = Nothing -- Fails if the previous monad fails (Just x) >>= f = f x -- Succeeds when both monads succeed
The monadic nature of the option type is useful for efficiently tracking failure and errors.[3]
Examples
[edit]Agda
[edit]This section needs expansion with: example usage. You can help by adding to it. (July 2022) |
In Agda, the option type is named Maybe
with variants nothing
and just a
.
ATS
[edit]In ATS, the option type is defined as
datatype option_t0ype_bool_type (a: t@ype+, bool) = | Some(a, true) of a | None(a, false) stadef option = option_t0ype_bool_type typedef Option(a: t@ype) = [b:bool] option(a, b)
#include "share/atspre_staload.hats" fn show_value (opt: Option int): string = case+ opt of | None() => "No value" | Some(s) => tostring_int s implement main0 (): void = let val full = Some 42 and empty = None in println!("show_value full → ", show_value full); println!("show_value empty → ", show_value empty); end
show_value full → 42 show_value empty → No value
C++
[edit]Since C++17, the option type is defined in the standard library as template<typename T> std::optional<T>
.
Coq
[edit]This section needs expansion with: example usage. You can help by adding to it. (July 2022) |
In Coq, the option type is defined as Inductive option (A:Type) : Type := | Some : A -> option A | None : option A.
.
Elm
[edit]This section needs expansion with: example usage. You can help by adding to it. (July 2022) |
In Elm, the option type is defined as type Maybe a = Just a | Nothing
.[4]
F#
[edit]In F#, the option type is defined as type 'a option = None | Some of 'a
.[5]
let showValue = Option.fold (fun _ x -> sprintf "The value is: %d" x) "No value" let full = Some 42 let empty = None showValue full |> printfn "showValue full -> %s" showValue empty |> printfn "showValue empty -> %s"
showValue full -> The value is: 42 showValue empty -> No value
Haskell
[edit]In Haskell, the option type is defined as data Maybe a = Nothing | Just a
.[6]
showValue :: Maybe Int -> String showValue = foldl (\_ x -> "The value is: " ++ show x) "No value" main :: IO () main = do let full = Just 42 let empty = Nothing putStrLn $ "showValue full -> " ++ showValue full putStrLn $ "showValue empty -> " ++ showValue empty
showValue full -> The value is: 42 showValue empty -> No value
Idris
[edit]In Idris, the option type is defined as data Maybe a = Nothing | Just a
.
showValue : Maybe Int -> String showValue = foldl (\_, x => "The value is " ++ show x) "No value" main : IO () main = do let full = Just 42 let empty = Nothing putStrLn $ "showValue full -> " ++ showValue full putStrLn $ "showValue empty -> " ++ showValue empty
showValue full -> The value is: 42 showValue empty -> No value
Nim
[edit]This section needs expansion with: the definition. You can help by adding to it. (July 2022) |
import std/options proc showValue(opt: Option[int]): string = opt.map(proc (x: int): string = "The value is: " & $x).get("No value") let full = some(42) empty = none(int) echo "showValue(full) -> ", showValue(full) echo "showValue(empty) -> ", showValue(empty)
showValue(full) -> The Value is: 42 showValue(empty) -> No value
OCaml
[edit]In OCaml, the option type is defined as type 'a option = None | Some of 'a
.[7]
let show_value = Option.fold ~none:"No value" ~some:(fun x -> "The value is: " ^ string_of_int x) let () = let full = Some 42 in let empty = None in print_endline ("show_value full -> " ^ show_value full); print_endline ("show_value empty -> " ^ show_value empty)
show_value full -> The value is: 42 show_value empty -> No value
Rust
[edit]In Rust, the option type is defined as enum Option<T> { None, Some(T) }
.[8]
fn show_value(opt: Option<i32>) -> String { opt.map_or("No value".to_owned(), |x| format!("The value is: {}", x)) } fn main() { let full = Some(42); let empty = None; println!("show_value(full) -> {}", show_value(full)); println!("show_value(empty) -> {}", show_value(empty)); }
show_value(full) -> The value is: 42 show_value(empty) -> No value
Scala
[edit]In Scala, the option type is defined as sealed abstract class Option[+A]
, a type extended by final case class Some[+A](value: A)
and case object None
.
object Main: def showValue(opt: Option[Int]): String = opt.fold("No value")(x => s"The value is: $x") def main(args: Array[String]): Unit = val full = Some(42) val empty = None println(s"showValue(full) -> ${showValue(full)}") println(s"showValue(empty) -> ${showValue(empty)}")
showValue(full) -> The value is: 42 showValue(empty) -> No value
Standard ML
[edit]This section needs expansion with: example usage. You can help by adding to it. (July 2022) |
In Standard ML, the option type is defined as datatype 'a option = NONE | SOME of 'a
.
Swift
[edit]In Swift, the option type is defined as enum Optional<T> { case none, some(T) }
but is generally written as T?
.[9]
func showValue(_ opt: Int?) -> String { return opt.map { "The value is: \($0)" } ?? "No value" } let full = 42 let empty: Int? = nil print("showValue(full) -> \(showValue(full))") print("showValue(empty) -> \(showValue(empty))")
showValue(full) -> The value is: 42 showValue(empty) -> No value
Zig
[edit]In Zig, add ? before the type name like ?i32
to make it an optional type.
Payload n can be captured in an if or while statement, such as if (opt) |n| { ... } else { ... }
, and an else clause is evaluated if it is null
.
const std = @import("std"); fn showValue(allocator: std.mem.Allocator, opt: ?i32) ![]u8 { return if (opt) |n| std.fmt.allocPrint(allocator, "The value is: {}", .{n}) else allocator.dupe(u8, "No value"); } pub fn main() !void { // Set up an allocator, and warn if we forget to free any memory. var gpa = std.heap.GeneralPurposeAllocator(.{}){}; defer std.debug.assert(gpa.deinit() == .ok); const allocator = gpa.allocator(); // Prepare the standard output stream. const stdout = std.io.getStdOut().writer(); // Perform our example. const full = 42; const empty = null; const full_msg = try showValue(allocator, full); defer allocator.free(full_msg); try stdout.print("showValue(allocator, full) -> {s}\n", .{full_msg}); const empty_msg = try showValue(allocator, empty); defer allocator.free(empty_msg); try stdout.print("showValue(allocator, empty) -> {s}\n", .{empty_msg}); }
showValue(allocator, full) -> The value is: 42 showValue(allocator, empty) -> No value
See also
[edit]References
[edit]- ^ Milewski, Bartosz (2015-01-13). "Simple Algebraic Data Types". Bartosz Milewski's Programming Cafe. Sum types. "We could have encoded Maybe as: data Maybe a = Either () a". Archived from the original on 2019-08-18. Retrieved 2019-08-18.
- ^ "A Fistful of Monads - Learn You a Haskell for Great Good!". www.learnyouahaskell.com. Retrieved 2019-08-18.
- ^ Hutton, Graham (Nov 25, 2017). "What is a Monad?". Computerphile Youtube. Archived from the original on 2021-12-20. Retrieved Aug 18, 2019.
- ^ "Maybe · An Introduction to Elm". guide.elm-lang.org.
- ^ "Options". fsharp.org. Retrieved 2024-10-08.
- ^ "6 Predefined Types and Classes". www.haskell.org. Retrieved 2022-06-15.
- ^ "OCaml library : Option". v2.ocaml.org. Retrieved 2022-06-15.
- ^ "Option in core::option - Rust". doc.rust-lang.org. 2022-05-18. Retrieved 2022-06-15.
- ^ "Apple Developer Documentation". developer.apple.com. Retrieved 2020-09-06.