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In programming languages (especially functional programming languages) and , 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 (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
In , it may be written as: A^{?} = A + 1. This expresses the fact that for a given set of values in A, an option type adds exactly one additional value (the empty value) to the set of valid values for A. This is reflected in programming by the fact that in languages having , option types can be expressed as the tagged union of the encapsulated type plus a .

In the Curry–Howard correspondence, option types are related to the for ∨: x∨1=1.

An option type can also be seen as a collection containing either one or zero elements.

The option type is also a where:

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.


Examples

Agda
In Agda, the option type is named with variants and .


C++
Since C++17, the option type is defined in the standard library as .


Coq
In Coq, the option type is defined as .


Elm
In Elm, the option type is defined as .


F#
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
In Haskell, the option type is defined as .

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
In Idris, the option type is defined as .

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
import std/options

proc showValue(opt: Optionint): 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
In OCaml, the option type is defined as .

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
In Rust, the option type is defined as .

fn show_value(opt: Option) -> 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
In Scala, the option type is defined as , a type extended by and .

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
In Standard ML, the option type is defined as .


Swift
In Swift, the option type is defined as but is generally written as .

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
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 , 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

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