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 or ), or which encapsulates the original data type (often written or ). A distinct, but related concept outside of functional programming, which is popular in object-oriented programming, is called nullable types (often expressed as ). The core difference between option types and nullable types is that option types support nesting (e.g. ≠ ), while nullable types do not (e.g. = ).

Theoretical aspects

In type theory, it may be written as:. 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 tagged unions, option types can be expressed as the tagged union of the encapsulated type plus a unit type. In the Curry–Howard correspondence, option types are related to the annihilation law 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 monad where: The monadic nature of the option type is useful for efficiently tracking failure and errors.

Examples

Agda

In Agda, the option type is named Maybe with variants nothing and just a.

ATS

In ATS, the option type is defined as

C++

Since C++17, the option type is defined in the standard library as template<typename T> std::optional<T>.

Coq

In Coq, the option type is defined as Some : A -> option A.

Elm

In Elm, the option type is defined as Nothing.

F#

In F#, the option type is defined as Some of 'a.

Haskell

In Haskell, the option type is defined as Just a.

Idris

In Idris, the option type is defined as Just a.

Nim

OCaml

In OCaml, the option type is defined as Some of 'a.

Rust

In Rust, the option type is defined as enum Option<T> { None, Some(T) }.

Scala

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.

Standard ML

In Standard ML, the option type is defined as SOME of 'a.

Swift

In Swift, the option type is defined as enum Optional<T> { case none, some(T) } but is generally written as T?.

Zig

In Zig, add ? before the type name like to make it an optional type. Payload n can be captured in an if or while statement, such as if (opt), and an else clause is evaluated if it is.