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

In type theory. 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. An option type is a particular case of a tagged union, where the Nothing is taken as (nullary constructor for a) singleton type. Tagged unions can generally be implemented by a combination of union types and record typess using occurrence typing.

The option type is also a monad where:

The monadic nature of the option type is useful for efficiently tracking failure and errors.

Examples

Ada

Ada does not implement option-types directly, however it provides discriminated types which can be used to parameterize a record. To implement an Option type, a Boolean type is used as the discriminant; the following example provides a generic to create an option type from any non-limited constrained type:

Example usage:

Agda

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

ATS

In ATS, the option type is defined as

C++

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

In C++23, support for monadic operations for is available.

Elm

In Elm, the option type is defined as .

F#

In F#, the option type is defined as .

Haskell

In Haskell, the option type is defined as .

Idris

In Idris, the option type is defined as .

Java

In Java, the option type is defined the standard library by the class.

Nim

OCaml

In OCaml, the option type is defined as .

Rocq

In Rocq, the option type is defined as .

Rust

In Rust, the option type is defined as .

Scala

In Scala, the option type is defined as , a type extended by and .

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 .

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.

See also

• Result type

• Tagged union

• Nullable type

• Null object pattern

• Exception handling

• Pattern matching

References