Option type

In programming languages (more so 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 (named None or Nothing), or which encapsulates the original data type A (written Just A or Some A). Outside of functional programming, these are termed nullable types.

Names and definitions

In different programming languages, the option type has various names and definitions.

• In Agda, it is named Maybe with variants nothing and just a.
• In C++17 it is defined as the template class std::optional<T>.
• In C#, it is defined as T?.
• In Coq, it is defined as Inductive option (A:Type) : Type := | Some : A -> option A | None : option A..
• In Haskell, it is named Maybe, and defined as data Maybe a = Nothing | Just a.
• In Idris, it is also defined as data Maybe a = Nothing | Just a.
• In Java, since version 8, it is defined as parameterized final class Optional<T>.
• In Julia, the option type is named Nullable{T}.
• In Kotlin, it is defined as T?. ^[1]
• In OCaml, it is defined as type 'a option = None | Some of 'a.
• In Rust, it is defined as enum Option<T> { None, Some(T) }.
• In Scala, it is defined as parameterized abstract class '.. Option[A] = if (x == null) None else Some(x)...
• In Standard ML, it is defined as datatype 'a option = NONE | SOME of 'a.
• In Swift, it is defined as enum Optional<T> { case none, some(T) } but is generally written as T? and is initialized with either a value or nil.

In type theory, it may be written as: $A^{?} = A + 1$.

In languages having tagged unions, as in most functional programming languages, option types can be expressed as the tagged union of a unit type plus the encapsulated 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 monad

The option type is a monad under these functions:

$\text{return}\colon A \to A^{?} = a \mapsto \text{Just} \, a$

$\text{bind}\colon A^{?} \to (A \to B^{?}) \to B^{?} = a \mapsto f \mapsto \begin{cases} \text{Nothing} & \text{if} \ a = \text{Nothing}\\ f \, a' & \text{if} \ a = \text{Just} \, a' \end{cases}$

We may also describe the option monad in terms of functions return, fmap and join, where the latter two are given by:

$\text{fmap} \colon (A \to B) \to A^{?} \to B^{?} = f \mapsto a \mapsto \begin{cases} \text{Nothing} & \text{if} \ a = \text{Nothing}\\ \text{Just} \, f \, a' & \text{if} \ a = \text{Just} \, a' \end{cases}$

$\text{join} \colon {A^{?}}^{?} \to A^{?} = a \mapsto \begin{cases} \text{Nothing} & \text{if} \ a = \text{Nothing}\\ \text{Nothing} & \text{if} \ a = \text{Just} \, \text{Nothing}\\ \text{Just} \, a' & \text{if} \ a = \text{Just} \, \text{Just} \, a' \end{cases}$

The option monad is an additive monad: it has Nothing as a zero constructor and the following function as a monadic sum:

$\text{mplus} \colon A^{?} \to A^{?} \to A^{?} = a_1 \mapsto a_2 \mapsto \begin{cases} \text{Nothing} & \text{if} \ a_1 = \text{Nothing} \and a_2 = \text{Nothing}\\ \text{Just} \, a'_2 & \text{if} \ a_1 = \text{Nothing} \and a_2 = \text{Just} \, a'_2 \\ \text{Just} \, a'_1 & \text{if} \ a_1 = \text{Just} \, a'_1 \end{cases}$

The resulting structure is an idempotent monoid.

Examples

Scala

Scala implements Option as a parameterized type, so a variable can be an Option, accessed as follows:^[2]

// Defining variables that are Options of type Int
val res1: Option[Int] = Some(42)
val res2: Option[Int] = None

// sample 1 :  This function uses pattern matching to deconstruct Options
def compute(opt: Option[Int]) = opt match {
  case None => "No value"
  case Some(x) => "The value is: " + x
}

// sample 2 :  This function uses monad method
def compute(opt: Option[Int]) = opt.fold("No Value")(v => "The value is:" + v )

println(compute(res1))  // The value is: 42
println(compute(res2))  // No value

Two main ways to use an Option value exist. The first, not the best, is the pattern matching, as in the first example. The second, the best practice, is the monad method, as in the second example. In this way, a program is safe, as it can generate no exception or error (e.g., by trying to obtain the value of an Option variable that is equal to None). Thus, it essentially works as a type-safe alternative to the null value.

F#

(* This function uses pattern matching to deconstruct Options *)
let compute = function
  | None   -> "No value"
  | Some x -> sprintf "The value is: %d" x

printfn "%s" (compute <| Some 42)(* The value is: 42 *)
printfn "%s" (compute None)      (* No value         *)

Haskell

-- This function uses pattern matching to deconstruct Maybes
compute :: Maybe Int -> String
compute Nothing  = "No value"
compute (Just x) = "The value is: " ++ show x

main :: IO ()
main = do
    print $ compute (Just 42) -- The value is: 42
    print $ compute Nothing -- No value

Swift

func compute(x: Int?) -> String {
  // This function uses optional binding to deconstruct optionals
  if let y = x {
    return "The value is: \(y)"
  } else {
    return "No value"
  }
}

print(compute(42)) // The value is: 42
print(compute(nil)) // No value

func compute(x: Int?) -> String {
  // This function explicitly unwraps an optional after comparing to nil
  return nil == x ? "No value" : "The value is: \(x!)"
}

print(compute(42)) // The value is: 42
print(compute(nil)) // No value

func compute(x: Int?) -> String {
  // This function uses pattern matching to deconstruct optionals
  switch x {
  case .none: 
    return "No value"
  case .some(let y): 
    return "The value is: \(y)"
  }
}

print(compute(42)) // The value is: 42
print(compute(nil)) // No value

Rust

Rust allows using either pattern matching or optional binding to deconstruct the Option type:

fn main() {
    // This function uses pattern matching to deconstruct optionals
    fn compute(x: Option<i32>) -> String {
        match x {
            Some(a) => format!("The value is: {}", a),
            None    => format!("No value")
        }
    }

    println!("{}", compute(Some(42))); // The value is: 42
    println!("{}", compute(None)); // No value
}

fn main() {
    // This function uses optional binding to deconstruct optionals
    fn compute(x: Option<i32>) -> String {
        if let Some(a) = x {
            format!("The value is: {}", a)
        } else {
            format!("No value")
        }
    }

    println!("{}", compute(Some(42))); // The value is: 42
    println!("{}", compute(None)); // No value
}

Julia

Julia requires explicit deconstruction to access a nullable value:

function compute(x::Nullable{Int})
    if !isnull(x)
        println("The value is: $(get(x))")
    else
        println("No value")
    end
end

julia> compute(Nullable(42))
The value is: 42
julia> compute(Nullable{Int}())
No value

See also

• Monad (functional_programming)
• Tagged union
• Nullable type
• Null Object pattern
• Sentinel value
• exception handling

References

1. "Null Safety - Kotlin Programming Language". Retrieved 2016-08-01.
2. Martin Odersky; Lex Spoon; Bill Venners (2008). Programming in Scala. Artima Inc. pp. 282–284. ISBN 978-0-9815316-0-1. Retrieved 6 September 2011.