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 (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 (Maybe (Maybe A) ≠ Maybe A), while nullable types do not (String?? = String?).
Theoretical aspects
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.
An option type can also be seen as a collection containing either one or zero elements.
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]
Names and definitions
In different programming languages, the option type has various names and definitions.
- In Agda, it is named
Maybewith variantsnothingandjust a. - 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 defined as
data Maybe a = Nothing | Just a. - 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
sealed abstract class Option[+A], a type extended byfinal case class Some[+A](value: A)andcase object None. - 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 asT?.
Examples
Ada
Ada does not implement option-types directly, however it provides discriminated types which can be used to parameterize a record. To implement a 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:
Generic
-- Any constrained & non-limited type.
Type Element_Type is private;
Package Optional_Type is
-- When the discriminant, Has_Element, is true there is an element field,
-- when it is false, there are no fields (hence the null keyword).
Type Optional( Has_Element : Boolean ) is record
case Has_Element is
when False => Null;
when True => Element : Element_Type;
end case;
end record;
end Optional_Type;
Scala
Scala implements Option as a parameterized type, so a variable can be an Option, accessed as follows:[4]
// 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 built-in fold 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 a monadic approach, 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.
OCaml
OCaml implements Option as a parameterized variant type. Options are constructed and deconstructed as follows:
(* Deconstructing options *)
let compute opt = match opt with
| None -> "No value"
| Some x -> "The value is: " ^ string_of_int x
print_endline (compute None) (* "No value" *)
print_endline (compute (Some 42)) (* "The value is: 42" *)
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 {
// y is now the non-optional `Int` content of `x`, if it has any
return "The value is: \(y)"
} else {
return "No value"
}
}
print(compute(42)) // The value is: 42
print(compute(nil)) // No value
Rust
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
}
Nim
import options #Module must be imported
proc compute(a : Option[int])=
if(a.isSome): echo "The Value is: " & $a.get
else: echo "The Value is None"
var
a : Option[int] = option(42)
b : Option[int]
compute(a) #The Value is: 42
compute(b) #The Value is None
References
- 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. Retrieved Aug 18, 2019.
- 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.