Option type
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Inprogramming languages(especiallyfunctional programminglanguages) andtype theory,anoption typeormaybe typeis apolymorphic typethat 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 namedNone
orNothing
), or which encapsulates the original data typeA
(often writtenJust A
orSome A
).
A distinct, but related concept outside of functional programming, which is popular inobject-oriented programming,is callednullable types(often expressed asA?
). 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
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Intype 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 havingtagged unions,option types can be expressed as the tagged union of the encapsulated type plus aunit type.[1]
In theCurry–Howard correspondence,option types are related to theannihilation lawfor ∨: x∨1=1.[how?]
An option type can also be seen as acollectioncontaining either one or zero elements.[original research?]
The option type is also amonadwhere:[2]
return=Just-- Wraps the value into a maybe
Nothing>>=f=Nothing-- Fails if the previous monad fails
(Justx)>>=f=fx-- Succeeds when both monads succeed
The monadic nature of the option type is useful for efficiently tracking failure and errors.[3]
Examples
[edit]Agda
[edit]![]() | This sectionneeds expansionwith: example usage. You can help byadding to it.(July 2022) |
In Agda, the option type is namedMaybe
with variantsnothing
andjusta
.
C++
[edit]Since C++17, the option type is defined in the standard library astemplate<typenameT>std::optional<T>
.
Coq
[edit]![]() | This sectionneeds expansionwith: example usage. You can help byadding to it.(July 2022) |
In Coq, the option type is defined asInductiveoption(A:Type):Type:=|Some:A->optionA|None:optionA.
.
Elm
[edit]![]() | This sectionneeds expansionwith: example usage. You can help byadding to it.(July 2022) |
In Elm, the option type is defined astypeMaybea=Justa|Nothing
.[4]
F#
[edit]![]() | This sectionneeds expansionwith: the definition. You can help byadding to it.(July 2022) |
letshowValue=
Option.fold(fun_x->sprintf"The value is: %d"x)"No value"
letfull=Some42
letempty=None
showValuefull|>printfn"showValue full -> %s"
showValueempty|>printfn"showValue empty -> %s"
showValue full -> The value is: 42
showValue empty -> No value
Haskell
[edit]In Haskell, the option type is defined asdataMaybea=Nothing|Justa
.[5]
showValue::MaybeInt->String
showValue=foldl(\_x->"The value is:"++showx)"No value"
main::IO()
main=do
letfull=Just42
letempty=Nothing
putStrLn$"showValue full ->"++showValuefull
putStrLn$"showValue empty ->"++showValueempty
showValue full -> The value is: 42
showValue empty -> No value
Idris
[edit]In Idris, the option type is defined asdataMaybea=Nothing|Justa
.
showValue:MaybeInt->String
showValue=foldl(\_,x=>"The value is"++showx)"No value"
main:IO()
main=do
letfull=Just42
letempty=Nothing
putStrLn$"showValue full ->"++showValuefull
putStrLn$"showValue empty ->"++showValueempty
showValue full -> The value is: 42
showValue empty -> No value
Nim
[edit]![]() | This sectionneeds expansionwith: the definition. You can help byadding to it.(July 2022) |
importstd/options
procshowValue(opt:Option[int]):string=
opt.map(proc(x:int):string="The value is:"&$x).get("No value")
let
full=some(42)
empty=none(int)
echo"showValue(full) ->",showValue(full)
echo"showValue(empty) ->",showValue(empty)
showValue(full) -> The Value is: 42
showValue(empty) -> No value
OCaml
[edit]In OCaml, the option type is defined astype'aoption=None|Someof'a
.[6]
letshow_value=
Option.fold~none:"No value"~some:(funx->"The value is:"^string_of_intx)
let()=
letfull=Some42in
letempty=Nonein
print_endline("show_value full ->"^show_valuefull);
print_endline("show_value empty ->"^show_valueempty)
show_value full -> The value is: 42
show_value empty -> No value
Rust
[edit]In Rust, the option type is defined asenumOption<T>{None,Some(T)}
.[7]
fnshow_value(opt:Option<i32>)->String{
opt.map_or("No value".to_owned(),|x|format!("The value is: {}",x))
}
fnmain(){
letfull=Some(42);
letempty=None;
println!("show_value(full) -> {}",show_value(full));
println!("show_value(empty) -> {}",show_value(empty));
}
show_value(full) -> The value is: 42
show_value(empty) -> No value
Scala
[edit]In Scala, the option type is defined assealedabstractclassOption[+A]
,a type extended byfinalcaseclassSome[+A](value:A)
andcaseobjectNone
.
objectMain:
defshowValue(opt:Option[Int]):String=
opt.fold("No value")(x=>s "The value is:$x")
defmain(args:Array[String]):Unit=
valfull=Some(42)
valempty=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
[edit]![]() | This sectionneeds expansionwith: example usage. You can help byadding to it.(July 2022) |
In Standard ML, the option type is defined asdatatype'aoption=NONE|SOMEof'a
.
Swift
[edit]In Swift, the option type is defined asenumOptional<T>{casenone,some(T)}
but is generally written asT?
.[8]
funcshowValue(_opt:Int?)->String{
returnopt.map{"The value is:\($0)"}??"No value"
}
letfull=42
letempty:Int?=nil
print("showValue(full) ->\(showValue(full))")
print("showValue(empty) ->\(showValue(empty))")
showValue(full) -> The value is: 42
showValue(empty) -> No value
Zig
[edit]In Zig, add? before the type name like?i32
to make it an optional type.
Payloadncan be captured in aniforwhilestatement, such asif(opt)|n|{...}else{...}
,and anelseclause is evaluated if it isnull
.
conststd=@import("std");
fnshowValue(allocator:std.mem.Allocator,opt:?i32)![]u8{
returnif(opt)|n|
std.fmt.allocPrint(allocator,"The value is: {}",.{n})
else
allocator.dupe(u8,"No value");
}
pubfnmain()!void{
// Set up an allocator, and warn if we forget to free any memory.
vargpa=std.heap.GeneralPurposeAllocator(.{}){};
deferstd.debug.assert(gpa.deinit()==.ok);
constallocator=gpa.allocator();
// Prepare the standard output stream.
conststdout=std.io.getStdOut().writer();
// Perform our example.
constfull=42;
constempty=null;
constfull_msg=tryshowValue(allocator,full);
deferallocator.free(full_msg);
trystdout.print("showValue(allocator, full) -> {s}\n",.{full_msg});
constempty_msg=tryshowValue(allocator,empty);
deferallocator.free(empty_msg);
trystdout.print("showValue(allocator, empty) -> {s}\n",.{empty_msg});
}
showValue(allocator, full) -> The value is: 42
showValue(allocator, empty) -> No value
See also
[edit]References
[edit]- ^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".Archivedfrom the original on 2019-08-18.Retrieved2019-08-18.
- ^"A Fistful of Monads - Learn You a Haskell for Great Good!".www.learnyouahaskell.com.Retrieved2019-08-18.
- ^Hutton, Graham (Nov 25, 2017)."What is a Monad?".Computerphile Youtube.Archivedfrom the original on 2021-12-20.RetrievedAug 18,2019.
- ^"Maybe · An Introduction to Elm".guide.elm-lang.org.
- ^"6 Predefined Types and Classes".www.haskell.org.Retrieved2022-06-15.
- ^"OCaml library: Option".v2.ocaml.org.Retrieved2022-06-15.
- ^"Option in core::option - Rust".doc.rust-lang.org.2022-05-18.Retrieved2022-06-15.
- ^"Apple Developer Documentation".developer.apple.com.Retrieved2020-09-06.