In the area of
mathematical logic
Mathematical logic is the study of formal logic within mathematics. Major subareas include model theory, proof theory, set theory, and recursion theory. Research in mathematical logic commonly addresses the mathematical properties of forma ...
and
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
known as
type theory
In mathematics, logic, and computer science, a type theory is the formal presentation of a specific type system, and in general type theory is the academic study of type systems. Some type theories serve as alternatives to set theory as a founda ...
, a unit type is a
type that allows only one value (and thus can hold no information). The carrier (underlying set) associated with a unit type can be any
singleton set. There is an
isomorphism
In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word i ...
between any two such sets, so it is customary to talk about ''the'' unit type and ignore the details of its value. One may also regard the unit type as the type of 0-
tuple
In mathematics, a tuple is a finite ordered list (sequence) of elements. An -tuple is a sequence (or ordered list) of elements, where is a non-negative integer. There is only one 0-tuple, referred to as ''the empty tuple''. An -tuple is defi ...
s, i.e. the
product of no types.
The unit type is the
terminal object in the
category of types and typed functions. It should not be confused with the ''zero'' or
bottom type, which allows ''no'' values and is the
initial object in this category. Similarly, the
Boolean is the type with ''two'' values.
The unit type is implemented in most
functional programming
In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions tha ...
languages. The
void type that is used in some imperative programming languages serves some of its functions, but because its carrier set is empty, it has some limitations (as detailed below).
In programming languages
Several computer
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
s provide a unit type to specify the result type of a
function with the sole purpose of causing a
side effect, and the argument type of a function that does not require arguments.
*In
Haskell,
Rust
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
, and
Elm, the unit type is called
()
and its only value is also
()
, reflecting the 0-tuple interpretation.
*In
ML descendants (including
OCaml
OCaml ( , formerly Objective Caml) is a general-purpose, multi-paradigm programming language
Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms.
...
,
Standard ML
Standard ML (SML) is a general-purpose, modular, functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of ...
, and
F#), the type is called
unit
but the value is written as
()
.
*In
Scala, the unit type is called
Unit
and its only value is written as
()
.
*In
Common Lisp
Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fr ...
the type named is a unit type which has one value, namely the symbol . This should not be confused with the ''type'', which is the
bottom type.
* In
Python, there is a type called
NoneType
which allows the single value of
None
.
* In
Swift, the unit type is called
Void
or
()
and its only value is also
()
, reflecting the 0-tuple interpretation.
* In
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
, the unit type is called
Void
and its only value is
null
.
* In
Go, the unit type is written
struct
and its value is
struct
.
* In
PHP, the unit type is called null, which only value is NULL itself.
* In
JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
, both
Null
(its only value is
null
) and
Undefined
(its only value is
undefined
) are built-in unit types.
* in
Kotlin,
Unit
is a singleton with only one value: the
Unit
object.
* In
Ruby
A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
,
nil
is the only instance of the
NilClass
class.
* In
C++, the
std::monostate
unit type was added in C++17. Before that, it is possible to define a custom unit type using an empty struct such as
struct empty
.
Void type as unit type
In
C,
C++,
C#, and
D,
void
is used to designate a function that does not return anything useful, or a function that accepts no arguments. The unit type in C is conceptually similar to an empty
struct
, but a struct without members is not allowed in the C language specification (this is allowed in C++). Instead, '
void
' is used in a manner that simulates some, but not all, of the properties of the unit type, as detailed below. Like most imperative languages, C allows functions that do not return a value; these are specified as having the void return type. Such functions are called procedures in other imperative languages like
Pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Frenc ...
, where a syntactic distinction, instead of type-system distinction, is made between functions and procedures.
Difference in calling convention
The first notable difference between a true unit type and the void type is that the unit type may always be the type of the argument to a function, but the void type cannot be the type of an argument in C, despite the fact that it may appear as the sole argument in the list. This problem is best illustrated by the following program, which is a compile-time error in C:
void f(void)
void g(void)
int main(void)
This issue does not arise in most programming practice in C, because since the
void
type carries no information, it is useless to pass it anyway; but it may arise in
generic programming, such as C++
templates, where
void
must be treated differently from other types. In C++ however, empty classes are allowed, so it is possible to implement a real unit type; the above example becomes compilable as:
class unit_type ;
const unit_type the_unit;
unit_type f(unit_type)
unit_type g(unit_type)
int main()
(For brevity, we're not worried in the above example whether
the_unit
is really a
singleton; see
singleton pattern for details on that issue.)
Difference in storage
The second notable difference is that the void type is special and can never be stored in a
record type
Record type is a family of typefaces designed to allow medieval manuscripts (specifically those from England) to be published as near- facsimiles of the originals. The typefaces include many special characters intended to replicate the variou ...
, i.e. in a struct or a class in C/C++. In contrast, the unit type can be stored in records in functional programming languages, i.e. it can appear as the type of a field; the above implementation of the unit type in C++ can also be stored. While this may seem a useless feature, it does allow one for instance to elegantly implement a
set
Set, The Set, SET or SETS may refer to:
Science, technology, and mathematics Mathematics
*Set (mathematics), a collection of elements
*Category of sets, the category whose objects and morphisms are sets and total functions, respectively
Electro ...
as a
map to the unit type; in the absence of a unit type, one can still implement a set this way by storing some dummy value of another type for each key.
In Generics
In Java Generics, type parameters must be reference types. The wrapper type
Void
is often used when a unit type parameter is needed. Although the
Void
type can never have any instances, it does have one value,
null
(like all other reference types), so it acts as a unit type. In practice, any other non-instantiable type, e.g.
Math
, can also be used for this purpose, since they also have exactly one value,
null
.
public static Void f(Void x)
public static Void g(Void x)
public static void main(String[] args)
Null type
Statically typed languages give a type to every possible expression. They need to associate a type to the null expression.
A type will be defined for null an it will only have this value.
For example in D, it's possible to declare functions that return the only value the unit type for null has
typeof(null) returnThatSpecialThing()
null, is the only value that typeof(null), a unit type, can have.
See also
*
Singleton pattern (where a particular class has only one instance, but narrowly-typed non-nullable references to it are usually not held by other classes)
References
*
*
{{Data types
Data types
Type theory
Articles with example Java code