Unit type

This article is about the notion used incomputer programmingandtype theory.For types of measurement units, seeUnits of measurement.For other uses, seeUnit (disambiguation).

In the area ofmathematical logicandcomputer scienceknown astype theory,aunit typeis atypethat allows only one value (and thus can hold no information). The carrier (underlying set) associated with a unit type can be anysingleton set.There is anisomorphismbetween any two such sets, so it is customary to talk abouttheunit type and ignore the details of its value. One may also regard the unit type as the type of 0-tuples,i.e. theproductof no types.

The unit type is theterminal objectin thecategoryof types and typed functions. It should not be confused with thezeroorempty type,which allowsnovalues and is theinitial objectin this category. Similarly, theBooleanis the type withtwovalues.

The unit type is implemented in mostfunctional programminglanguages. Thevoid typethat 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 computerprogramming languagesprovide a unit type to specify the result type of afunctionwith the sole purpose of causing aside effect,and the argument type of a function that does not require arguments.

• InHaskell,Rust,andElm,the unit type is called()and its only value is also(),reflecting the 0-tuple interpretation.
• InMLdescendants (includingOCaml,Standard ML,andF#), the type is calledunitbut the value is written as().
• InScala,the unit type is calledUnitand its only value is written as().
• InCommon Lispthe type namedNULLis a unit type which has one value, namely the symbolNIL.This should not be confused with theNILtype,which is thebottom type.
• InPython,there is a type calledNoneTypewhich allows the single value ofNone.In Python's optional static type annotations, this type is represented asNone.
• InSwift,the unit type is calledVoidor()and its only value is also(),reflecting the 0-tuple interpretation.
• InJava,the unit type is calledVoidand its only value isnull.
• InGo,the unit type is writtenstruct{}and its value isstruct{}{}.
• InPHP,the unit type is called null, which only value is NULL itself.
• InJavaScript,bothNull(its only value isnull) andUndefined(its only value isundefined) are built-in unit types.
• inKotlin,Unitis a singleton with only one value: theUnitobject.
• InRuby,nilis the only instance of theNilClassclass.
• InC++,thestd::monostateunit type was added in C++17. Before that, it is possible to define a custom unit type using an empty struct such asstruct empty{}.

Void type as unit type

InC,C++,C#,D,andPHP,voidis 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 emptystruct,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 likePascal,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:

voidf(void){}
voidg(void){}

intmain(void)
{
f(g());// compile-time error here
return0;
}

This issue does not arise in most programming practice in C, because since thevoidtype carries no information, it is useless to pass it anyway; but it may arise ingeneric programming,such as C++templates,wherevoidmust 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:

classunit_type{};
constunit_typethe_unit;

unit_typef(unit_type){returnthe_unit;}
unit_typeg(unit_type){returnthe_unit;}

intmain()
{
f(g(the_unit));
return0;
}

(For brevity, we're not worried in the above example whetherthe_unitis really asingleton;seesingleton patternfor details on that issue.)

Difference in storage

The second notable difference is that the void type is special and can never be stored in arecord type,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 asetas amapto 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 typeVoidis often used when a unit type parameter is needed. Although theVoidtype 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.

publicstaticVoidf(Voidx){returnnull;}
publicstaticVoidg(Voidx){returnnull;}

publicstaticvoidmain(String[]args)
{
f(g(null));
}

Null type

Statically typed languages give a type to every possible expression. They need to associate a type to thenullexpression. A type will be defined fornulland it will only have this value.

For example in D, it's possible to declare functions that may only returnnull:

typeof(null)returnThatSpecialThing(){
returnnull;
}

nullis the only value thattypeof(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

• Pierce, Benjamin C.(2002).Types and Programming Languages.MIT Press.pp. 118–119.ISBN0-262-16209-1.
• unit typeat thenLab