A unit type is a mathematical type that allows only one value.

The carrier (underlying set) associated with a unit type can be any singleton set. There is an isomorphism 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-tuples, i.e. the cartesian product of no types.

When applying category theory to types, the unit type is a terminal object in many set-based categories. The unit type should not be confused with the zero or bottom type, which allows no values and is the initial object in these categories.

Unit type in programming languages

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Several computer programming languages 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 the functional programming languages, Haskell, and Clean, the unit type is called () and its only value is also (), reflecting the 0-tuple interpretation. In ML, the type is called unit but the value is written as ().

In C, C++, C#, and Java, void is a unit type. These languages do not provide any way to declare an object or represent a value with type void. However, this functionality is not needed, since the unit type carries no information. Type theory