High Quality Content by WIKIPEDIA articles! In type theory, a theory within mathematical logic, the bottom type is the type that has no values. It is also called the zero or empty type, and is sometimes denoted with falsum (?). A function whose return type is bottom cannot return any value. In the Curry–Howard correspondence, the bottom type corresponds to falsity. The bottom type is a subtype of all types. It is used to represent the return type of a function that does not return a value: for instance, one which loops forever, signals an exception, or exits. In a Turing-complete language, it is not generally possible to assert that a function will halt and return rather than looping forever. For instance, a function whose type states that it will return an integer may in fact not return. In order to theoretically account for this possibility, the bottom type is considered a subtype of integer (and, indeed, all types): a function declared as returning integer does not violate the type system if it instead loops forever.

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