In Programming Languages
Most commonly used languages don't have a way to explicitly denote the empty type. There are a few notable exceptions.
Haskell does not support empty data types. However, in GHC, there is a flag -XEmptyDataDecls to allow the definition data Empty (with no constructors). The type Empty is not quite empty, as it contains non-terminating programs and the undefined constant. The undefined constant is often used when you want something to have the empty type, because undefined matches any type (so is kind of a "subtype" of all types), and attempting to evaluate undefined will cause the program to abort, therefore it never returns an answer.
In Common Lisp the symbol NIL, amongst its other uses, is also the name of a type that has no values. It is the complement of T which is the top type. The type named NIL is sometimes confused with the type named NULL, which has one value, namely the symbol NIL itself.
In Scala, the bottom type is denoted as Nothing. Besides its use for functions that just throw exceptions or otherwise don't return normally, it's also used for covariant parameterized types. For example, Scala's List is a covariant type constructor, so List is a subtype of List for all types A. So Scala's Nil, the object for marking the end of a list of any type, belongs to the type List.
Read more about this topic: Bottom Type
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