.. _nullary-types:

Data types with no constructors
-------------------------------

.. extension:: EmptyDataDecls
    :shortdesc: Allow definition of empty ``data`` types.

    :since: 6.8.1

    :status: Included in :extension:`GHC2021` and :extension:`Haskell2010`

    Allow definition of empty ``data`` types.

With the :extension:`EmptyDataDecls` extension, GHC lets you declare a
data type with no constructors.

You only need to enable this extension if the language you're using
is Haskell 98, in which a data type must have at least one constructor.
Haskell 2010 relaxed this rule to allow data types with no constructors,
and thus :extension:`EmptyDataDecls` is enabled by default when the
language is Haskell 2010.

For example: ::

      data S      -- S :: Type
      data T a    -- T :: Type -> Type

Syntactically, the declaration lacks the "= constrs" part. The type can be
parameterised over types of any kind, but if the kind is not ``Type`` then an
explicit kind annotation must be used (see :ref:`kinding`).

Such data types have only one value, namely bottom. Nevertheless, they
can be useful when defining "phantom types".

In conjunction with the :extension:`EmptyDataDeriving` extension, empty data
declarations can also derive instances of standard type classes
(see :ref:`empty-data-deriving`).