6.4.1. Data types with no constructors¶
-
EmptyDataDecls
¶ Since: 6.8.1 Status: Included in GHC2021
andHaskell2010
Allow definition of empty
data
types.
With the 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 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 Explicitly-kinded quantification).
Such data types have only one value, namely bottom. Nevertheless, they can be useful when defining “phantom types”.
In conjunction with the EmptyDataDeriving
extension, empty data
declarations can also derive instances of standard type classes
(see Deriving instances for empty data types).