6.6.1. Deriving instances for empty data types¶

EmptyDataDeriving¶

Since:	8.4.1

Allow deriving instances of standard type classes for empty data types.

One can write data types with no constructors using the EmptyDataDecls flag (see Data types with no constructors), which is on by default in Haskell 2010. What is not on by default is the ability to derive type class instances for these types. This ability is enabled through use of the EmptyDataDeriving flag. For instance, this lets one write:

data Empty deriving (Eq, Ord, Read, Show)

This would generate the following instances:

instance Eq Empty where
  _ == _ = True

instance Ord Empty where
  compare _ _ = EQ

instance Read Empty where
  readPrec = pfail

instance Show Empty where
  showsPrec _ x = case x of {}

The EmptyDataDeriving flag is only required to enable deriving of these four “standard” type classes (which are mentioned in the Haskell Report). Other extensions to the deriving mechanism, which are explained below in greater detail, do not require EmptyDataDeriving to be used in conjunction with empty data types. These include:

StandaloneDeriving (see Stand-alone deriving declarations)
Type classes which require their own extensions to be enabled to be derived, such as DeriveFunctor (see Deriving instances of extra classes (Data, etc.))
DeriveAnyClass (see Deriving any other class)