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.. _deriving:
Deriving mechanism
==================
Haskell 98 allows the programmer to add a deriving clause to a data type
declaration, to generate a standard instance declaration for specified class.
GHC extends this mechanism along several axes:
* The derivation mechanism can be used separately from the data type
declaration, using the :ref:`standalone deriving mechanism
<stand-alone-deriving>`.
* In Haskell 98, the only derivable classes are ``Eq``,
``Ord``, ``Enum``, ``Ix``, ``Bounded``, ``Read``, and ``Show``. :ref:`Various
language extensions <deriving-extra>` extend this list.
* Besides the stock approach to deriving instances by generating all method
definitions, GHC supports two additional deriving strategies, which can
derive arbitrary classes:
* :ref:`Generalised newtype deriving <newtype-deriving>` for newtypes and
* :ref:`deriving any class <derive-any-class>` using an empty instance
declaration.
The user can optionally declare the desired :ref:`deriving strategy
<deriving-strategies>`, especially if the compiler chooses the wrong
one :ref:`by default <default-deriving-strategy>`.
.. toctree::
:maxdepth: 1
empty_data_deriving
deriving_inferred
standalone_deriving
deriving_extra
newtype_deriving
derive_any_class
deriving_strategies
deriving_via
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