.. _stand-alone-deriving:

Stand-alone deriving declarations
---------------------------------

.. extension:: StandaloneDeriving
    :shortdesc: Enable standalone deriving.

    :since: 6.8.1

    :status: Included in :extension:`GHC2021`

    Allow the use of stand-alone ``deriving`` declarations.

GHC allows stand-alone ``deriving`` declarations, enabled by
:extension:`StandaloneDeriving`: ::

      data Foo a = Bar a | Baz String

      deriving instance Eq a => Eq (Foo a)

The syntax is identical to that of an ordinary instance declaration
apart from (a) the keyword ``deriving``, and (b) the absence of the
``where`` part.

However, standalone deriving differs from a ``deriving`` clause in a
number of important ways:

-  The standalone deriving declaration does not need to be in the same
   module as the data type declaration. (But be aware of the dangers of
   orphan instances (:ref:`orphan-modules`).

-  In most cases, you must supply an explicit context (in the example the
   context is ``(Eq a)``), exactly as you would in an ordinary instance
   declaration. (In contrast, in a ``deriving`` clause attached to a
   data type declaration, the context is inferred.)

   The exception to this rule is that the context of a standalone deriving
   declaration can infer its context when a single, extra-wildcards constraint
   is used as the context, such as in: ::

         deriving instance _ => Eq (Foo a)

   This is essentially the same as if you had written ``deriving Eq`` after
   the declaration for ``data Foo a``. Using this feature requires the use of
   :extension:`PartialTypeSignatures` (:ref:`partial-type-signatures`).

-  Unlike a ``deriving`` declaration attached to a ``data`` declaration,
   the instance can be more specific than the data type (assuming you
   also use :extension:`FlexibleInstances`, :ref:`instance-rules`). Consider
   for example ::

         data Foo a = Bar a | Baz String

         deriving instance Eq a => Eq (Foo [a])
         deriving instance Eq a => Eq (Foo (Maybe a))

   This will generate a derived instance for ``(Foo [a])`` and
   ``(Foo (Maybe a))``, but other types such as ``(Foo (Int,Bool))``
   will not be an instance of ``Eq``.

-  Unlike a ``deriving`` declaration attached to a ``data`` declaration,
   GHC does not restrict the form of the data type. Instead, GHC simply
   generates the appropriate boilerplate code for the specified class,
   and typechecks it. If there is a type error, it is your problem. (GHC
   will show you the offending code if it has a type error.)

   The merit of this is that you can derive instances for GADTs and
   other exotic data types, providing only that the boilerplate code
   does indeed typecheck. For example: ::

         data T a where
            T1 :: T Int
            T2 :: T Bool

         deriving instance Show (T a)

   In this example, you cannot say ``... deriving( Show )`` on the data
   type declaration for ``T``, because ``T`` is a GADT, but you *can*
   generate the instance declaration using stand-alone deriving.

   The down-side is that, if the boilerplate code fails to typecheck,
   you will get an error message about that code, which you did not
   write. Whereas, with a ``deriving`` clause the side-conditions are
   necessarily more conservative, but any error message may be more
   comprehensible.

-  Under most circumstances, you cannot use standalone deriving to create an
   instance for a data type whose constructors are not all in scope. This is
   because the derived instance would generate code that uses the constructors
   behind the scenes, which would break abstraction.

   The one exception to this rule is :extension:`DeriveAnyClass`, since
   deriving an instance via :extension:`DeriveAnyClass` simply generates
   an empty instance declaration, which does not require the use of any
   constructors. See the `deriving any class <#derive-any-class>`__ section
   for more details.

In other ways, however, a standalone deriving obeys the same rules as
ordinary deriving:

-  A ``deriving instance`` declaration must obey the same rules
   concerning form and termination as ordinary instance declarations,
   controlled by the same flags; see :ref:`instance-decls`.

-  The stand-alone syntax is generalised for newtypes in exactly the
   same way that ordinary ``deriving`` clauses are generalised
   (:ref:`newtype-deriving`). For example: ::

         newtype Foo a = MkFoo (State Int a)

         deriving instance MonadState Int Foo

   GHC always treats the *last* parameter of the instance (``Foo`` in
   this example) as the type whose instance is being derived.
