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.. _stand-alone-deriving:
Stand-alone deriving declarations
---------------------------------
.. extension:: StandaloneDeriving
:shortdesc: Enable standalone deriving.
:since: 6.8.1
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.
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