.. _derive-any-class: Deriving any other class ------------------------ .. extension:: DeriveAnyClass :shortdesc: Enable deriving for any class. :since: 7.10.1 Allow use of any typeclass in ``deriving`` clauses. With :extension:`DeriveAnyClass` you can derive any other class. The compiler will simply generate an instance declaration with no explicitly-defined methods. This is mostly useful in classes whose `minimal set <#minimal-pragma>`__ is empty, and especially when writing `generic functions <#generic-programming>`__. As an example, consider a simple pretty-printer class ``SPretty``, which outputs pretty strings: :: {-# LANGUAGE DefaultSignatures, DeriveAnyClass #-} class SPretty a where sPpr :: a -> String default sPpr :: Show a => a -> String sPpr = show If a user does not provide a manual implementation for ``sPpr``, then it will default to ``show``. Now we can leverage the :extension:`DeriveAnyClass` extension to easily implement a ``SPretty`` instance for a new data type: :: data Foo = Foo deriving (Show, SPretty) The above code is equivalent to: :: data Foo = Foo deriving Show instance SPretty Foo That is, an ``SPretty Foo`` instance will be created with empty implementations for all methods. Since we are using :extension:`DefaultSignatures` in this example, a default implementation of ``sPpr`` is filled in automatically. Note the following details - In case you try to derive some class on a newtype, and :extension:`GeneralizedNewtypeDeriving` is also on, :extension:`DeriveAnyClass` takes precedence. - The instance context is determined by the type signatures of the derived class's methods. For instance, if the class is: :: class Foo a where bar :: a -> String default bar :: Show a => a -> String bar = show baz :: a -> a -> Bool default baz :: Ord a => a -> a -> Bool baz x y = compare x y == EQ And you attempt to derive it using :extension:`DeriveAnyClass`: :: instance Eq a => Eq (Option a) where ... instance Ord a => Ord (Option a) where ... instance Show a => Show (Option a) where ... data Option a = None | Some a deriving Foo Then the derived ``Foo`` instance will be: :: instance (Show a, Ord a) => Foo (Option a) Since the default type signatures for ``bar`` and ``baz`` require ``Show a`` and ``Ord a`` constraints, respectively. Constraints on the non-default type signatures can play a role in inferring the instance context as well. For example, if you have this class: :: class HigherEq f where (==#) :: f a -> f a -> Bool default (==#) :: Eq (f a) => f a -> f a -> Bool x ==# y = (x == y) And you tried to derive an instance for it: :: instance Eq a => Eq (Option a) where ... data Option a = None | Some a deriving HigherEq Then it will fail with an error to the effect of: :: No instance for (Eq a) arising from the 'deriving' clause of a data type declaration That is because we require an ``Eq (Option a)`` instance from the default type signature for ``(==#)``, which in turn requires an ``Eq a`` instance, which we don't have in scope. But if you tweak the definition of ``HigherEq`` slightly: :: class HigherEq f where (==#) :: Eq a => f a -> f a -> Bool default (==#) :: Eq (f a) => f a -> f a -> Bool x ==# y = (x == y) Then it becomes possible to derive a ``HigherEq Option`` instance. Note that the only difference is that now the non-default type signature for ``(==#)`` brings in an ``Eq a`` constraint. Constraints from non-default type signatures never appear in the derived instance context itself, but they can be used to discharge obligations that are demanded by the default type signatures. In the example above, the default type signature demanded an ``Eq a`` instance, and the non-default signature was able to satisfy that request, so the derived instance is simply: :: instance HigherEq Option - :extension:`DeriveAnyClass` can be used with partially applied classes, such as :: data T a = MKT a deriving( D Int ) which generates :: instance D Int a => D Int (T a) where {} - :extension:`DeriveAnyClass` can be used to fill in default instances for associated type families: :: {-# LANGUAGE DeriveAnyClass, TypeFamilies #-} class Sizable a where type Size a type Size a = Int data Bar = Bar deriving Sizable doubleBarSize :: Size Bar -> Size Bar doubleBarSize s = 2*s The ``deriving( Sizable )`` is equivalent to saying :: instance Sizeable Bar where {} and then the normal rules for filling in associated types from the default will apply, making ``Size Bar`` equal to ``Int``.