6.6.7. Deriving strategies

DerivingStrategies
Since

8.2.1

Allow multiple deriving, each optionally qualified with a strategy.

In most scenarios, every deriving statement generates a typeclass instance in an unambiguous fashion. There is a corner case, however, where simultaneously enabling both the GeneralizedNewtypeDeriving and DeriveAnyClass extensions can make deriving become ambiguous. Consider the following example

{-# LANGUAGE DeriveAnyClass, GeneralizedNewtypeDeriving #-}
newtype Foo = MkFoo Bar deriving C

One could either pick the DeriveAnyClass approach to deriving C or the GeneralizedNewtypeDeriving approach to deriving C, both of which would be equally as valid. GHC defaults to favoring DeriveAnyClass in such a dispute, but this is not a satisfying solution, since that leaves users unable to use both language extensions in a single module.

To make this more robust, GHC has a notion of deriving strategies, which allow the user to explicitly request which approach to use when deriving an instance. To enable this feature, one must enable the DerivingStrategies language extension. A deriving strategy can be specified in a deriving clause

newtype Foo = MkFoo Bar
  deriving newtype C

Or in a standalone deriving declaration

deriving anyclass instance C Foo

DerivingStrategies also allows the use of multiple deriving clauses per data declaration so that a user can derive some instance with one deriving strategy and other instances with another deriving strategy. For example

newtype Baz = Baz Quux
  deriving          (Eq, Ord)
  deriving stock    (Read, Show)
  deriving newtype  (Num, Floating)
  deriving anyclass C

Currently, the deriving strategies are:

6.6.7.1. Default deriving strategy

If an explicit deriving strategy is not given, multiple strategies may apply. In that case, GHC chooses the strategy as follows:

  1. Stock type classes, i.e. those specified in the report and those enabled by language extensions, are derived using the stock strategy, with the following exception:

    • For newtypes, Eq, Ord, Ix and Bounded are always derived using the newtype strategy, even without GeneralizedNewtypeDeriving enabled. (There should be no observable difference to instances derived using the stock strategy.)

    • Also for newtypes, Functor, Foldable and Enum are derived using the newtype strategy if GeneralizedNewtypeDeriving is enabled and the derivation succeeds.

  2. For other any type class:

    1. When DeriveAnyClass is enabled, use anyclass.

    2. When GeneralizedNewtypeDeriving is enabled and we are deriving for a newtype, then use newtype.

    If both rules apply to a deriving clause, then anyclass is used and the user is warned about the ambiguity. The warning can be avoided by explicitly stating the desired deriving strategy.