Data.Derive.TopDown.Derive

 data A a b = A a (B b) deriving (Show)
 data B a = B a deriving (Show)

 derivings ''Eq makeEq ''A 

If you enable -ddump-splices, you will get:

 Data\Derive\TopDown\Test.hs:1:1: Splicing declarations
    derives ''Eq makeEq ''A
  ======>
    Data\Derive\TopDown\Test.hs:18:1-25
    instance Eq a_1627720873 => Eq (B a_1627720873) where
      (==) (B x1) (B y1) = (x1 == y1)
    instance (Eq a_1627720874, Eq b_1627720875) =>
             Eq (A a_1627720874 b_1627720875) where
      (==) (A x1 x2) (A y1 y2) = ((x1 == y1) && (x2 == y2))

This will make sense if you have a deep composited data types, nomally an AST of a language. For now, you have to specify both of ''Eq and makeEq, I suppose ''Eq will be enough. To look what typeclasses you can derive, see derive library on hackage.

Documentation

derivings :: Name -> Derivation -> Name -> Q [Dec]

deriving from top