ghci

Prelude Language.Haskell.TH.Cleanup Generics.Deriving.TH
λ data X = A | B
Prelude Language.Haskell.TH.Cleanup Generics.Deriving.TH
λ putStrLn $(simplifiedTH  =<< deriveAll0 ''X)
instance Generic (X :: *)
    where type Rep (X :: *) = D1 ('MetaData "X" "Ghci3" "interactive" 'False) (:+: (C1 ('MetaCons "A" 'PrefixI 'False) U1) (C1 ('MetaCons "B" 'PrefixI 'False) U1))
          from val_0 = case val_0 of
                           y_1 -> M1 (case y_1 of
                                          A -> L1 (M1 U1)
                                          B -> R1 (M1 U1))
          to val_2 = case val_2 of
                         M1 y_3 -> case y_3 of
                                       L1 (M1 (U1)) -> A
                                       R1 (M1 (U1)) -> B