SYB traversal for GADT

gadt.hs

{-# LANGUAGE GADTs, EmptyDataDecls, MultiParamTypeClasses, TypeFamilies #-}
{-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}

import Data.Generics
import Data.Typeable
import Data.Data

data HasHoles
data Complete
deriving instance Typeable HasHoles
deriving instance Data HasHoles
deriving instance Typeable Complete
deriving instance Data Complete
type family Holes a b :: *
canHaveHolesT :: a -> b -> Holes a b
canHaveHolesT _ _ = undefined
type instance Holes HasHoles Complete = HasHoles
type instance Holes Complete HasHoles = HasHoles
type instance Holes HasHoles HasHoles = HasHoles
type instance Holes Complete Complete = HasHoles

data Expression k a where
  EQuote  :: a -> String -> Expression HasHoles a
  IntLit  :: a -> Int -> Expression Complete a
  EArith  :: (Typeable k1, Typeable k2)
          => a -> ArithOp -> Expression k1 a -> Expression k2 a ->
                             Expression (Holes k1 k2) a
deriving instance Typeable2 (Expression)
data ArithOp = OpAdd
                      | OpSub
                      | OpMul
                      | OpDiv
                   deriving (Data, Typeable)

instance (Typeable k_a3db, Data a_a3dc) => Data (Expression k_a3db a_a3dc) where
  gfoldl k_a3dj z_a3dk (EQuote a1_a3dl a2_a3dm)
    = ((z_a3dk EQuote `k_a3dj` a1_a3dl) `k_a3dj` a2_a3dm)
  gfoldl k_a3dn z_a3do (IntLit a1_a3dp a2_a3dq)
    = ((z_a3do IntLit `k_a3dn` a1_a3dp) `k_a3dn` a2_a3dq)
  gfoldl
    k_a3dr
    z_a3ds
    (EArith a1_a3dt a2_a3du a3_a3dv a4_a3dw)
    = ((((z_a3ds EArith `k_a3dr` a1_a3dt) `k_a3dr` a2_a3du)
        `k_a3dr` a3_a3dv)
       `k_a3dr` a4_a3dw)