{-# LANGUAGE DeriveDataTypeable, RankNTypes#-}
{-# OPTIONS -Wall #-}
import Data.Data
import Data.Maybe(maybeToList)
import Data.Generics.Schemes

data ABC 
    = ABC Int Double String
    deriving (Eq, Show, Data, Typeable)

data A
    = A String
    | Abc B C deriving (Eq, Show, Data, Typeable)

data B
    = B Double
    | Bca C A deriving (Eq, Show, Data, Typeable)

data C      
    = C Int
    | Cab A B deriving (Eq, Show, Data, Typeable)

abc1 :: ABC
abc1 = ABC 2012 1.1 "Tatsu"


a1 :: A
a1 = Abc 
     (Bca (Cab (A "Happy") (B 4.2)) (Abc (B 5.6) (C 512))) 
     (Cab (Abc (B 8.4) (C 208)) (Bca (C 2012) (A "New Year!")))


recMapQ' :: forall a a1 a2. (Data a1, Data a2) => (a1->a) -> a2 -> [a]
recMapQ' f x = (maybeToList $ fmap f (cast x)) ++ concat (gmapQ (recMapQ' f) x)


recMapQ :: forall a a1 a2. (Data a1, Data a2) => (a1->a) -> a2 -> [a]
recMapQ f x = case fmap f (cast x) of
  Just y  -> [y]
  Nothing -> concat (gmapQ (recMapQ f) x)


recMapT :: forall a b. (Data a, Data b) => (b -> b) -> a -> a
recMapT f x = 
  let y = gmapT (recMapT f) x in
  case (>>= cast) $ fmap f $ cast x of
    Just z  -> z
    Nothing -> y


caster :: (Typeable a) => (a -> a) -> (forall b. Typeable b => b -> b)
caster f x =
  case (cast =<<) $ fmap f $ cast x of
    Just y  -> y
    Nothing -> x

main :: IO ()
main = do
  print a1     
  print $ gsize a1
  print $ recMapQ ("Very " ++) a1
  print $ recMapT ("Very " ++) a1
--  print $ everywhere (caster $ ("Very " ++)) a1
--  print $ everywhere'(caster $ ("Very " ++)) a1

Error occurred
ERROR "t.hs" - Can't find imported module "Data.Generics.Schemes"