content: {-# LANGUAGE TypeFamilies #-}
  
  module Main where
  
  import Prelude hiding (lookup)
  import Data.Char (ord)
  import qualified Data.Map as Map
  import Data.Kind (Type)
  
  
  -- Generic maps as toplevel indexed data types
  ----------------------------------------------
  
  data family GMap k :: Type -> Type
  data instance GMap Int          v = GMapInt    (Map.Map Int v)
  data instance GMap Char         v = GMapChar   (GMap Int v)
  data instance GMap ()           v = GMapUnit   (Maybe v)
  data instance GMap (a, b)       v = GMapPair   (GMap a (GMap b v))
  data instance GMap (Either a b) v = GMapEither (GMap a v) (GMap b v)
  
  class GMapKey k where
    empty       :: GMap k v
    lookup      :: k -> GMap k v -> Maybe v
    insert      :: k -> v -> GMap k v -> GMap k v
  
  instance GMapKey Int where
    empty                  = GMapInt Map.empty
    lookup k (GMapInt m)   = Map.lookup k m
    insert k v (GMapInt m) = GMapInt (Map.insert k v m)
  
  instance GMapKey Char where
    empty                   = GMapChar empty
    lookup k (GMapChar m)   = lookup (ord k) m
    insert k v (GMapChar m) = GMapChar (insert (ord k) v m)
  
  instance GMapKey () where
    empty                    = GMapUnit Nothing
    lookup () (GMapUnit v)   = v
    insert () v (GMapUnit _) = GMapUnit $ Just v
  
  instance (GMapKey a, GMapKey b) => GMapKey (a, b) where
    empty                         = GMapPair empty
    lookup (a, b) (GMapPair gm)   = lookup a gm >>= lookup b
    insert (a, b) v (GMapPair gm) = GMapPair $ case lookup a gm of
                                      Nothing  -> insert a (insert b v empty) gm
                                      Just gm2 -> insert a (insert b v gm2  ) gm
  
  instance (GMapKey a, GMapKey b) => GMapKey (Either a b) where
    empty                                   = GMapEither empty empty
    lookup (Left  a) (GMapEither gm1  _gm2) = lookup a gm1
    lookup (Right b) (GMapEither _gm1 gm2 ) = lookup b gm2
    insert (Left  a) v (GMapEither gm1 gm2) = GMapEither (insert a v gm1) gm2
    insert (Right a) v (GMapEither gm1 gm2) = GMapEither gm1 (insert a v gm2)
  
  
  -- Test code
  -- ---------
  
  nonsence :: GMap Bool String
  nonsence = undefined
  
  myGMap :: GMap (Int, Either Char ()) String
  myGMap = insert (5, Left 'c') "(5, Left 'c')"    $
           insert (4, Right ()) "(4, Right ())"    $
           insert (5, Right ()) "This is the one!" $
           insert (5, Right ()) "This is the two!" $
           insert (6, Right ()) "(6, Right ())"    $
           insert (5, Left 'a') "(5, Left 'a')"    $
           empty
  main = putStrLn $ maybe "Couldn't find key!" id $ lookup (5, Right ()) myGMap
  