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{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GADTs #-}
-- From https://www.haskell.org/haskellwiki/GHC/Type_families#An_associated_data_type_example
import qualified Data.IntMap
import Prelude hiding (lookup)
import Data.Char (ord)
class GMapKey k where
data GMap k :: * -> *
empty :: GMap k v
lookup :: k -> GMap k v -> Maybe v
insert :: k -> v -> GMap k v -> GMap k v
-- An Int instance
instance GMapKey Int where
data GMap Int v = GMapInt (Data.IntMap.IntMap v)
empty = GMapInt Data.IntMap.empty
lookup k (GMapInt m) = Data.IntMap.lookup k m
insert k v (GMapInt m) = GMapInt (Data.IntMap.insert k v m)
-- A Char instance
instance GMapKey Char where
data GMap Char v = GMapChar (GMap Int v)
empty = GMapChar empty
lookup k (GMapChar m) = lookup (ord k) m
insert k v (GMapChar m) = GMapChar (insert (ord k) v m)
-- A Unit instance
instance GMapKey () where
data GMap () v = GMapUnit (Maybe v)
empty = GMapUnit Nothing
lookup () (GMapUnit v) = v
insert () v (GMapUnit _) = GMapUnit $ Just v
-- Product and sum instances
instance (GMapKey a, GMapKey b) => GMapKey (a, b) where
data GMap (a, b) v = GMapPair (GMap a (GMap b v))
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
data GMap (Either a b) v = GMapEither (GMap a v) (GMap b v)
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 b) v (GMapEither gm1 gm2) = GMapEither gm1 (insert b v gm2)
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
-- (Type) Synonym Family
type family Elem c
type instance Elem [e] = e
-- type instance Elem BitSet = Char
data family T a
data instance T Int = T1 Int | T2 Bool
newtype instance T Char = TC Bool
data family G a b
data instance G [a] b where
G1 :: c -> G [Int] b
G2 :: G [a] Bool
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