Associated.hs

{-# 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