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{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
-- Based on https://www.haskell.org/haskellwiki/GHC/Type_families#Detailed_definition_of_data_families
module DataFamilies
(
GMap
, GMapKey(type GMapK)
) where
-- Type 1, Top level
data family GMap k :: * -> *
data family Array e
data family ArrayK :: * -> *
-- Type 2, associated types
class GMapKey k where
data GMapK k :: * -> *
class C a b c where { data T1 c a :: * } -- OK
-- class C a b c where { data T a a :: * } -- Bad: repeated variable
-- class D a where { data T a x :: * } -- Bad: x is not a class variable
class D a where { data T2 a :: * -> * } -- OK
-- Instances
data instance GMap (Either a b) v = GMapEither (GMap a v) (GMap b v)
data family T3 a
data instance T3 Int = A
data instance T3 Char = B
nonsense :: T3 a -> Int
-- nonsense A = 1 -- WRONG: These two equations together...
-- nonsense B = 2 -- ...will produce a type error.
nonsense = undefined
instance (GMapKey a, GMapKey b) => GMapKey (Either a b) where
data GMapK (Either a b) v = GMapEitherK (GMap a v) (GMap b v)
-- data GMap () v = GMapUnit (Maybe v)
-- deriving Show
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