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{-# LANGUAGE OverloadedRecordFields, DataKinds, PolyKinds, GADTs,
StandaloneDeriving, TypeFamilies, UndecidableInstances,
MagicHash #-}
import GHC.Prim (Proxy#, proxy#)
import GHC.Records
data T (a :: x -> *)(b :: x) :: * where
MkT :: a b -> T a b
deriving instance Show (a b) => Show (T a b)
data U (a :: x -> *)(b :: x)(c :: y -> *)(d :: y)
= MkU { bar :: T a b, baz :: T c d }
deriving Show
data V (a :: x -> *)(b :: x)(c :: x -> *)(d :: x)
= MkV { bar :: T a b, baz :: T c d }
deriving Show
data F (f :: * -> *) = MkF
deriving Show
-- Updates to fields of U may change kinds:
-- x :: U F f [] Bool
x = setField (proxy# :: Proxy# "bar") (MkU (MkT [3]) (MkT [False])) (MkT MkF)
-- Updates to fields of V may not, but may change types:
-- y :: V Maybe Int [] Bool
y = setField (proxy# :: Proxy# "bar") (MkV (MkT [3]) (MkT [False])) (MkT (Just 6))
main = do print x
print y
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