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{-# LANGUAGE DataKinds
, DatatypeContexts
, FlexibleInstances
, GADTs
, MultiParamTypeClasses
, TypeFamilies
, TypeApplications
#-}
import GHC.Records (HasField(..))
type family B where B = Bool
data T = MkT { foo :: Int, bar :: B }
data U a b = MkU { baf :: a }
data family V a b c d
data instance V x Int y [z] = MkVInt { baz :: (x, y, z, Bool) }
data W a where
MkW :: { woo :: a } -> W [a]
data Eq a => X a = MkX { xoo :: a }
data Y a = Eq a => MkY { yoo :: a }
t = MkT 42 True
u :: U Char Char
u = MkU 'x'
v = MkVInt (42, 'x', True, False)
w = MkW True
x = MkX True
y = MkY True
-- A virtual foo field for U
instance HasField "foo" (U a b) [Char] where
getField _ = "virtual"
main = do print (getField @"foo" t)
print (getField @"bar" t)
print (getField @"baf" u)
print (getField @"foo" u)
print (getField @"baz" v)
print (getField @"woo" w)
print (getField @"xoo" x)
print (getField @"yoo" y)
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