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import Data.Kind
data HList (ts :: [Type]) where
HNil :: HList '[]
(:#) :: t -> HList ts -> HList (t ': ts)
infixr 5 :#
type family All (c :: Type -> Constraint) (ts :: [Type]) :: Constraint where
All c '[] = ()
All c (t ': ts) = (c t, All c ts)
instance All Eq ts => Eq (HList ts) where
HNil == HNil = True
(a :# as) == (b :# bs) = a == b && as == bs
-- Exercise 5.3-iii
-- Rewrite the `Ord` and `Show` instances in terms of `All`:
instance (All Eq ts, All Ord ts) => Ord (HList ts) where
compare HNil HNil = EQ
compare (a :# as) (b :# bs) = compare a b <> compare as bs
instance All Show ts => Show (HList ts) where
show HNil = "HNil"
show (a :# as) = show a <> " :# " <> show as
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