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{-# LANGUAGE ImportQualifiedPost #-}
-- Exercise 13.2-i
-- Provide a generic instance for the `Ord` class.
import GHC.Generics qualified as G
class GOrd a where
gcompare :: a x -> a x -> Ordering
instance GOrd G.U1 where
gcompare G.U1 G.U1 = EQ
instance GOrd G.V1 where
gcompare _ _ = EQ
instance Ord a => GOrd (G.K1 _1 a) where
gcompare (G.K1 a) (G.K1 b) = compare a b
instance (GOrd a, GOrd b) => GOrd (a G.:+: b) where
gcompare (G.L1 a1) (G.L1 a2) = gcompare a1 a2
gcompare (G.R1 b1) (G.R1 b2) = gcompare b1 b2
gcompare (G.L1 _) (G.R1 _) = LT
gcompare (G.R1 _) (G.L1 _) = GT
instance (GOrd a, GOrd b) => GOrd (a G.:*: b) where
-- Don't forget `Ord` is a monoid! :)
gcompare (a1 G.:*: b1) (a2 G.:*: b2) = gcompare a1 a2 <> gcompare b1 b2
instance GOrd a => GOrd (G.M1 _x _y a) where
gcompare (G.M1 a1) (G.M1 a2) = gcompare a1 a2
genericCompare :: (G.Generic a, GOrd (G.Rep a)) => a -> a -> Ordering
genericCompare a b = gcompare (G.from a) (G.from b)
-- Example usage:
data Foo a b c = F0 | F1 a | F2 b c deriving (Eq, G.Generic)
instance (Ord a, Ord b, Ord c) => Ord (Foo a b c) where
compare = genericCompare
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