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akhileshs/contravariant.hs

Created April 29, 2017 18:32
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contravariant.hs
class Contravariant f where
fmap :: (b -> a) -> f a -> f b
newtype Predicate a = Predicate { runPredicate :: a -> Bool }
instance Contravariant Predicate where
fmap f (Predicate a) = Predicate (a . f)
data MyOrdering = LT | EQ | GT
newtype Comparison a =
Comparison { runComparison :: a -> a -> MyOrdering }
instance Contravariant Comparison where
fmap f (Comparison c) = Comparison (\a b -> c (f a) (f b))
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