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chris-martin/EqBy.hs

Created August 22, 2017 02:28
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EqBy.hs
data EqBy a = EqBy (a -> Bool) a
instance Eq (EqBy a) where
EqBy f _ == EqBy _ y = f y
eqBy :: (a -> a -> Bool) -> a -> EqBy a
eqBy f x = EqBy (f x) x
equalBy :: (a -> a -> Bool) -> [a] -> [a] -> Bool
equalBy f xs ys = fmap (eqBy f) xs == fmap (eqBy f) ys
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