reflection for dynamic Eq instances

ReflectEq.hs

{-# LANGUAGE FlexibleContexts     #-}
{-# LANGUAGE ScopedTypeVariables  #-}
{-# LANGUAGE TypeApplications     #-}
{-# LANGUAGE UndecidableInstances #-}

import           Data.Proxy
import           Data.Reflection

data EqDict a = EQD { withEqDict :: a -> a -> Bool }

newtype HasEq s a = HasEq a

instance Reifies s (EqDict a) => Eq (HasEq s a) where
    HasEq x == HasEq y = withEqDict (reflect (Proxy @s)) x y

eqBy :: (a -> a -> Bool) -> [a] -> [a] -> Bool
eqBy f xs ys = reify (EQD f) $ \(Proxy :: Proxy s) ->
    map (HasEq @s) xs == map (HasEq @s) ys

main :: IO ()
main = print $ eqBy (==) [1..10] [1..10]