DataFamilyDerivingVia.hs

{-# LANGUAGE DerivingVia, ScopedTypeVariables, StandaloneDeriving, TypeApplications, TypeFamilies #-}
module DataFamilyDerivingVia where

import Data.Coerce

data family Foo a

newtype ByBar a = ByBar a

class Baz a where
  baz :: a -> a

instance Baz (ByBar a) where
  baz = id

-- deriving via ByBar (Foo a) instance Baz (Foo a)

instance Baz (Foo a) where
  baz = coerce @(ByBar (Foo a) -> ByBar (Foo a)) @(Foo a -> Foo a) baz