Quantified constraints vs. (closed) type families
问题 I am trying to use this blogpost's approach to higher-kinded data without dangling Identity functors for the trival case together with quantified-constraint deriving: {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE QuantifiedConstraints, StandaloneDeriving, UndecidableInstances #-} module HKD2 where import Control.Monad.Identity type family HKD f a where HKD Identity a = a HKD f a = f a data Result f = MkResult { foo :: HKD f Int , bar :: HKD f Bool } deriving instance (forall a. Show a => Show