-- A solution, but it couples Entity to GetMessage

import Data.Kind

main :: IO ()
main = pure ()

-- making `a` the second variable
data Entity (c :: Type -> Constraint) = forall a. (c a, GetMessage a) => MkEntity a

class HasDataID a where
  getDependencies :: a -> Entity HasDataID

instance HasDataID (Entity HasDataID) where
  getDependencies (MkEntity x) = getDependencies x

data Foo = Foo Bar

data Bar = Bar

instance HasDataID Foo where
  getDependencies (Foo bar) = MkEntity bar




class GetMessage a where
  getMessage :: Int -> a -> String

instance GetMessage (Entity HasDataID) where
  getMessage int (MkEntity x) = getMessage int x

instance GetMessage Foo where
  getMessage nr foo = "foo: " <> show nr 

instance HasDataID Bar where
  getDependencies bar = MkEntity $ Foo bar

instance GetMessage Bar where
  getMessage nr bar = "bar: " <> show nr