GHas.hs

data Path = L Path | R Path | Here

data MPath = NotFound | Conflict | Found Path

type family Search a (f :: k -> Type) :: MPath where
  -- ...
  Search a (f :*: g) = OrPath (Search a f) (Search a g)
  Search a (K1 i c) = Check a c

type family Check a c where
  Check a a = 'Found 'Here
  Check a b = 'NotFound

type family OrPath (p1 :: MPath) (p2 :: MPath) :: MPath where
  OrPath ('Found p) 'NotFound = 'Found ('L p)
  OrPath 'NotFound ('Found p) = 'Found ('R p)
  OrPath 'NotFound 'NotFound = 'NotFound
  OrPath _ _ = 'Conflict
  
class GHas (path :: Path) (part :: Type) (f :: k -> Type) where
  ghas_ :: ...
  
instance GHas p part f => GHas ('L p) part (f :*: g) where ...
instance GHas p part g => GHas ('R p) part (f :*: g) where ...


ghas :: (Generic a, Search part (Rep a) ~ 'Found p, GHas p part f) => ...
ghas = ghas_