Need to figure out how to get this route to work...

routes.hs

-- The problem is that I need to model the following routes:
-- * /profile/:username
-- * /profile/:username/favourites
-- 
-- And I'm a little stuck and I'm not sure whether it is my encoding of the routes, 
-- that I'm missing an important combinator or Obelisk just doesn't handle this yet.

newtype Username = Username { unUsername :: Text } deriving (Eq, Ord, Show)
makeWrapped ''Username

data FrontendRoute :: * -> * where
  FrontendRoute_Home :: FrontendRoute ()
  FrontendRoute_Profile :: FrontendRoute (Username, Maybe (R ProfileRoute))
 
data ProfileRoute :: * -> * where
  ProfileRoute_Favourites :: ProfileRoute ()

backendRouteEncoder
  :: Encoder (Either Text) Identity (R (Sum BackendRoute (ObeliskRoute FrontendRoute))) PageName
backendRouteEncoder = handleEncoder (const (InL BackendRoute_Missing :/ ())) $
  pathComponentEncoder $ \case
    InL backendRoute -> case backendRoute of
      BackendRoute_Missing -> PathSegment "missing" $ unitEncoder mempty
    InR obeliskRoute -> obeliskRouteSegment obeliskRoute $ \case
      FrontendRoute_Home -> PathEnd $ unitEncoder mempty
      FrontendRoute_Profile -> PathSegment "profile" $ unicorn

unicorn
  :: (MonadError Text parse, check ~ parse)
  => Encoder check parse (Username, Maybe (R ProfileRoute)) PageName
unicorn = segmentParam unwrappedEncoder profileRMayEnc -- This may be nonsensical. Follow reasoning below
  where
    -- I know how to do these two things separately! But it doesn't feel like this is the way to go
    -- because both encoders would encode into query params and could clash horribly
    usernameEnc :: (Applicative check, MonadError Text parse) => Encoder check parse Username PageName
    usernameEnc = singlePathSegmentEncoder . unwrappedEncoder
    profileRMayEnc :: (MonadError Text check, check ~ parse) => Encoder check parse (Maybe (R ProfileRoute)) PageName
    profileRMayEnc = maybeEncoder (unitEncoder mempty) $ pathComponentEncoder $ \case
      ProfileRoute_Favourites -> PathEnd $ unitEncoder mempty
    -- It feels like you want something that can peel a pathPart off and handle the rest as a sub
    -- like the maybeEncoder, but it's a 1 or more encoder that doesn't lose the params.
    segmentParam 
      :: MonadError Text parse 
      => Encoder check parse a Text 
      -> Encoder check parse b PageName 
      -> Encoder check parse (a,b) PageName
    segmentParam = undefined
    -- But I don't know whether this makes sense or could be implemented lawfully
    -- This kinda looks like chainEncoder, but chainEncoder doesn't feel like it

    -- I may just missing the implication of an important instance on Encoder
    -- * Category (Encoder check parse)
    -- * Associative (Encoder check parse) (,)
    -- * Monoidal (Encoder check parse) (,)
    -- * Cat.Functor f (Encoder check parse) (Encoder check parse)
    -- * PFunctor (,) (Encoder check parse) (Encoder check parse)
    -- * QFunctor (,) (Encoder check parse) (Encoder check parse)
    -- * Bifunctor (,) (Encoder check parse) (Encoder check parse)