Graph.hs:36:14:
    Couldn't match type `Int' with `Maybe Int'
    When using functional dependencies to combine
      Control.Monad.State.Class.MonadState s (StateT s m),
        arising from the dependency `m -> s'
        in the instance declaration in `Control.Monad.State.Class'
      Control.Monad.State.Class.MonadState
        (IM.IntMap (Maybe Int))
        (StateT
           (IM.IntMap Int)
           (Control.Monad.Trans.Reader.ReaderT
              (IM.IntMap (IM.IntMap Int)) Data.Functor.Identity.Identity)),
        arising from a use of `get' at Graph.hs:36:14-16
    In a stmt of a 'do' block: distance <- get
    In the expression:
      do { γ <- ask;
           distance <- get;
           let candidates :: [Vertex]
               candidates = ...
               ....;
           case newVertex of {
             Just (v, d) -> do { ... }
             Nothing -> return False } }



---------------
step ∷ StateT (IM.IntMap Distance) (Reader Graph) Bool
step = do
  γ ← ask
  distance ← get --line 36
  let candidates ∷ [Vertex]
      candidates = concatMap nub . map (\v → (IM.keys $ γ IM.! v) \\ (IM.keys distance)) $ IM.keys distance ∷ [Vertex]
      nearestWay ∷ Vertex → Distance
      nearestWay c = minimum . catMaybes . map (\v → (+ distance IM.! v) `fmap` IM.lookup c $ γ IM.! v) $ IM.keys distance
      newVertex ∷ Maybe (Vertex, Distance)
      newVertex = listToMaybe . sortBy (compare `on` snd) . map (\c → (c, nearestWay c)) candidates
  case newVertex of
    Just (v, d) → do
      put $ IM.insert v d distance
      return True
    Nothing → return False