T3169.hs:13:22: error:
    • Couldn't match type ‘elt’ with ‘Map b elt’
      Expected: Map a (Map b elt)
        Actual: Map (a, b) elt
      ‘elt’ is a rigid type variable bound by
        the type signature for:
          lookup :: forall elt. (a, b) -> Map (a, b) elt -> Maybe elt
        at T3169.hs:12:3-8
    • In the second argument of ‘lookup’, namely ‘m’
      In the expression: lookup a m :: Maybe (Map b elt)
      In the expression:
        case lookup a m :: Maybe (Map b elt) of {
          Just (m2 :: Map b elt) -> lookup b m2 :: Maybe elt }
    • Relevant bindings include
        m :: Map (a, b) elt (bound at T3169.hs:12:17)
        b :: b (bound at T3169.hs:12:13)
        lookup :: (a, b) -> Map (a, b) elt -> Maybe elt
          (bound at T3169.hs:12:3)