NoMatchErr.hs:19:7: error:
    • Couldn't match type: Memo d0
                     with: Memo d
      Expected: Memo d a -> Memo d a
        Actual: Memo d0 a -> Memo d0 a
      NB: ‘Memo’ is a non-injective type family
      The type variable ‘d0’ is ambiguous
    • In the ambiguity check for ‘f’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature: f :: (Fun d) => Memo d a -> Memo d a