Right, but the point is that you can't actually write the equivalent of that function in Haskell. You'd have to write something like this:

    defaultMap :: Map k v
    defaultMap = ...

    f :: TVar (Map k v) -> IO (Map k v)
    f = \case
      Nothing -> newTVarIO Map.empty
      Just m  -> pure defaultMap
  

which would make it hard to be unaware of the possibility that the dictionary you get each time you pass `Nothing` would be the same one. And then the type signature returns an IO action, so you would know that "anything could happen here", whereas there's no such indication in python.

Conversely, if you had your `f` function in Haskell has a type like `Maybe (Map k v) -> Map k v` then you know that it can't possibly be creating a mutable map and inserting it in the process somewhere without your knowledge. (Modulo `unsafePerformIO` and other evils like that).