Hello! I've just started learning Haskell and have a simple question regarding monads. Please, don't blame me if it is too obvious (any RTFM links are welcome). Suppose, we have two data types, X and Y. X represent local state, Y -- global one. In some sense, X is contained in Y. For example, Y maybe (X, Z) or [X] or Map Int X. Let's take that (as it is in my case) Y = Map Int X I have function that make some transormation of local state (X) and having some other side effects. f :: StateT X m a where m is another monad, something like WriterT Log IO. Now I want to convert it into function transorming global state, more specifically it's n-th part. So, I want to combine n :: Int and f into g :: StateT Y m a where (Y ! n) is transormed via f, and other parts of Y are not touched. How can this be done? I've looked through StateT documentation and tutorial, but had not found any function from StateT X to StateT Y where Y is not the same as X. With best regards, Alexander.