Is it possible to use the ST monad as a (drop-in) replacement for the State monad in the following situation? If not, is there a "best practice" for refactoring? I have a bunch of functions that return state actions: type MyState = ... foo1 :: T1 -> State MyState a foo2 :: T2 -> State MyState a ... foon :: Tn -> State MyState a And I'd like to refactor this to use the ST monad, mechanically, if possible. All uses of the MyState inside State are single-threaded. In my application, MyState is a record with 5 or so fields. One of those fields uses a list to keep track of some information, and I'd like to change that to STUArray, because it changes my bottleneck operations from O(n) to O(1). This, of course, requires having the ST monad around, in order to achieve the proper time complexity. Is there an easy way to do this? In the future, should I *start out* with the ST monad if I suspect I'll need to use an imperative data structure for efficiency reasons? I started out with State because I'm modeling a transition system, so it seemed natural. Any advice is appreciated. -- Denis