ExposedEval

An datastructure describing an computation which at some points delegates some work to the user. The user receives an intermediate state (type SI) and a value of type X by calling pre. The value needs to be transformed into another of type Y that is fed back to the computation, along with the intermediate state with the functions post. e.g. pseudo-code to run the whole computation:

(si, x) = pre() y = f(x) result = post(si, y)