State in Haskell

Some algorithms make critical internal use of updatable state, even though their external specification is purely functional. Based on earlier work on monads, we present a way of securely encapsulating stateful computations that manipulate multiple, named, mutable objects, in the context of a nonstrict, purely functional language. The security of the encapsulation is assured by the type system, using parametricity. The same framework is also used to handle input/output operations (state changes on the external world) and calls to C.