Update the module's private state. This is the preferred way of changing the state. The state will be locked until the body returns. The function is exception-safe, i.e. even if an error occurs or the thread is killed (e.g. because it deadlocked and therefore exceeded its time limit), the state from the last write operation will be restored. If the writer escapes, calling it will have no observable effect. withMS is not composable, in the sense that a readMS from within the body will cause a dead-lock. However, all other possibilies to access the state that came to my mind had even more serious deficiencies such as being prone to race conditions or semantic obscurities.