Managing state
A program can be divided into several parts which can execute concurrently. It is often necessary to share data or state among these concurrently running tasks. Thus, we arrive at the notion of having multiple observers for some data. If the data gets modified, we must ensure that the changes are visible to all observers. For example, suppose there are two threads that read data from a common variable. This data gets modified by one thread, and the change must be propagated to the other thread as soon as possible to avoid inconsistencies.
Programming languages that support mutability handle this problem by locking over a monitor, as we demonstrated with the locking form, and maintaining local copies of the data. In such languages, a variable is just a container for data. Whenever a concurrent task accesses a variable that is shared with other tasks, it copies the data from the variable. This is done in order to prevent unwanted overwriting of the variable by other tasks while...
