Homework 6: Universality of Processes

A semaphore is a data structure designed to track how many units of some resource are available. It also provides operations to increase or decrease the number, representing the creation or the consumption of that resource, respectively. A process that wishes to consume one unit of the resource, when none are available, will be blocked until one new unit of the resource is created. Semaphores are easily implementable in Concurrent Algol if we ignore the issue of fairness. The trick is to treat a process whose sole purpose is emitting a message as a token for one unit of resource. The way to take a token is to receive one message from that channel and the token process that is sending on that channel will die. The way to give back a token is to create one such token process. The number of available units is thus represented by the number of token processes floating around. Because we do not care about the content of the messages, but only the existence of the messages themselves. unit is a suitable type for the messages. Combing these two ideas, a token can be a process (only) emitting a nullary tuple.