In this chapter we integrate concurrency into the framework of Modernized Algol described in Chapter 35. The resulting language, called Concurrent Algol, ℒ{nat cmd ⇀∥}, illustrates the integration of the mechanisms of the process calculus described in Chapter 41 into a practical programming language. To avoid distracting complications, we drop assignables from Modernized Algol entirely. (There is no loss of generality, however, because free assignables are definable in Concurrent Algol using processes as cells.)

The process calculus described in Chapter 41 is intended as a self-standing model of concurrent computation. When viewed in the context of a programming language, however, it is possible to streamline the machinery to take full advantage of types that are in any case required for other purposes. In particular, the concept of a channel, which features prominently in Chapter 41, may be identified with the concept of a dynamic class as described in Chapter 34. More precisely, we take broadcast communication of dynamically classified values as the basic synchronization mechanism of the language. Being dynamically classified, messages consist of a payload tagged with a class, or channel. The type of the channel determines the type of the payload. Importantly, only those processes that have access to the channel may decode the message; all others must treat it as inscrutable data that may be passed around but not examined. In this way we can model not only the mechanisms described in Chapter 41, but also formulate an abstract account of encryption and decryption in a network using the methods described in Chapter 41.