Commands

A process is a sequence of commands, and let statements. Now that we’ve covered let statements, it’s time to look at commands. We’ll do that by using them in small, but realistic examples.

First, what even is a command? It’s similar to a statement in imperative programming, but there is an important difference. In imperative programming, a statement is usually an expression that’s evaluated not for its result, but for its side-effects. In contrast, commands in Par are not expressions, they are a distinct syntactic category.

Every command has a subject, a channel — usually a local variable — that the command is operating on. After executing the command, the subject changes its type. This is the important distinction from imperative statements. It’s the distinction that brings proper, and ergonomic session typing to this imperative-looking process syntax.

Let’s see that in action!