Recurse Center

Paper of the Week: Statecharts: A Visual Formalism for Complex Systems

David Albert

This is part of our “Paper of the Week” series. For more info, check out our introductory blog post.

This week’s paper is Statecharts: A Visual Formalism for Complex Systems by David Harel, a professor of computer science and applied mathematics at the Weizmann Institute of Science. It was published in the June 1987 issue of Science of Computer Programming.

This week’s paper was submitted by Hacker School resident Kevin Lynagh, who shared the following:

Harel’s Statecharts are a visual formalism for specifying the behavior of systems. They’re basically finite state machines, but with notions of hierarchy and concurrency.

The paper is very readable: Harel reverse engineers his digital wristwatch and draws the statechart specifying its behavior. It’s extremely fruitful to internalize the larger idea: That you can (and should!) think rigorously about the desired behavior of an artifact before diving straight into the code.

Here’s the abstract from the paper:

We present a broad extension of the conventional formalism of state machines and state diagrams, that is relevant to the specification and design of complex discrete-event systems, such as multi-computer real-time systems, communication protocols and digital control units. Our diagrams, which we call statecharts, extend conventional state-transition diagrams with essentially three elements, dealing, respectively, with the notions of hierarchy, concurrency and communication. These transform the language of state diagrams into a highly structured and economical description language. Statecharts are thus compact and expressive—small diagrams can express complex behavior—as well as compositional and modular. When coupled with the capabilities of computerized graphics, statecharts enable viewing the description at different levels of detail, and make even very large specifications manageable and comprehensible. In fact, we intend to demonstrate here that statecharts counter many of the objections raised against conventional state diagrams, and thus appear to render specification by diagrams an attractive and plausible approach. Statecharts can be used either as a stand-alone behavioral description or as part of a more general design methodology that deals also with the system’s other aspects, such as functional decomposition and data-flow specification. We also discuss some practical experience that was gained over the last three years in applying the statechart formalism to the specification of a particularly complex system.

Read Along

Read Along is a way for you to participate in Paper of the Week. If you want to take part, all you have to do is read the paper, make something small in response (code or prose), and email us a link to what you made by noon Eastern Time next Monday.

Sadly, we didn’t receive any Read Along submissions for Notation as a Tool of Thought. We’re looking forward to your submissions for this week’s paper.

Happy reading!