Home > Articles > Programming > Windows Programming

Domain-Specific Languages: An Introductory Example

In this excerpt from his book, Domain-Specific Languages, Martin Fowler offers a concrete example to demonstrate the different forms a DSL can take.
From the book

When I start to write, I need to swiftly explain what it is I’m writing about; in this case, to explain what a domain-specific language (DSL) is. I like to do this by showing a concrete example and following up with a more abstract definition. So, here I’m going to start with an example to demonstrate the different forms a DSL can take. In the next chapter I’ll try to generalize the definition into something more widely applicable.

1.1 Gothic Security

I have vague but persistent childhood memories of watching cheesy adventure films on TV. Often, these films would be set in some old castle and feature secret compartments or passages. In order to find them, heroes would need to pull the candle holder at the top of stairs and tap the wall twice.

Let’s imagine a company that decides to build security systems based on this idea. They come in, set up some kind of wireless network, and install little devices that send four-character messages when interesting things happen. For example, a sensor attached to a drawer would send the message D2OP when the drawer is opened. We also have little control devices that respond to four-character command messages—so a device can unlock a door when it hears the message D1UL.

At the center of all this is some controller software that listens to event messages, figures out what to do, and sends command messages. The company bought a job lot of Java-enabled toasters during the dot-com crash and is using them as the controllers. So whenever a customer buys a gothic security system, they come in and fit the building with lots of devices and a toaster with a control program written in Java.

For this example, I’ll focus on this control program. Each customer has individual needs, but once you look at a good sampling, you will soon see common patterns. Miss Grant closes her bedroom door, opens a drawer, and turns on a light to access a secret compartment. Miss Shaw turns on a tap, then opens either of her two compartments by turning on correct light. Miss Smith has a secret compartment inside a locked closet inside her office. She has to close a door, take a picture off the wall, turn her desk light on three times, open the top drawer of her filing cabinet—and then the closet is unlocked. If she forgets to turn the desk light off before she opens the inner compartment, an alarm will sound.

Although this example is deliberately whimsical, the underlying point isn’t that unusual. What we have is a family of systems that share most components and behaviors, but have some important differences. In this case, the way the controller sends and receives messages is the same across all the customers, but the sequence of events and commands differs. We want to arrange things so that the company can install a new system with the minimum of effort, so it must be easy for them to program the sequence of actions into the controller.

Looking at all these cases, it emerges that a good way to think about the controller is as a state machine. Each sensor sends an event that can change the state of the controller. As the controller enters a state, it can send a command message out to the network.

At this point, I should confess that originally in my writing it was the other way around. A state machine makes a good example for a DSL, so I picked that first. I chose a gothic castle because I get bored of all the other state machine examples.

1.1.1 Miss Grant’s Controller

Although my mythical company has thousands of satisfied customers, we’ll focus on just one: Miss Grant, my favorite. She has a secret compartment in her bedroom that is normally locked and concealed. To open it, she has to close the door, then open the second drawer in her chest and turn her bedside light on—in either order. Once these are done, the secret panel is unlocked for her to open.

I can represent this sequence as a state diagram (Figure 1.1).

If you haven’t come across state machines yet, they are a common way of describing behavior—not universally useful but well suited to situations like this. The basic idea is that the controller can be in different states. When you’re in a particular state, certain events will transition you to another state that will have different transitions on it; thus a sequence of events leads you from state to state. In this model, actions (sending of commands) occur when you enter a state. (Other kinds of state machines perform actions in different places.)

This controller is, mostly, a simple and conventional state machine, but there is a twist. The customers’ controllers have a distinct idle state that the system spends most of its time in. Certain events can jump the system back into this idle state even if it is in the middle of the more interesting state transitions, effectively resetting the model. In Miss Grant’s case, opening the door is such a reset event.

Figure 1.1 State diagram for Miss Grant’s secret compartment

Introducing reset events means that the state machine described here doesn’t quite fit one of the classical state machine models. There are several variations of state machines that are pretty well known; this model starts with one of these but the reset events add a twist that is unique to this context.

In particular, you should note that reset events aren’t strictly necessary to express Miss Grant’s controller. As an alternative, I could just add a transition to every state, triggered by doorOpened, leading to the idle state. The notion of a reset event is useful because it simplifies the diagram.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020