Home > Articles > Programming > Visual Studio

This chapter is from the book

Textual DSLs

Before talking about graphical DSLs, let's look briefly at textual DSLs. We'll see how Domain-Specific Development involves a particular way of thinking about a problem, and we'll look at how to implement this approach using textual languages.

Imagine that we are designing a graphical modeling tool and have the problem of defining a set of shapes that will be displayed on a screen to represent the various concepts that can be depicted by the tool. One way we might do this would be to invent a new textual language for defining the various shapes. A fragment of this language might look like this:

Define AnnotationShape Rectangle
      Width=1.5
      Height=0.3
      FillColor=khaki
      OutlineColor=brown
      Decorator Comment
            Position="Center"
      End Comment
End AnnotationShape

In order to process this language, a program must be written to parse and interpret this text. As a programming exercise from scratch, this is a big job. But a parser-generator might be used, which itself takes as input a description of the grammar of the new language, such as the following, based on BNF (the Backus Naur Form, originally developed for defining the Algol language):

Definitions ::= Definition*
      Definition ::= Define Id Shape
      Width Eq Number
      Height Eq Number
      FillColor Eq Color
      OutlineColor Eq Color
      Decorator*
End Id

Shape ::= Rectangle | RoundedRectangle | Ellipse

Eq ::= "="

Decorator ::= Decorator Id
      Position Eq Position
End Id

Position ::= Center|
             TopLeft |
             TopRight |
             BottomLeft |
             BottomRight

The definitions for Id, Number, and Color are not included here; it's assumed that they are built into the grammar-defining language.

We need an algorithm to convert this BNF into a parser for the language it describes. We'd either use an existing parser-generator such as Yacc, Bison, Antlr, or Happy, or an expert might write one by hand in a normal third-generation programming language such as C# or Java.

Notice that the BNF is itself a DSL. We might "bootstrap" the BNF language by describing its grammar in itself, causing it to generate a parser for itself. Perhaps the hand-written parser will be quite simple, and the generated parser would handle a more complicated version of BNF. This pattern of using languages to describe languages, and bootstrapping languages using themselves, is very common when defining domain-specific languages.

Implementing a textual DSL by implementing its grammar like this can be a difficult and error-prone task, requiring significant expertise in language design and the use of a parser-generator. Implementing a parser-generator is definitely an expert task, because a grammar might be ambiguous or inconsistent, or might require a long look-ahead to decide what to do. Furthermore, there is more to implementing a language than just implementing a parser. We'd really like an editor for the language that gives the kinds of facilities we expect from a programming language editor in a modern development environment, like text colorization, real-time syntax checking, and autocompletion. If you include these facilities, the task of implementing a textual language can get very large. Happily, there are alternative strategies for implementing a textual DSL that don't involve implementing a new grammar.

The first strategy is to use the facilities of a host language to emulate the capabilities of a domain-specific language. For example, the following C# code has the effect of defining the same shape as the previous example:

Shape AnnotationShape = new Shape(ShapeKind.Rectangle,
                                  1.5,
                                  0.3,
                                  Color.Khaki,
                                  Color.Brown);
Decorator Comment = new Decorator(Position.Center);
AnnotationShape.AddDecorator(Comment);

This kind of code is often called configuration code, because it uses previously defined classes and structures to create a specific configuration of objects and data for the problem that you want to solve. In effect, the definitions of these classes and structures are creating an embedded DSL, and the configuration code is using that DSL. The capabilities of modern languages to define abstractions such as classes, structures, enumerations, and even configurable syntax make them more amenable to this approach than earlier languages that lacked these facilities.

The second strategy is to use XML—Extensible Markup Language. There are many ways in which the definition can be expressed using XML. Here's a possible approach.

<?xml version="1.0" encoding="utf-8" ?>
<Shapes>
  <Shape name="AnnotationShape">
    <Kind>Rectangle</Kind>
    <Width>1.5</Width>
    <FillColor>Khaki</FillColor>
    <OutlineColor>Brown</OutlineColor>
    <Decorator name="Comment">
      <Position>Center</Position>
    </Decorator>
  </Shape>
</Shapes>

The syntax is obviously limited to what can be done using XML elements and attributes. Nevertheless, the tags make it obvious what each element is intended to represent, and the meaning of the document is quite clear. One great advantage of using XML for this kind of purpose is the widespread availability of tools and libraries for processing XML documents.

If we want to use standard XML tools for processing shape definitions, the experience will be much improved if we create a schema that allows us to define rules for how shape definitions are represented in XML documents. There are several technologies available for defining such rules for XML documents, including XML Schema from the World Wide Web Consortium (defined at www.w3.org/XML/Schema.html), RELAX NG from the OASIS consortium (defined at www.relaxng.org) and Schematron, which has been accepted as a standard by the International Organization for Standardization (ISO) and is defined at www.schematron.com. Schematron is supported in .NET: A version called Schematron.NET is downloadable from SourceForge, and it is possible to combine the facilities of XML Schema and Schematron. We'll use here the XML Schema approach, which is also supported by the .NET framework.

An XML Schema is an XML document written in a special form that defines a grammar for other XML documents. So, using an appropriate schema, we can specify exactly which XML documents are valid shape definition documents. Modern XML editors, such as the one in Visual Studio 2005, can use the XML schema to drive the editing experience, providing the user with real-time checking of document validity, colorization of language elements, auto-completion of tags, and tips about the document's meaning when you hover above the elements.

Here is one of many possible XML schemas for validating shape definition documents such as the one presented earlier. Writing such schemas is something of an art; you'll certainly observe that it is significantly more complicated than the BNF that we defined earlier, although it expresses roughly the same set of concepts.

<?xml version="1.0" encoding="utf-8"?>
<xs:schema
  xmlns="http://schemas.microsoft.com/dsltools/ch01"
  attributeFormDefault="unqualified"
  elementFormDefault="qualified"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://schemas.microsoft.com/dsltools/ch01">
  <xs:element name="Shapes">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" name="Shape">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Kind" type="kind" />
              <xs:element name="Width" type="xs:decimal" />
              <xs:element name="Height" type="xs:decimal" />
              <xs:element name="FillColor" type="xs:string" />
              <xs:element name="OutlineColor" type="xs:string" />
              <xs:element maxOccurs="unbounded" name="Decorator">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element name="Position" type="position" />
                  </xs:sequence>
                  <xs:attribute name="name" type="xs:string" use="required" />
                </xs:complexType>
              </xs:element>
            </xs:sequence>
            <xs:attribute name="name" type="xs:string" use="required" />
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name="position">
    <xs:restriction base="xs:string">
      <xs:enumeration value="Center" />
      <xs:enumeration value="TopLeft" />
      <xs:enumeration value="TopRight" />
      <xs:enumeration value="BottomLeft" />
      <xs:enumeration value="BottomRight" />
    </xs:restriction>
  </xs:simpleType>

  <xs:simpleType name="kind">
    <xs:restriction base="xs:string">
      <xs:enumeration value="Rectangle" />     
      <xs:enumeration value="RoundedRectangle" />
      <xs:enumeration value="Ellipse" />
    </xs:restriction>
  </xs:simpleType>

</xs:schema>

To summarize, in this section we have looked at three ways of defining a textual DSL: using a parser-generator, writing configuration code embedded in a host language, and using XML with a schema to help validate your documents and provide syntax coloring and autocompletion. A further option would be to define an equivalent to the DSL Tools that targeted textual languages.

Each of these approaches has its pros and cons, but they all share a common theme—investing some resources early in order to define a language that will make it easier to solve specific problems later. This is the basic pattern that also applies to graphical DSLs, as we shall see.

The DSL Tools themselves provide no facilities in version 1 for defining textual domain-specific languages. The Tools' authors have taken the view that XML provides a sufficiently good approach to start with, and so they have designed the DSL Tools to integrate XML-based textual DSLs with graphical DSLs.

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