Domain-Specific Languages

Martin Fowler / Rebecca Parsons  
Total pages
September 2010
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When carefully selected and used, Domain-Specific Languages (DSLs) may simplify complex code, promote effective communication with customers, improve productivity, and unclog development bottlenecks. In Domain-Specific Languages, noted software development expert Martin Fowler first provides the information software professionals need to decide if and when to utilise DSLs. Then, where DSLs prove suitable, Fowler presents effective techniques for building them, and guides software engineers in choosing the right approaches for their applications. This book’s techniques may be utilised with most modern object-oriented languages; the author provides numerous examples in Java and C#, as well as selected examples in Ruby. Wherever possible, chapters are organised to be self-standing, and most reference topics are presented in a familiar patterns format.

Armed with this wide-ranging book, developers will have the knowledge they need to make important decisions about DSLs—and, where appropriate, gain the significant technical and business benefits they offer.

The topics covered include:

  • How DSLs compare to frameworks and libraries, and when those alternatives are sufficient
  • Using parsers and parser generators, and parsing external DSLs
  • Understanding, comparing, and choosing DSL language constructs
  • Determining whether to use code generation, and comparing code generation strategies
  • Previewing new language workbench tools for creating DSLs

Table of Contents

Preface                            xix


Part I: Narratives                                              1

Chapter 1: An Introductory Example                             3

Gothic Security         3

The State Machine Model        5

Programming Miss Grant’s Controller         9

Languages and Semantic Model          16

Using Code Generation         19

Using Language Workbenches         22

Visualization        24


Chapter 2: Using Domain-Specific Languages                            27

Defining Domain-Specific Languages         27

Why Use a DSL?         33

Problems with DSLs         36

Wider Language Processing         39

DSL Lifecycle        40

What Makes a Good DSL Design?       42


Chapter 3: Implementing DSLs                                   43

Architecture of DSL Processing           43

The Workings of a Parser          47

Grammars, Syntax, and Semantics        49

Parsing Data        50

Macros       52


Chapter 4: Implementing an Internal DSL                         67

Fluent and Command-Query APIs          68

The Need for a Parsing Layer         71

Using Functions         72

Literal Collections       77

Using Grammars to Choose Internal Elements       79

Closures       80

Parse Tree Manipulation       82

Annotation       84

Literal Extension      85

Reducing the Syntactic Noise         85

Dynamic Reception         86

Providing Some Type Checking      87


Chapter 5: Implementing an External DSL                                 89

Syntactic Analysis Strategy          89

Output Production Strategy        92

Parsing Concepts        94

Mixing-in Another Language        100

XML DSLs        101


Chapter 6: Choosing between Internal and External DSLs                      105

Learning Curve       105

Cost of Building        106

Programmer Familiarity       107

Communication with Domain Experts         108

Mixing In the Host Language        108

Strong Expressiveness Boundary        109

Runtime Configuration        110

Sliding into Generality       110

Composing DSLs       111

Summing Up        111


Chapter 7: Alternative Computational Models                           113

A Few Alternative Models        116


Chapter 8: Code Generation                                            121

Choosing What to Generate         122

How to Generate        124

Mixing Generated and Handwritten Code        126

Generating Readable Code       127

Preparse Code Generation        128

Further Reading        128


Chapter 9: Language Workbenches                                        129

Elements of Language Workbenches       130

Schema Definition Languages and Meta-Models      131

Source and Projectional Editing         136

Illustrative Programming         138

Tools Tour         140

Language Workbenches and CASE tools           141

Should You Use a Language Workbench?        142


Part II: Common Topics                                                           145

Chapter 10: A Zoo of DSLs                                                           147

Graphviz      147

JMock      149

CSS       150

Hibernate Query Language (HQL)        151

XAML         152

FIT         155

Make et al.      156


Chapter 11: Semantic Model                                                 159

How It Works        159

When to Use It        162

The Introductory Example (Java)      163


Chapter 12: Symbol Table                                                       165

How It Works        166

When to Use It       168

Further Reading      168

Dependency Network in an External DSL (Java and ANTLR)       168

Using Symbolic Keys in an Internal DSL (Ruby)          170

Using Enums for Statically Typed Symbols (Java)       172


Chapter 13: Context Variable                                     175

How It Works        175

When to Use It          176

Reading an INI File (C#)       176


Chapter 14: Construction Builder                                      179

How It Works        179

When to Use It        180

Building Simple Flight Data (C#)        180


Chapter 15: Macro                                                   183

How It Works        184

When to Use It       192


Chapter 16: Notification                                          193

How It Works           194

When to Use It       194

A Very Simple Notification (C#)         194

Parsing Notification (Java)          195


Part III: External DSL Topics                                              199

Chapter 17: Delimiter-Directed Translation                                  201

How It Works           201

When to Use It           204

Frequent Customer Points (C#)        205

Parsing Nonautonomous Statements with Miss Grant’s Controller (Java)       211


Chapter 18: Syntax-Directed Translation                                         219

How It Works              220

When to Use It         227

Further Reading       227


Chapter 19: BNF                                       229

How It Works          229

When to Use It       238


Chapter 20: Regex Table Lexer (by Rebecca Parsons)                                239

How It Works       240

When to Use It      241

Lexing Miss Grant’s Controller (Java)      241


Chapter 21: Recursive Descent Parser (by Rebecca Parsons)                    245

How It Works      246

When to Use It      249

Further Reading      249

Recursive Descent and Miss Grant’s Controller (Java)     250


Chapter 22: Parser Combinator (by Rebecca Parsons)                         255

How It Works       256

When to Use It         261

Parser Combinators and Miss Grant’s Controller (Java)      261


Chapter 23: Parser Generator                                      269

How It Works       269

When to Use It     272

Hello World (Java and ANTLR)      272


Chapter 24: Tree Construction                                       281

How It Works        281

When to Use It       284

Using ANTLR’s Tree Construction Syntax (Java and ANTLR)       284

Tree Construction Using Code Actions (Java and ANTLR)      292


Chapter 25: Embedded Translation                                    299

How It Works        299

When to Use It         300

Miss Grant’s Controller (Java and ANTLR)         300


Chapter 26: Embedded Interpretation                               305

How It Works       305

When to Use It      306

A Calculator (ANTLR and Java)      306


Chapter 27: Foreign Code                                                309

How It Works        309

When to Use It        311

Embedding Dynamic Code (ANTLR, Java, and Javascript)       311


Chapter 28: Alternative Tokenization                                       319

How It Works       319

When to Use It         326


Chapter 29: Nested Operator Expression                                 327

How It Works         327

When to Use It        331


Chapter 30: Newline Separators                                          333

How It Works       333

When to Use It       335


Chapter 31: External DSL Miscellany                                 337

Syntactic Indentation      337

Modular Grammars       339


Part IV: Internal DSL Topics                                           341

Chapter 32: Expression Builder                                       343

How It Works        344

When to Use It        344

A Fluent Calendar with and without a Builder (Java)        345

Using Multiple Builders for the Calendar (Java)          348


Chapter 33: Function Sequence                                        351

How It Works           351

When to Use It       352

Simple Computer Configuration (Java)      352


Chapter 34: Nested Function                                             357

How It Works      357

When to Use It       359

The Simple Computer Configuration Example (Java)      360

Handling Multiple Different Arguments with Tokens (C#)      361

Using Subtype Tokens for IDE Support (Java)     363

Using Object Initializers (C#)      365

Recurring Events (C#)     366


Chapter 35: Method Chaining                                 373

How It Works      373

When to Use It        377

The Simple Computer Configuration Example (Java)      378

Chaining with Properties (C#)     381

Progressive Interfaces (C#)      382


Chapter 36: Object Scoping                                          385

How It Works        386

When to Use It      386

Security Codes (C#)        387

Using Instance Evaluation (Ruby)          392

Using an Instance Initializer (Java)       394


Chapter 37: Closure                                                397

How It Works          397

When to Use It        402


Chapter 38: Nested Closure                                              403

How It Works         403

When to Use It        405

Wrapping a Function Sequence in a Nested Closure (Ruby)       405

Simple C# Example (C#)        408

Using Method Chaining (Ruby)       409

Function Sequence with Explicit Closure Arguments (Ruby   411

Using Instance Evaluation (Ruby)     412


Chapter 39: Literal List                                               417

How It Works       417

When to Use It      417


Chapter 40: Literal Map                                         419

How It Works      419

When to Use It      420

The Computer Configuration Using Lists and Maps (Ruby)      420

Evolving to Greenspun Form (Ruby)       422


Chapter 41: Dynamic Reception                                 427

How It Works      428

When to Use It       429

Promotion Points Using Parsed Method Names (Ruby)        430

Promotion Points Using Chaining (Ruby)        434

Removing Quoting in the Secret Panel Controller (JRuby)       438


Chapter 42: Annotation                                                  445

How It Works      446

When to Use It      449

Custom Syntax with Runtime Processing (Java)        449

Using a Class Method (Ruby)        451

Dynamic Code Generation (Ruby)      452


Chapter 43: Parse Tree Manipulation                                       455

How It Works        455

When to Use It        456

Generating IMAP Queries from C# Conditions (C#)        457


Chapter 44: Class Symbol Table                                   467

How It Works            468

When to Use It       469

Statically Typed Class Symbol Table (Java)       469


Chapter 45: Textual Polishing                               477

How It Works        477

When to Use It          478

Polished Discount Rules (Ruby)          478


Chapter 46: Literal Extension                            481

How It Works           481

When to Use It        482

Recipe Ingredients (C#)        483


Part V: Alternative Computational Models                      485

Chapter 47: Adaptive Model                      487

How It Works     488

When to Use It     492


Chapter 48: Decision Table             495

How It Works     495

When to Use It     497

Calculating the Fee for an Order (C#)     497


Chapter 49: Dependency Network          505

How It Works      506

When to Use It     508

Analyzing Potions (C#)     508


Chapter 50: Production Rule System                 513

How It Works     514

When to Use It      517

Validations for club membership (C#)     517

Eligibility Rules: extending the club membership (C#)     521


Chapter 51: State Machine                   527

How It Works      527

When to Use It     529

Secret Panel Controller (Java)     530


Part VI: Code Generation                   531

Chapter 52: Transformer Generation                    533

How It Works       533

When to Use It      535

Secret Panel Controller (Java generating C)     535


Chapter 53: Templated Generation                   539

How It Works     539

When to Use It      541

Generating the Secret Panel State Machine with Nested Conditionals (Velocity and Java generating C)      541


Chapter 54: Embedment Helper                   547

How It Works     548

When to Use It     549

Secret Panel States (Java and ANTLR)     549

Should a Helper Generate HTML? (Java and Velocity)     552


Chapter 55: Model-Aware Generation          555

How It Works     556

When to Use It     556

Secret Panel State Machine (C)     557

Loading the State Machine Dynamically (C)     564


Chapter 56: Model Ignorant Generation          567

How It Works     567

When to Use It      568

Secret Panel State Machine as Nested Conditionals (C)    568


Chapter 57: Generation Gap                          571

How It Works     571

When to Use It    573

Generating Classes from a Data Schema (Java and a Little Ruby)    573


Bibliography                        579

Index                        581

Back Cover

Designed as a wide-ranging guide to Domain Specific Languages (DSLs) and how to approach building them, this book covers a variety of different techniques available for DSLs. The goal is to provide readers with enough information to make an informed choice about whether or not to use a DSL and what kinds of DSL techniques to employ. Part I is a 150-page narrative overview that gives you a broad understanding of general principles.  The reference material in Parts II through VI provides the details and examples you will need to get started using the various techniques discussed.  Both internal and external DSL topics are covered, in addition to alternative computational models and code generation. Although the general principles and patterns presented can be used with whatever programming language you happen to be using, most of the examples are in Java or C#.


Martin Fowler is Chief Scientist at ThoughtWorks. He describes himself as “an author, speaker, consultant, and general loudmouth on software development. I concentrate on designing enterprise software—looking at what makes a good design and what practices are needed to come up with good design.” Fowler’s books include Patterns of Enterprise Application Architecture; UML Distilled, Third Edition; and (with Kent Beck, John Brant, and William Opdyke) Refactoring: Improving the Design of Existing Code. All are published by Addison-Wesley.