Developing Domain-Specific Languages for the JVM

1. Developing Domain-Specific Languages for the JVM Travis Dazell Systems Architect Digi-Key Corporation

2. Outline • What is a Domain Specific Language (DSL)? • Internal Versus External DSLs • Internal DSLs • Java Example without a DSL • Improved Java Example using Internal DSL Patterns • Groovy for Internal DSLs • External DSLs • Writing a new language to solve our example problem • ANTLR • JetBrains MPS • Scala Parser Combinators

3. What is a DSL? • DSLs have been around for a long time • SQL, CSS, Regular Expressions • DSLs are small languages, focused on a specific problem • DSLs are easy to use and understand, even for a non-programmer • Often accomplished through layers of abstraction • The DSL language relates directly to the problem domain

4. Example Problem: Golf Course Scheduler • To demonstrate how DSLs can be used, we’ll look at writing a simple language for scheduling tee times at a golf course

5. Our Goal • Before TeeTimeteeTime = new TeeTime(); teeTime.setTime(“Sep-23-2013 2:15 PM”); Golfer golfer = new Golfer(); golfer.setFirstName(“Travis”); golfer.setLastName(“Dazell”); golfer.setNumberOfGuests(3); teeTime.setGolfer(golfer); TeeTimeScheduler.schedule(teeTime); • After (as a DSL) new tee time at Sep-23-2013 2:15 PM for Travis Dazell and 3 guests

6. Java Example with the Builder Pattern Live Coding Demo

7. Can We Make the Java DSL Even Better? • An internal Java DSL is limited by the inherent language constraints • Semi-colons are required • Parentheses for invoking methods are not optional • Dots cannot be avoided when dispatching methods • We often need to build a significant amount of indirection to achieve an expressive DSL • Example: Builder Pattern

8. Let’s Make the DSL Groovy Live Coding Demo

9. Solving the Problem with a Groovy DSL • Optional semicolons • We can omit parentheses in many cases • Closures • methodMissing • Metaprogramming • DSL Descriptors for syntax highlighting and help hovers

10. Using External DSLs • What happens when we need to step outside of a host language? • External DSLs are custom languages • We write a grammar to define the syntax for our language • We need to write a lexerand parser • We may need to perform semantic analysis • We need to write an interpreter or code generator • Thankfully, there are tools that make this easier

11. ANTLR Live Coding Demo

12. ANTLR Review • We define our EBNF grammar • We can mix Java code into our grammar for processing input scripts • Note that ASTs can be constructed for more complex processing • ANTLR will generate the lexer and parser for us

13. Scala Parser Combinators Live Coding Demo

14. Scala Parser Combinators Review • You have the privilege of working entirely within Scala • You define parsing rules in an EBNF-like format • You combine each of your parsing rules to form your DSL syntax • Like the other examples, you can interpret the input and generate whatever output you need

15. JetBrains MPS Live Coding Demo

16. JetBrains MPS Review • Takes DSL design to a more abstract level • You work on the concepts and structure of your DSL, instead of the low-level grammar • You do very little, if any, traditional coding in the IDE • You can generate all kinds of output. We auto-generated Java code in this example • You can test your DSLs within the MPS IDE or export your generated Java code to another IDE for integration

17. Applying DSLs in Your Projects • Adapt legacy code • Wrapper APIs • Develop a DSL for a particular sub-system (i.e. domain) of your application • Flesh-out requirements and test cases • Using a DSL to code your business rules can help bridge the gap from requirements to implementation • You can show DSL source code to a business analyst and they’ll understand it completely • Enhancing custom IT tasks • Operators, System Administrators

18. References • References • DebasishGhosh, DSLs In Action, Manning, 2011 • Fergal Dearle, Groovy for Domain-Specific Languages, PACKT, 2010 • Cay S. Horstmann, Scala for the Impatient, Addison-Wesley, 2012 • Martin Fowler, Domain-Specific Languages, 2010 • Markus Voelter, DSL Engineering: Designing, Implementing, and Using Domain-Specific Languages, 2013

19. Related JavaOne Sessions • Attend these JavaOne sessions to learn more • Embedded DSL: Groovy and Scala Fair Duel • Monday 4:30 PM – 6:30 PM, Hilton Continental Ballroom 1/2/3 • BOF2893: Scala Parser Combinators • Tuesday 7:30 PM – 8:15 PM, Hilton Yosemite B/C • CON5389: Groovy DSLs: Beginner to Expert • Wednesday 11:30 AM-12:30 PM, Hilton Golden Gate 4/5 • CON2077: Integrating JVM Languages • Thursday 12:30 PM – 1:30 PM, Hilton Yosemite B/C

20. Conclusion • DSLs allow you to develop software using syntax that fits within the problem domain • Internal DSLs can be written in many JVM languages • Other JVM languages are also well-suited for internal DSL development, such as JRuby, Clojure, and Scala • External DSLs give you the flexibility to design the language any way you want • More involved, but thanks to tools like ANTLR, JetBrains MPS, and Scala Parser Combinators, this isn’t difficult

21. Miscellaneous • Contact Information: • travis.dazell@digikey.com • Code examples are available on GitHub: • https://github.com/travisdazell • Follow me: • Blog: travisdazell.blogspot.com • Twitter: @travisdazell

22. Questions?