1 Extending Java And Developing DSLs With Open Source Language Workbench JetBrains MPS Konstantin Solomatov JetBrains Lead Developer for JetBrains MPS Project

Agenda  Extending languages. Why bother?  Problems and solutions Composability Language support  MPS’ approach Idea behind it Hands-on demo  MPS & domain-specific languages 2

Why Bother?  Specific domains = specific entities  Drawbacks of general-purpose languages: Limited set of generic constructs Domains are described too verbosely Low code maintainability  Domain-specific extensions Higher abstraction level Higher maintainability 3

Implicit Language Extensions  Domain-specific constructs in Java: synchronized block in Java enhanced for in Java typesafe enums in Java  Desired constructs: closures yield better support for regexps whatever else you are missing 4

Language Extensions  Existing extensions Cover domain-specific needs Are built into existing languages  Further extensions Hard to integrate Too long to wait until JCP accepts them Risks of incompatibility 5

MPS DEMO Closure & UI Language samples. 6

Agenda  Extending Languages. Why bother?  Problems and Solutions Composability Language support  MPS’ Approach Idea behind it Hands-on Demo  MPS & Domain-Specific Languages 7

Composability Definition: Components are composable when they can be used together even if created by different parties 8

Composability Desired Goal Java Libraries  Hibernate  Spring  Joda Time Java Extensions  DB Language  DI Language  Dates Language 9

Composability Libraries vs. Extensions  Libraries are composable: Java + Hibernate=> OK Java + Spring => OK Java + Hibernate, Spring=> OK  Language extensions are not Java + extension A => OK Java + extension B => OK Java + extensions A, B=> possible ambiguity 10

Composability Grammar Ambiguity Sample With String Interpolation  Extension “A”: int resultsCount = ; string s = “Found ${resultCount} results”;  Extension “B”: int resultCount = ; string t = “Found {resultCount} results”;  “A” + “B” => ambiguity for the compiler: string a = “Account balance is ${balance}”; 11

Language Support Requirements  Generic infrastructure Abstract grammar Type system Compiler Etc.  IDE infrastructure Editor Error highlighting Refactoring Version control Etc. 12

Language Support Problems  Time- and resource-consuming task Complicated algorithms for processing code Highly qualified developers required  Resulting infrastructure composability Type systems to be compatible with different extensions Refactorings should support potential extensions Etc. 13

Existing Solutions Pros & Cons Composability Language Infrastructure LISPYesNo Internal DSLs in Ruby / Groovy YesNo XText FrameworkNoYes 14

Common Problem  Existing solutions provide: Eithercomposable language extensions Ora decent infrastructure Not both 15

Agenda  Extending Languages. Why bother?  Problems and Solutions Composability Language support  MPS’ Approach Idea behind it Hands-on Demo  MPS & Domain-Specific Languages 16

Our Solution to the Problem  JetBrains MPS Workbench for defining and extending languages IDE for new languages and extensions 17

Idea Behind It Text-based grammars lead to ambiguities Another approach needed MPS works with Abstract Syntax Tree directly 18

Idea Behind It Editing Abstract Syntax Tree  Former attempts Diagram-based editing Limited domain of application  MPS solution Keeps user in habitual environment Supports text-like editing 19

Idea Behind It Projectional Editor 20  Each node of a syntax tree has its projection in the MPS Editor

Idea Behind It Projectional Editor 21  Each node – in a dedicated cell  A node is shown in its text/symbol representation  Instantly synchronized editor and syntax tree

Idea Behind It Projection vs. Text  Text-like projectional editor  Has its pros & cons  Reasonable learning curve: approx. 2 weeks 22

MPS DEMO Projectional editor in use. 23

Idea Behind It Language Support  User-defined language infrastructure Abstract grammar Type system Editor Compiler Etc.  Programming assistance Completion Find usages Rename Etc. 24

Idea Behind It Existing Languages & Extensions  Java™ re-implemented with MPS + Extensions Collections language Dates language Closures language Regexp language Etc.  Language definition languages Implemented with themselves i.e. bootstrapped  Misc languages (XML, etc.) 25

Hands-On Demo Goal Java ReadWriteLock l = … l.readLock().lock(); try { //code } finally { l.readLock().unlock(); } Java + Extension ReadWriteLock l = … read (l) { //code } 26

Hands-On Demo Workflow  We will define Structure Editor Typesystem Generator 27

MPS DEMO Adding lock statement to Java 28

Agenda  Extending Languages. Why bother?  Problems and Solutions Composability Language support  MPS’ Approach Idea behind it Hands-on Demo  MPS & Domain-Specific Languages 29

Domain-Specific Languages  MPS suits nicely for creating DSLs  Common language parts can be reused (e.g., expression language)  You can embed general-purpose languages (e.g., Java) inside DSLs 30

Domain-Specific Languages Example 31

Existing Applications  YouTrack – bug tracking system Completely developed with MPS Used internally Beta program  Languages for Web Development DNQ Webr Spring language 32

Licensing/Pricing  Almost fully open-sourced  Apache 2.0 License MPS is absolutely FREE 33

Q & A 34

More info - MPS page - MPS blog 35

36 Konstantin Solomatov Lead Developer for JetBrains MPS Project Thank You