by Mario Fusco twitter: @mariofusco Real world DSL making technical and business people speaking the same language

What is a Domain Specific Language? A computer programming language of limited expressiveness focused on a particular domain computer programming language limited expressiveness particular domain

2 principles driving toward DSL development Why use a Domain Specific Language?

The only purpose of languages, even programming ones IS COMMUNICATION Communication is king

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Written once, read many times Always code as if the person who will maintain your code is a maniac serial killer that knows where you live

"Any fool can write code that a computer can understand. Good programmers write code that humans can understand“ Martin Fowler

Pros & Cons of DSLs + Expressiveness  Communicativity + Conciseness + Readability  Maintainability  Modificability + Higher level of abstraction + Higher productivity in the specific problem domain ̶ Language design is hard ̶ Upfront cost ̶ Additional indirection layer  Performance concerns ̶ Lack of adequate tool support ̶ Yet-another-language-to-learn syndrome

DSL taxonomy External DSL  a language having custom syntactical rules separate from the main language of the application it works with Internal DSL  a particular way of employing a general-purpose language, using only a small subset of the language's features Language workbench  a specialized IDE for defining and building DSL, usually in a visual way, used both to determine the structure of the DSL and to provide an editing environment for people using it

Internal DSL External DSL Language Workbench DSL Types Comparison + learning curve + cost of building + programmers familiarity + IDE support (autocompletion …) + composability + flexibility + readability + clear separation between business (DSL) and host language + helpful when business code is written by a separate team (domain experts) ̶ syntactic noise ̶ needs to be recompiled (if host language is static) ̶ need to learn of grammars and language parsing ̶ boundary between DSL and host language ̶ easier to grow out of control + visual + rapid development + IDE support for external DSL ̶ tools immaturity ̶̶ vendor lock-in

What is an internal DSL? A (business) internal DSL is a fluent interface built on top of a clearly defined Command & Query API

The Command & Query API public interface Car { void setColor(Color color); void addEngine(Engine engine); void add Transmission(Transmission transmission); } public interface Engine { enum Type { FUEL, DIESEL, ELECTRIC, HYDROGEN, METHANE } void setType(Type type); void setPower(int power); void setCylinder(int cylinder); } public interface Transmission { enum Type { MANUAL, AUTOMATIC, CONTINOUSLY_VARIABLE } void setType(Type type); void setNumberOfGears(int gears); }

Let's build a car Car car = new CarImpl(); Car.setColor(Color.WHITE); Engine engine1 = new EngineImpl(); engine1.setType(Engine.Type.FUEL); engine1.setPower(73); engine1.setCylinder(4); car.addEngine(engine1); Engine engine2 = new EngineImpl(); engine2.setType(Engine.Type.ELECTRIC); engine2.setPower(60); car.addEngine(engine2); Transmission transmission = new TransmissionImpl(); transmission.setType(Transmission.Type.CONTINOUSLY_VARIABLE); car.setTransmission(transmission);

… but you don't expect a (non-technical) domain expert to read this, don't you? Quite good …

"Domain users shouldn't be writing code in our DSL but it must be designed for them to understand and validate“ Debasish Ghosh

Object Initialization new CarImpl() {{ color(Color.WHITE); engine(new EngineImpl {{ type(Engine.Type.FUEL); power(73); cylinder(4); }}); engine(new EngineImpl {{ type(Engine.Type.FUEL); power(60); }}); transmission(new TransmissionImpl {{ type(Transmission.Type.CONTINOUSLY_VARIABLE); }}); }}; ̶ syntactial noise ̶ explicit use of constructors + clear hierarchic structure + small implementation effort ̶ unclear separation between Command API and DSL

Functions Sequence car(); color(Color.WHITE); engine(); type(Engine.Type.FUEL); power(73); cylinder(4); engine(); type(Engine.Type.ELECTRIC); power(60); transmission(); type(Transmission.Type.CONTINOUSLY_VARIABLE); end(); + works well for defining the items of a (top level) list ̶ use of global context variable(s) ̶ hierarchy defined only by identation convention + lowest possible syntactic noise ̶ impossible to enforce right sequence of global function invocation

Methods Chaining car() .color(Color.WHITE) .engine() .type(Engine.Type.FUEL) .power(73) .cylinder(4) .engine() .type(Engine.Type.ELECTRIC) .power(60) .transmission() .type(Transmission.Type.CONTINOUSLY_VARIABLE) .end(); + method names act as keyword argument ̶ hierarchy defined only by identation convention + object scoping + works good with optional parameter

Nested Functions car( color(Color.WHITE), transmission( type(Transmission.Type.CONTINOUSLY_VARIABLE), ), engine( type(Engine.Type.FUEL), power(73), cylinder(4) ), engine( type(Engine.Type.ELECTRIC), power(60) ) ); + no need for context variables ̶ higher punctuation noise + hierarchic structure is echoed by function nesting ̶ inverted evaluation order ̶ arguments defined by position rather than name ̶ rigid list of arguments or need of methods overloading

Is my DSL good (concise, expressive, readable) enough? You cooked the meal … … now eat it! DSL design is an iterative process

Mixed Strategy car( color(Color.WHITE), transmission() .type(Transmission.Type.CONTINOUSLY_VARIABLE), engine() .type(Engine.Type.FUEL) .power(73) .cylinder(4), engine() .type(Engine.Type.ELECTRIC) .power(60) ); car( ... ); function sequence for top level lists method chaining for arguments definition nested function for top level object creation

Advanced DSL examples

Hibernate Criteria Queries List cats = session .createCriteria(Cat.class) .add( Restrictions.like("name", "Fritz%") ) .add( Restrictions.between("weight", min, max) ) .addOrder( Order.desc("age") ) .setMaxResults(50) .list();

jMock Turtle turtle = context.mock(Turtle.class); // The turtle will be told to turn 45 degrees once only oneOf(turtle).turn(45); // The turtle can be told to flash its LEDs any number // of times or not at all allowing(turtle).flashLEDs(); // The turtle can be asked about its pen any number of times // and will always return PEN_DOWN allowing(turtle).queryPen(); will(returnValue(PEN_DOWN)); // The turtle will be told to stop at least once. atLeast(1).of(turtle).stop();

lambdaj Plain Java version: List automaticCars = new ArrayList(); for (Car car : cars) { if (car.getTransmission().getType() == Transmission.TYPE.AUTOMATIC) automaticCars.add(car); } lambdaj version: List salesOfAFerrari = select(cars, having( on(Car.class).getTransmission().getType(), equalTo(Transmission.TYPE.AUTOMATIC) ));

Mario Fusco Red Hat – Senior Software Engineer mario.fusco@gmail.com twitter: @mariofusco Q A