Practical Groovy Domain-Specific Languages

Practical Domain-Specific Languages in Groovy Guillaume Laforge Groovy Project Manager SpringSource

Guillaume Laforge Groovy Project Manager JSR-241 Spec Lead Head of Groovy Development at SpringSource Initiator of the Grails framework Co-author of Groovy in Action Speaker: JavaOne, QCon, JavaZone, Sun TechDays, Devoxx, The Spring Experience, JAX, Dynamic Language World, IJTC, and more... 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 2

A few words about Groovy Groovy is a dynamic language for the JVM • with a Meta Object Protocol • compiles directly to bytecode, seamless Java interop Open Source ASL 2 project hosted at Codehaus Relaxed grammar derived from Java 5 • + borrowed good ideas from Ruby, Python, Smalltalk Fast... for a dynlang on the JVM Closures, properties, optional typing, BigDecimal by default, nice wrapper APIs, and more... 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 3

end a The context and Ag the usual issues we face Some real-life examples of Domain- Specific Languages Groovy’s DSL capabilities Integrating a DSL in your application Considerations to remember when designing your own DSL 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 4

The context

Subject Matter Experts, Business analysts...

Developer producing LOLCODE HAI CAN HAS STDIO? I HAS A VAR IM IN YR LOOP UP VAR!!1 VISIBLE VAR IZ VAR BIGGER THAN 10? KTHXBYE IM OUTTA YR LOOP KTHXBYE

Lots of languages...

And in the end... ...nobody understands each other

Expressing requirements...

DSL: a potential solution? Use a more expressive language than a general purpose one Share a common metaphore of understanding between developers and subject matter experts Have domain experts help with the design of the business logic of an application Avoid cluttering business code with too much boilerplate technical code Cleanly separate business logic from application code Let business rules have their own lifecycle 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 11

Towards more readibility (1) 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 12

Towards more readibility (1) 20% 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 12

Towards more readibility (2) 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 13

Towards more readibility (2) 80% 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 13

A collection of DSLs In our everyday life, we’re surrounded by DSLs • Technical dialects • Notations • Business languages 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 15

Technical dialects 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 16

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Notations 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 19

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Visual!

Business languages 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 23

Real-life Groovy examples Anti-malaria drug resistance simulation Human Resources employee skills representation Insurance policies risk calculation engine Loan acceptance rules engine for a financial platform Mathematica-like lingua for nuclear safety simulations Market data feeds evolution scenarios and more... 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 24

A flexible & malleable syntax No need to write full-blown classes, use scripts Optional typing (def) • in scripts, you can even omit the def keyword Native syntax constructs Parentheses & semi-colons are optional Named arguments BigDecimal by default for decimal numbers Closures for custom control structures Operator overloading 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 26

Scripts vs classes Hide all the boilerplate technical code • an end-user doesn’t need to know about classes • public class Rule { public static void main(String[] args) { System.out.println(“Hello”); } } • println “Hello” 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 27

Optional typing No need to bother with types or even generics • unless you want to! Imagine an interest rate lookup table method returning some generified type: • Rate<LoanType, Duration, BigDecimal>[] lookupTable() { ... } def table = lookupTable() No need to repeat the horrible generics type info! 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 28

Native syntax constructs Lists • [Monday, Tuesday, Wednesday] Maps • [CA: ‘California’, TX: ‘Texas’] Ranges • def bizDays = Monday..Friday • def allowedAge = 18..65 • You can create your own custom ranges 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 29

Optional parens & semis Make statements and expressions look more like natural languages • move(left); • move left 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 30

Named arguments In Groovy you can mix named and unnamed arguments for method parameters • named params are actually put in a map parameter • plus optional parens & semis take 1.pill, of: Chloroquinine, after: 6.hours Corresponds to a method signature like: • def take(Map m, MedicineQuantity mq) 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 31

BigDecimal by default Main reason why financial institutions often decide to use Groovy for their business rules! • Although these days rounding issues are overrated! Java vs Groovy for a simple interpolation equation BigDecimal uMinusv = c.subtract(a); BigDecimal vMinusl = b.subtract(c); BigDecimal uMinusl = a.subtract(b); return e.multiply(uMinusv) .add(d.multiply(vMinusl)) .divide(uMinusl, 10, BigDecimal.ROUND_HALF_UP); (d * (b - c) + e * (c - a)) / (a - b) 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 32

Custom control structures, thanks to closures When closures are last, they can be put “out” of the parentheses surrounding parameters unless (account.balance > 100.euros, { account.debit 100.euros }) unless (account.balance > 100.euros) { account.debit 100.euros } Signature def unless(boolean b, Closure c) 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 33

Operator overloading a + b a.plus(b) Currency amounts a - b a.minus(b) • 15.euros + 10.dollars a * b a.multiply(b) a / b a.divide(b) Distance handling a % b a.modulo(b) • 10.kilometers - 10.meters a ** b a.power(b) a | b a.or(b) Workflow, concurrency a & b a.and(b) • taskA | taskB & taskC a ^ b a.xor(b) a[b] a.getAt(b) Credit an account a << b a.leftShift(b) •account << 10.dollars account += 10.dollars a >> b a.rightShift(b) account.credit 10.dollars +a a.positive() -a a.negative() ~a a.bitwiseNegate() 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 34

Groovy’s dynamic heart: The MOP! MetaObject Protocol

Groovy’s MOP All the accesses to methods, properties, constructors, operators, etc. can be intercepted thanks to the MOP While Java’s behavior is hard-wired at compile-time in the class Groovy’s runtime behavior is adaptable at runtime through the metaclass. Different hooks for changing the runtime behavior • GroovyObject, custom MetaClass implementation, categories, ExpandoMetaClass 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 36

GroovyObject All instances of classes created in Groovy implement the GroovyObject interface: • getProperty(String name) • setProperty(String name, Object value) • invokeMethod(String name, Object[] params) • getMetaClass() • setMetaClass(MetaClass mc) A GO can have “pretended” methods and properties 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 37

MetaClass The core of Groovy’s MOP system • invokeConstructor() • invokeMethod() and invokeStaticMethod() • invokeMissingMethod() • getProperty() and setProperty() • getAttribute() and setAttribute() • respondsTo() and hasProperty() MetaClasses can change the behavior of existing third- party classes — even from the JDK 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 38

ExpandoMetaClass A DSL for MetaClasses! MoneyAmount.metaClass.constructor = { ... } Number.metaClass.getDollars = { ... } Distance.metaClass.toMeters = { ... } Distance.metaClass.static.create = { ... } To avoid repetition of Type.metaClass, you can pass a closure to metaClass { ... } The delegate variable in closure represents the current instance, and it the default parameter 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 39

The Builder pattern

A builder for HR softskills { ideas { capture 2 formulate 3 } ... } knowhow { languages { java 4 groovy 5 } ... } 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 41

Builders Builders are... • a mechanism for creating any tree-structered graph • the realization of the GoF builder pattern at the syntax level in Groovy • simply a clever use of chained method invocation, closures, parentheses omission, and use of the GroovyObject methods Existing builders • XML, Object graph, Swing, Ant, JMX, and more... 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 42

The clever trick GroovyObject#invokeMethod() is used to catch all non- existing method calls in the context of the builder The nesting of closures visually shows the level of nesting / depth in the tree builder.m1(attr1:1, attr2:2, { builder.m2(..., {...}) } becomes equivalent to builder.m1(attr1:1, attr2:2) { m2(...) {...} } thanks to parens omission 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 43

Adding properties to numbers Three possible approaches • create a Category • a category is a kind of decorator for default MCs • create a custom MetaClass • a full-blown MC class to implement and to set on the POGO instance • use ExpandoMetaClass • friendlier DSL approach but with a catch 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 44

With a Category class DistanceCategory { static Distance getMeters(Integer self) { new Distance(self, Unit.METERS) } } use(DistanceCategory) { 100.meters } Interesting scope: thread-bound & lexical Have to surround with “use” • but there are ways to hide it 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 45

With an ExpandoMetaClass Number.metaClass.getMeters = {-> new Distance(delegate, Unit.METERS) } 100.meters Works for the class hierarchy for POJOs, and a flag exists to make it work for POGOs too But the catch is it’s really a global change, so beware EMC enhancements collisions 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 46

Compile-time metaprogramming Groovy 1.6 introduced AST Transformations Compile-time == No runtime performance penalty! Transformation 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 47

AST Transformations Two kinds of transformations • Global transformations • applicable to all compilation units • Local transformations • applicable to marked program elements • using specific marker annotations 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 48

Global transformations Implement ASTTransformation Annotate the transfo specifying a compilation phase @GroovyASTTransformation(phase=CompilePhase.CONVERSION) public class MyTransformation implements ASTTransformation { public void visit(ASTNode[] nodes, SourceUnit unit) { ... } } For discovery, create the file META-INF/services/ org.codehaus.groovy.transform.ASTTransformation Add the fully qualified name of the class in that file 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 49

Local transformations Same approach as Globale transformations But you don’t need the META-INF file Instead create an annotation to specify on which element the transformation should apply @Retention(RetentionPolicy.SOURCE) @Target([ElementType.METHOD]) @GroovyASTTransformationClass( [quot;fqn.MyTransformationquot;]) public @interface WithLogging {...} 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 50

Example: the Spock framework Changing the semantics of the original code But keeping a valid Groovy syntax @Speck class HelloSpock { def quot;can you figure out what I'm up to?quot;() { expect: name.size() == size where: name << [quot;Kirkquot;, quot;Spockquot;, quot;Scottyquot;] size << [4, 5, 6] } } Check out http://www.spockframework.org 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 51

Various integration mechanisms Java 6’s javax.script.* APIs (aka JSR-223) Spring’s language namespace Groovy’s own mechanisms But a key idea is to externalize those DSL programs • DSL programs can have their own lifecycle • no need to redeploy an application because of a rule change • business people won’t see the technical code 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 53

Java 6’s javax.script.* API Groovy 1.6 provides its own implementation of the javax.script.* API ScriptEngineManager mgr = new ScriptEngineManager(); ScriptEngine engine = mgr.getEngineByName(“Groovy”); String result = (String)engine.eval(“2+3”); 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 54

Spring’s lang namespace POGOs (Plain Old Groovy Objects) can be pre-compiled as any POJO and used interchangeably with POJOs in a Spring application But Groovy scripts & classes can be loaded at runtime through the <lang:groovy/> namespace and tag Reloadable on change Customizable through a custom MetaClass <lang:groovy id=quot;eventsquot; script-source=quot;classpath:dsl/ eventsChart.groovyquot; customizer-ref=quot;eventsMetaClassquot; /> 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 55

Groovy’s own mechanisms Eval • for evaluating simple expressions GroovyShell • for more complex scripts and DSLs GroovyClassLoader • the most powerful mechanism 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 56

Eval Simple mechanism to evaluate math-like formulas Eval.me ( ‘3*4’) Eval.x (1, ‘3*x + 4’) Eval.xy (1, 2, ‘x + y’) Eval.xyz(1, 2, 3, ‘x * y - z’) 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 57

GroovyShell A Binding provides a context of execution • can implement lazy evaluation if needed A base script class can be specified def binding = new Binding() binding.mass = 22.3 binding.velocity = 10.6 def shell = new GroovyShell(binding) shell.evaluate(“mass * velocity ** 2 / 2”) 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 58

GroovyClassLoader Most powerful mechanism • could also visit or change the AST • scripts & classes can be loaded from elsewhere • more control on compilation GroovyClassLoader gcl = new GroovyClassLoader(); Class clazz = gcl.parseClass( new File(“f.groovy”)); GroovyObject instance = (GroovyObject)clazz.newInstance(); instance.setMetaClass(customMC); 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 59

Externalize business rules Although Groovy DSLs can be embedded in normal Groovy classes, you should externalize them Store them elsewhere • in a database, an XML file, etc. Benefits • Business rules are not entangled in technical application code • Business rules can have their own lifecycle, without requiring application redeployments 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 60

Start small, with key concepts Beware overengineering!

Grow your language progressively

Get your hands dirty Play with the end-users

Let your DSL ﬂy, it’s not yours, it’s theirs!

Tight feedback loop Iterative process

Stay humble. You can’t get it right the ﬁrst time. Don’t design alone at your desk Involve the end users from the start

Playing it safe in a sandbox

Various levels of sandboxing Groovy supports the usual Java Security Managers Use metaprogramming tricks to prevent calling / instanciating certain classes Create a special GroovyClassLoader AST code visitor to filter only the nodes of the AST you want to keep • ArithmeticShell in Groovy’s samples 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 69

Test, test, test! Don’t just test for nominal cases • Explicitely test for errors! Ensure end-users get meaninful error messages 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 70

end a Ag Summary Questions & Answers 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 71

Summary Groovy’s a great fit for Domain-Specific Languages • Malleable & flexible syntax • Full object-orientation Metaprogramming capabilities • Runtime metaprogramming • Compile-time metaprogramming Groovy’s very often used for mission-critical DSLs 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 72

? I kan haz my cheezburgr naw? Or do ya reely haz keshtionz?

Appendix 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 74

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http://www.flickr.com/photos/forezt/192554677/sizes/o/ http://keremkosaner.files.wordpress.com/2008/04/softwaredevelopment.gif http://www.jouy.inra.fr http://www.flickr.com/photos/ejpphoto/408101818/sizes/o/ http://www.flickr.com/photos/solaro/2127576608/sizes/l/ http://www.flickr.com/photos/biggreymare/2846899405/sizes/l/ 2009 CommunityOne Conference: EAST | developers.sun.com/events/communityone 76

Practical Groovy Domain-Specific Languages

by Guillaume Laforge

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