Practical Domain-Specific Languages with Groovy

Guillaume Laforge

Groovy Project Manager

SpringSource

glaforge@gmail.com

jeudi 2 juillet 2009

• Groovy Project Manager

• JSR-241 Spec Lead

• Head of Groovy Developmentat 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, GR8Conf, DSL DevCon and more...

Guillaume Laforge

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jeudi 2 juillet 2009

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...

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• The context and 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

Agenda

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The contextjeudi 2 juillet 2009

Subject Matter Experts,Business analysts...

jeudi 2 juillet 2009

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

Developer producing LOLCODE

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Lots of languages...

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

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Expressing requirements...

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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

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Towards more readability (1)

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Towards more readability (1)

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Towards more readability (1)

20%

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Towards more readability (2)

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Towards more readability (2)

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Towards more readability (2)

80%

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• The context and 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

Agenda

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• In our everyday life, we’re surrounded by DSLs

– Technical dialects

– Notations

– Business languages

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A collection of DSLs

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Technical dialects

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SQL

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^[\w-\.]+@([\w-]){2,4}$

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Notations

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1. e4 e52. Nf3 Nc63. Bb5 a6

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L2 U F-1 B L2 F B -1 U L2 jeudi 2 juillet 2009

Visual!jeudi 2 juillet 2009

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Business languages

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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...

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• The context and 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

Agenda

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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

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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”

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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!

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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

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Optional parens & semis

• Make statements and expressions look more like natural languages

– move(left);

– move left

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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)

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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)

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• 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)

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Custom control structuresThanks to closures

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Operator overloading

• Currency amounts–15.euros + 10.dollars

• Distance handling–10.kilometers - 10.meters

• Workflow, concurrency–taskA | taskB & taskC

• Credit an account–account << 10.dollarsaccount += 10.dollarsaccount.credit 10.dollars

a + b a.plus(b)

a - b a.minus(b)

a * b a.multiply(b)

a / b a.divide(b)

a % b a.modulo(b)

a ** b a.power(b)

a | b a.or(b)

a & b a.and(b)

a ^ b a.xor(b)

a[b] a.getAt(b)

a << b a.leftShift(b)

a >> b a.rightShift(b)

+a a.positive()

-a a.negative()

~a a.bitwiseNegate()

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Groovy’s dynamic heart:

The MOP!MetaObject Protocol

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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

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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

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Adding properties to numbers with an ExpandoMetaClass

• Number.metaClass.getMeters = {-> new Distance(delegate, Unit.METERS) }

100.meters

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The Builder pattern

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The Groovy MarkupBuilder

•def mkp = new MarkupBuilder()mkp.html { head { title “Groovy in Action” } body { div(width: ‘100’) { p(class: ‘para) { span “Best book ever!” } } }}

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A builder for HR

• softskills { ideas { capture 2 formulate 3 } ...}knowhow { languages { java 4 groovy 5 } ...}

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A builder for HR

• softskills { ideas { capture 2 formulate 3 } ...}knowhow { languages { java 4 groovy 5 } ...}

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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...

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Compile-time metaprogramming

• Groovy 1.6 introduced AST Transformations

• Compile-time == No runtime performance penalty!

Transformation

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AST Transformations

• Two kinds of transformations

– Global transformations

• applicable to all compilation units

– Local transformations

• applicable to marked program elements

• using specific marker annotations

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Example #1: @Singleton

• Let’s revisit this evil (anti-)pattern! public class Evil { public static final Evil instance = new Evil (); private Evil () {} Evil getInstance() { return instance; } }

• In Groovy! @Singleton class Evil {}

• Also a “lazy” version! @Singleton(lazy = true) class Evil {}

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• You can delegate to fields of your classes

–class Employee { def doTheWork() { “done” }}class Manager { @Delegate Employee slave = new Employee()}def god = new Manager()assert god.doTheWork() == “done”

• Damn manager who will get all the praise...

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Example #2: @DelegateNot just for managers!

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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

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Local transformations

• Same approach as Global 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( ["fqn.MyTransformation"])public @interface WithLogging {...}

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• The context and 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

Agenda

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• 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

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Various integration mechanisms

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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”);

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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="events" script-source="classpath:dsl/eventsChart.groovy" customizer-ref="eventsMetaClass" />

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Groovy’s own mechanisms

• Eval

– for evaluating simple expressions

• GroovyShell

– for more complex scripts and DSLs

• GroovyClassLoader

– the most powerful mechanism

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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’)

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• 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 = 22.3binding.velocity = 10.6def shell = new GroovyShell(binding)shell.evaluate(“mass * velocity ** 2 / 2”)

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GroovyShell

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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);

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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

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• The context and 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

Agenda

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Start small, with key concepts

Beware overengineering!

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Grow your language progressively

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Get your hands dirty

Play with the end-usersjeudi 2 juillet 2009

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

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Tight feedback loop

Iterative process

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Stay humble.

You can’t get it right the first time.

Don’t design alone at your deskInvolve the end users from the start

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Playing it safein a sandbox

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Various levels of sandboxing

• Groovy supports the usual Java Security Managers

• Use metaprogramming tricks to prevent calling / instantiating 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

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Test, test, test!

• Don’t just test for nominal cases– Explicitly test for errors!

• Ensure end-users get meaningful error messages

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• Summary

• Questions & Answers

Agenda

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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

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

?jeudi 2 juillet 2009

Appendix

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• http://www.flickr.com/photos/wheatfields/420088151/sizes/l/

• http://www.flickr.com/photos/therefromhere/518053737/sizes/l/

• http://www.flickr.com/photos/romainguy/230416692/sizes/l/

• http://www.flickr.com/photos/addictive_picasso/2874279971/sizes/l/

• http://www.flickr.com/photos/huangjiahui/3127634297/sizes/l/

• http://www.flickr.com/photos/25831000@N08/3064515804/sizes/o/

• http://www.flickr.com/photos/lanier67/3147696168/sizes/l/

• http://www.flickr.com/photos/ktb/4916063/sizes/o/

• http://www.flickr.com/photos/nathonline/918128338/sizes/l/

• http://www.flickr.com/photos/kevinsteele/39300193/sizes/l/

• http://commons.wikimedia.org/wiki/File:Brueghel-tower-of-babel.jpg

• http://commons.wikimedia.org/wiki/File:Platypus.jpg

• http://www.flickr.com/photos/joaomoura/2317171808/sizes/l/

• http://www.flickr.com/photos/wiccked/132687067/

• http://www.flickr.com/photos/xcbiker/386876546/sizes/l/

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• http://www.flickr.com/photos/pietel/152403711/sizes/o/

• 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/

• http://www.flickr.com/photos/timsamoff/252370986/sizes/l/

• http://www.flickr.com/photos/29738009@N08/2975466425/sizes/l/

• http://www.flickr.com/photos/howie_berlin/180121635/sizes/o/

• http://www.flickr.com/photos/yogi/1281980605/sizes/l/

• http://www.flickr.com/photos/dorseygraphics/1336468896/sizes/l/

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