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...

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


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

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

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


Towards more readibility (1)



20%



80%


end a
The context and Ag
Specific Languages
Integrating a DSL
in your application


A collection of DSLs

In our everyday life, we’re surrounded by DSLs

• Technical dialects
• Notations
• Business languages


Technical dialects


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


Notations


Business languages


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...


end a
The context and Ag
Specific Languages
Integrating a DSL
in your application


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


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”


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!


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


Optional parens & semis

Make statements and expressions
look more like natural languages

• move(left);

• move left


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)


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)


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)


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()

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


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


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


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


A builder for HR

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


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...


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


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


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


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


Compile-time
metaprogramming
Groovy 1.6 introduced AST Transformations
Compile-time == No runtime performance penalty!

Transformation

AST Transformations

Two kinds of transformations
• Global transformations
• applicable to all compilation units

• Local transformations
• applicable to marked program elements
• using specific marker annotations


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


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 {...}


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


end a
The context and Ag
Specific Languages
Integrating a DSL
in your application


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


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”);


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; />


Groovy’s own mechanisms

Eval
• for evaluating simple expressions

GroovyShell
• for more complex scripts and DSLs

GroovyClassLoader
• the most powerful mechanism


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’)


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”)


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);


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


end a
The context and Ag
Specific Languages
Integrating a DSL
in your application


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

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


Test, test, test!

Don’t just test for nominal cases
• Explicitely test for errors!

Ensure end-users get meaninful error messages


end a
Ag
Summary

Questions & Answers


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


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

Appendix


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


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