Building DSLs with Groovy

Building DSLs with Groovy:
A Real World Case Study
Sten Anderson

Agenda
What is a DSL?
Groovy’s DSL-friendly language features
Case study: City of Chicago

DSL: Domain Specific Language
Small, Specific “Mini-language” geared toward a Domain
Favor conciseness and readability in a narrow domain over Turing
Completeness
External
• Involve Lexer/Parser
• Ant, SQL
Internal (Embedded DSLs/Fluent Interfaces)
• Defined inside a more general purpose language
• Gant/Rake
DSL vs. Framework vs. API?
• What is OpenGL?

Why use Groovy to Write DSLs?
Groovy is a natural extension for Java Developers
• IDE support for “free”
At a minimum Groovy is “less verbose Java”
At a maximum Groovy lets you change the language itself
• dynamic methods and properties and operator overloading
Groovy has built-in DSLs (Builders)

Groovy’s DSL-Friendly Features
Reduced Syntax
Closures
Metaprogramming
• Dynamic methods and properties
• ExpandoMetaclass

Language Feature Example: Text Adventure Interpreter

class Game {
void go(dir) {
println "You go $dir"
}
void look() {
println "You look around and see nothing"
}
void take(it) {
println "You take the $it"
}
...
}

…And Make it Groovy…
new GroovyShell().evaluate(“game.${playerInput}”)
go(“north”);
look();
take(“dagger”);

Reduced Syntax
go “north”
look()
take “dagger”

Metaprogramming: Dynamic Properties
class Game {
...
def propertyMissing(String name) {
name
}
go north
look()
take dagger

Metaprogramming: Dynamic Properties
def propertyMissing(String name) {
if (metaClass.respondsTo(this, name)) {
this."$name"()
}
name
}
go north
look
take dagger

Remember This?
new GroovyShell().evaluate(“game.${playerInput}”)
Not very “Groovy”

Closures and “with”
new GroovyShell().evaluate(“game.with{${playerInput}}”)
3.times {
go north
}
look
take dagger

One more thing... (the Synonym Problem)
look
> You see a dagger on the ground
get dagger
> I don’t understand that
grab dagger
Just let me have the %$!#@ dagger!

Metaprogramming: Dynamic Methods
def methodMissing(String name, args) {
if (["grab", “hold", "yoink"].contains(name)) {
this.take(args[0])
}
}
look
> You see a dagger on the ground
grab dagger
> You take the dagger
You’re darn right I do!

def methodMissing(String name, args) {
def funcName = ThesaurusLookup.contains(name)
if (funcName) {
Game.metaClass."$name" = { item ->
this.”$funcName”(item)
}
this.”$name”(args[0])
}
}
Metaprogramming: ExpandoMetaclass

How Far We’ve Come
go north
look
take dagger
grab fish
game.go(“north”);
game.look();
game.take(“dagger”);
game.take(“fish”);
From this… To this…

Case Study: Budget Books for Fun and (non) Profit

The Challenge
Replace Mainframe “Budget Book Generator” application
Did not interact with the live budget data
Over 3 hours to produce a book
Very manual process (print the book, rescan as a PDF of images)
Somewhat dated look

The Solution
Java SE Swing Application deployed locally via Webstart
Time to produce final PDF reduced from three hours to a few minutes
Operated Directly with the Budget Data
PDF as native output: searchable, indexable

BookSection
PageElement
Book Object Model

BookSection
PageElement
TextElement TableHeaderElement TableRowElement …

TextElement
TableHeader
Separator
TableSection
TableRow
TableTotal

How do we go from Budget Objects to Book Objects?
•Departments
•Organizations
•Appropriations
•Positions
•BookSections
•PageElements
Can we write a DSL to create a book?

Building Drafts with ContentBuilder
interface ContentBuilder
- Draft build(ContentData)

ContentBuilder: First Attempt
Java

BookSection bookSection = new BookSection ("Positions");
TableHeaderElement header = new TableHeaderElement();
header.addHeader(new TableHeader(0.05));
header.addHeader(new TableHeader(0.3, "Position",
Formatter.LeftAligned));
bookSection.addPageElement(header);
for (TableSection section : tableSections) {
TableSectionElement sectionElement = new TableSectionElement();
sectionElement.addColumn(new TableColumn(2, section.getName());
bookSection.addPageElement(sectionElement);
...

Remember to fill out your Objects in Triplicate
1 2 3

header.addHeader(new TableHeader(0.3, "Position",
Formatter.LeftAligned));
bookSection.addPageElement(header);
Declared here
Added here

header.addHeader(
new TableHeader(0.3, "Position", Formatter.LeftAligned));
•What do these arguments mean?
•What is “0.05”?
•What is “Position”?

ContentBuilder: Second Attempt
Java with method chaining

Method Chaining: Return the “Current” Object
public void setWidth(double width) { this.width = width;}
public TableHeader setWidth(double width) {
this.width = width;
return this;
}
•Instead of:
•We have:

bookSection.addPageElement(new TableHeader().
addHeader(new TableHeader().setWidth(0.05)).
addHeader(new TableHeader().
setWidth(0.3).
setName("Position").
setFormatter(Formatter.LeftAligned)));
for (TableSection section : tableSections) {
bookSection.addPageElement(new TableSectionElement().
addColumn(new TableColumn().
setColumnSpan(2).
setName(section.getName()));
...
}

Let’s Take a Step Back…
Both Java versions “work”, but are verbose and not overly readable
Need better support for building complex object graphs at the
language level
• The structure of the code should mirror the structure of the objects
(declarative syntax)
• Ceremony vs. Essence
We need a “Builder”

Well, Groovy has Builders
MarkupBuilder
• Tag-based markup (XML, HTML)
SwingBuilder
• GUIs
GraphicsBuilder
• Java2D
NodeBuilder
• General Object graph (Nodes)
MetaBuilder
• Builder of Builders

MarkupBuilder
def htmlBuilder = new MarkupBuilder()
htmlBuilder.html {
body {
h1 "Hi There!"
}
}
<html>
<body>
<h1>Hi There!</h1>
</body>
</html>

SwingBuilder
def builder = new SwingBuilder()
builder.frame (title:"Swing Builder Test",
size:[300,100],
location:[500,200]) {
flowLayout()
button(text:"Press Me")
}.show()

groovy.util.BuilderSupport
void setParent(Object parent, Object child);
Object createNode(Object name);
Object createNode(Object name, Object value);
Object createNode(Object name, Map attributes);
Object createNode(Object name, Map attributes, Object value);

createNode(Object name, Map attributes, Object value)
h1 "Hi There!"
button text:"Press Me“, visible:true

Reduced Syntax
foo [a1:“val1”, b2:true] foo a1:“val1”, b2:true
foo (a1:“val1”, b2:true, {bar()})
foo (a1:“val1”, b2:true) {bar()}

Instead of this:
frame ([title:"Swing Builder Test"], {
flowLayout()
})
frame (title:"Swing Builder Test") {
flowLayout()
}
You have this:

Better Still: FactoryBuilderSupport
Used by SwingBuilder
Register “Factories” that know how to create your objects
Convenience AbstractFactory requires only one method:
• Object newInstance ( FactoryBuilderSupport builder,
Object name,
Object value,
Map attributes )

A Look Inside SwingBuilder
public SwingBuilder() {
...
registerFactory("button", new RichActionWidgetFactory(JButton));
...
}

BookSectionBuilder (extends FactoryBuilderSupport)
public BookSectionBuilder() {
registerFactory("bookSection", new BookSectionFactory());
registerFactory("textElement",
new PageElementFactory(TextElement.class));
...
}

BookSectionFactory (extends AbstractFactory)
public Object newInstance(FactoryBuilderSupport builder, Object name,
Object value, Map attributes) {
return new BookSection(attributes.get(“name”).toString());
}
public void setParent( FactoryBuilderSupport builder, Object parent,
Object child ) {
((BookSection) parent).newBookSection((BookSection) child);
}

PageElementFactory (extends AbstractFactory)
private Class<? extends PageElement> clazz;
public Object newInstance(FactoryBuilderSupport builder, Object name,
Object value, Map attributes) {
return clazz.newInstance();
}
public void setParent( FactoryBuilderSupport builder, Object parent,
Object child ) {
((BookSection) parent).newPageElement((PageElement) child);
}

Groovy Builder (Final Version)
bookSection (name:"Positions") {
tableHeaderElement(styleName:”header.large”, preTablePadding:10) {
tableHeader width:0.05
tableHeader name:”Position”, width:0.3,formatter:LeftAligned
}
tableSections.each { section ->
tableSectionElement (styleName:"table.subheader.1") {
tableColumn columnSpan:2, section.name
}
...

...
positions.each { position ->
tableRowElement (styleName:"table.row") {
tableColumn position.accountId
tableColumn position.title
}
}
tableTotalElement(styleName:"table.total") {
tableColumn columnSpan:2, "Position Total"
}
} // table section
tableFooterElement()
} // book section

for (Position position : positions) {
TableRowElement row = new TableRowElement();
row.setStyleName(“table.row”);
row.addColumn(new TableColumn(position.getAccountId()));
row.addColumn(new TableColumn(position.getTitle()));
bookSection.newPageElement(row);
}
positions.each { position ->
tableRowElement (styleName:"table.row") {
tableColumn position.accountId
tableColumn position.title
}
}
Before:
After:

Free Things “Built-In” to the Builder
Wiring of map attributes to object properties
Preservation of Object hierarchy
Automatic management of closure delegates so that the “Builder” is
always in scope

Closures: What’s a Delegate?
def htmlBuilder = new MarkupBuilder()
htmlBuilder.html {
body {
h1 "Hi There!"
}
}
“html”, “body”, and “h1” don’t exist in scope, but rather are handled
by the Builder
htmlBuilder needs to be set as the closure’s Delegate

…But the Methods Don’t Exist in the Builder Either
def xmlBuilder = new MarkupBuilder()
xmoBuilder.invoices {
invoice {
lineitem ...
}
}
The Builder “pretends” to have these methods
invokeMethod is overridden to write out the markup or (in the case of
SwingBuilder/BookSectionBuilder) dispatch the method to the correct
Factory
(Metaprogramming: Dynamic Methods)

First Write a Factory
private static class ChartFactory extends AbstractFactory {
public Object newInstance(...) {
return new Chart(value.toString());
}
public void setParent(...) {
TableCell cell = (TableCell) parent;
cell.addContent(child.toString());
}
}

Then Register the Factory…
public BookSectionBuilder() {
registerFactory("bookSection", new BookSectionFactory());
registerFactory("textElement",
new PageElementFactory(TextElement.class));
...
registerFactory("chart", new ChartFactory());
}

…And It’s Ready to use in the Builder
tableRowElement {
tableColumn (columnSpan:2) {
chart dept.fundPercent
}
tableColumn {
chart allFundTotal
}
}

Is a DSL Right for Me?
Are the people who maintain your project different than those who
initially developed it?
DSLs give the “gift of documentation”
Is your project part of a domain?
Using Dynamic Languages like Groovy make DSLs more Painless

Resources
Writing an Adventure game DSL in LISP: http://www.lisperati.com/casting.html
http://groovy.codehaus.org/FactoryBuilderSupport
http://docs.codehaus.org/display/GROOVY/MetaBuilder
http://docs.codehaus.org/display/GROOVY/Writing+Domain-Specific+Languages
http://www.agiledeveloper.com/blog/PermaLink.aspx?guid=ce021e6d-b0c2-4d83-8b69-
c163c290983d
http://blogs.citytechinc.com/sanderson/
King’s Quest and Zork images taken from Wikipedia.

“Groovy is like Gravy, except that it has the „Groove‟ in it.”
-Meredith Anderson (5 Year Old)

Building DSLs with Groovy

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