In pursuit of expressivity: Groovy and Scala compared

In pursuit of expressivity:
Groovy and Scala compared

Chris Richardson
Author of POJOs in Action
Founder of CloudFoundry.com
chris@chrisrichardson.net
@crichardson

Overall presentation goal

Examine the good,
the bad and the ugly
aspects of
developing with
Groovy and Scala

4/3/11 Copyright (c) 2011 Chris Richardson. All rights reserved.
Slide 2

About Chris
•  Grew up in England and live in Oakland, CA
•  Over 25+ years of software development
experience including 14 years of Java
•  Speaker at JavaOne, SpringOne, NFJS,
JavaPolis, Spring Experience, etc.
•  Organize the Oakland JUG (
http://bit.ly/ebjava) and the Groovy Grails
meetup (http://bit.ly/oakgrails)

http://www.theregister.co.uk/2009/08/19/springsource_cloud_foundry/

Slide 3

Agenda
o  Evolution of programming
languages
o  Favorite Groovy features
o  The frustration of using Groovy
o  Scala: expressiveness and compile-
time checking
o  The trouble with Scala

Slide 4

Lots of programming languages
This site lists 8512 languages, complete with 17837 bibliographic records
featuring 11064 extracts from those references. It is in effect a family
tree of languages with 5445 links, making it not only the biggest
programming language family tree around, but also one of the largest
idea-genealogical projects undertaken.

http://hopl.murdoch.edu.au/

Slide 5

The Elite v. The Masses

The Elite Mainstream

o  Lisp o  Thought Lisp and
n  Invented in the 1960s Smalltalk were:
n  Dynamic functional n  Weird (Too different
language from Algol)
n  Dynamic object-oriented n  Too resource hungry
language (mid-1980s) (probably were for the
o  Smalltalk time)
n  Invented in the 1970s o  So the mainstream used:
n  Dynamic object-oriented
Pascal, C, C++, ….
language o  Not so safe, manual
o  Safe, garbage collection, memory management,
rich, … primitive, …

Slide 6

LISP/CLOS – an early (late 1980s)
dynamic language
(defclass Account ()
((account-id :accessor account-id :initarg :account-id)
(balance :accessor account-balance :initarg :balance)) Develop by adding and
) changing program
elements in a running VM
(defmethod debit ((Account account) amount)
(decf (account-balance account) amount))

(defmethod credit ((Account account) amount)
(incf (account-balance account) amount))

CL-USER 5 > (setq a (make-instance 'account :account-id "abc123" :balance 10.0))
#<ACCOUNT 200C05AF>

CL-USER 6 > (describe a)

#<ACCOUNT 200C05AF> is an ACCOUNT
ACCOUNT-ID "abc123"
BALANCE 10.0

CL-USER 7 > (debit a 5)
5.0


#<ACCOUNT 200C05AF> is an ACCOUNT
ACCOUNT-ID "abc123"
BALANCE 5.0

7

… a very dynamic language

(defclass Account ()
((account-id :accessor account-id :initarg :account-id) Existing instances
(balance :accessor account-balance :initarg :balance) are updated when
(description :accessor account-description :initarg :description)) the class is redefined
)


#<ACCOUNT 2168DCBF> is an ACCOUNT
ACCOUNT-ID "abc123"
BALANCE 10.0
DESCRIPTION #<unbound slot>

CL-USER 10 > (setf (account-description a) "checking account")
"checking account"


#<ACCOUNT 2168DCBF> is an ACCOUNT
ACCOUNT-ID "abc123"
BALANCE 10.0
DESCRIPTION "checking account"

8

Along came Java
o  Provided the masses with:
n  Garbage collection
n  Safety, reliability and security

o  And 10+ years passed…

Slide 9

Ruby on Rails woke up the Java
community

o  Ruby on Rails:
n  Significantly more productive
n  Simplicity of Convention over
Configuration
o  Motivated the Java community to
improve: frameworks such as Spring
improved considerably
o  And…

Slide 10

…Highlighted Problems with Java
o  Tedious collection processing
o  Painful object construction code
n  No literals for collections
n  Need to use Builders for creating hierarchies
o  Tedious XML processing
o  Types:
n  Verbose declarations
n  Generics are complicated
o  Fixed set of control constructs
o  Limited support for DSLs
=>
o  Reopened the debate about programming
languages
o  Dynamic languages became interesting again

Slide 11

Dynamic languages on the JVM
o  JRuby
n  http://jruby.codehaus.org
n  Ruby on the JVM
o  Jython
n  http://www.jython.org/Project
n  Python on the JVM
o  Clojure
n  http://clojure.org/
n  Functional programming language
n  Software Transactional Memory
n  …
o  Groovy
n  http://groovy.codehaus.org/
n  Java-compatible, dynamic language

Slide 12

Small but growing

Slide 13

Static languages on the JVM
o  (Java)
o  Fortress
o  Gosu
o  Scala
o  …

http://www.is-research.de/info/vmlanguages/?s=static

Slide 14

Scala and Gosu adoption – Smaller
but growing

Slide 15

Agenda
o  Evolution of programming languages
time checking

Slide 16

About Groovy
o  Object-oriented, dynamic language
o  Made popular by the Grails
framework:
n  Rails-like productivity
n  Using robust Java frameworks including
Spring and Hibernate

Slide 17

The good news
o  Easy to learn
n  Designed to be Java compatible
n  Interactive Shell
o  Great support for DSLs
o  Closures:
n  Easy collection processing
n  New control constructs
o  Property/method missing mechanism
o  Good XML handling
o  Meta-classes for dynamically defining
new program elements => foundation
for Grails
Slide 18

Intentionally compatible with Java
o  Groovy objects are Java objects
o  Groovy collections are Java collections
o  Groovy can call Java
n  e.g. leverage all those Java libraries
o  Java can call Groovy, e.g. Groovy
objects can be:
n  Spring beans
n  Hibernate objects
o  Easy to write a module of a project
Groovy => Easy to incrementally adopt
Slide 19

Groovy can look like Java
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

class EC2RequestExecutor { Java-like syntax

Log logger = LogFactory.getLog(getClass())

public String calculateRFC2104HMAC(String data, String key) {
try {
SecretKeySpec signingKey = new SecretKeySpec(key.getBytes("UTF8"),
HMAC_SHA1_ALGORITHM)
Mac mac = Mac.getInstance(HMAC_SHA1_ALGORITHM)
mac.init(signingKey)
byte[] rawHmac = mac.doFinal(data.getBytes())
return new String(Base64.encodeBase64(rawHmac))
}
catch (Exception e) {
throw new RuntimeException("Failed to generate HMAC : ", e)
}
}

Slide 20

Groovy is concise and expressive
def configureAsMaster() {
writeFile fileName: "/etc/my.cnf", templateName: "/templates/master.my.cnf"

restartService "mysqld"

exec command: "mysql -u root",
templateName: "/templates/createSchema.sql", No parens
templateArgs: [schemaSpec: schemaSpec] Keyword parameters

executeSchemaScripts()
}

class TomcatServer { tomcatServer.contexts
No get…()
def getContexts() {
webApps.context
}
}

Slide 21

An interactive shell

Great for experimentation and learning

Slide 22

Literals for lists and maps

def myList = ["a", "b", "c"]

def params = ['Action': 'RunInstances',
'MinCount': n.toString(), 'MaxCount': n.toString(),
' ImageId': awsProperties."imageId.${instanceType}",
'KeyName': awsProperties.keyName,
' InstanceType': instanceType] + extraParams

def response = requestor.executeRequest(params)

Slide 23

GStrings
o  Templating mechanism
o  Embedded Groovy expressions
o  Multi-line strings

def schemaScript = """
DROP SCHEMA IF EXISTS ${schemaSpec.name};
CREATE SCHEMA ${schemaSpec.name};
"""

Slide 24

Closures – collection processing

public EC2Server findInstance(String instanceId) {
def server = servers.find {instanceId == it.instanceId}
if (server)
return server
else throw new RuntimeException(….)
}

assertEquals(["aa", "bb", "cc"],
["a", "b", "c"].collect { elem -> elem + elem }

Simplified collection processing
No ugly anonymous inner classes

Slide 25

Closures – new control structures
public class Ssh {

def withTunnel(String publicDnsName, int localPort, int remotePort, Closure closure) {
SshConnection sshConnection = makeSshConnection(publicDnsName);
try {
sshConnection.connect()
…
return closure.call()
} finally {
sshConnection.close()
}
}
}

class EC2 {

String snapshotEbsVolume(String hostName, EbsVolume ebsVolume, String schemaName) {
int localPort = PortUtil.allocatePort()
def result = ssh.withTunnel(hostName, localPort, 3306) {
…
}
return (String)result
}

}

Slide 26

Closures details
groovy:000> x = { println z }
===> groovysh_evaluate
$_run_closure1@658ba380
groovy:000> x.delegate = [z: 99]
===> Foo@564a5320
groovy:000> x.call()
Used to resolve
99 unbound methods and
properties
===> null
For example, Builders
set the delegate

Slide 27

Simple reusable code
interface Quackable {
void quack();
Java:
} complexity of
class QuackableContainer<T extends Quackable>
generics

void add(T element) {
element.quack();
…
}
…
}

class QuackableContainer { Groovy: just assume that
void add(element) { there is a quack() method
element.quack()
…
}

Slide 28

Method Missing
public class ClassWithMissingMethods {

def methodMissing(String name, args) {
Closure method = makeMethodImplementation(name)
if (method) {
method(args)
} else {
throw new MissingMethodException(name, getClass(), args)
}
}

def makeMethodImplementation(String name) {
….
}

}

def foo = new ClassWithMissingMethods()
foo.nonExistentMethod()

Slide 29

Groovy metaclasses
public class StringMetaclassExampleTest extends GroovyTestCase {

void testAddMethodToString() {
String.metaClass.doubleString = { -> delegate + delegate }
String.metaClass.static.makePowerString = { String s, int n ->
def result = s
n.times { result = result.doubleString() }
result
}
assertEquals "JavaZoneJavaZone", "JavaZone".doubleString()
assertEquals "OsloOsloOsloOslo", String.makePowerString("Oslo", 2)
}
}

Runtime definition of program elements

The foundation of many powerful features

Slide 30

XML and GPath expressions
<RunInstancesResponse>
…
<instancesSet>
<item>
<instanceId>i-4ef21327</instanceId>
<imageId>ami-3795705e</imageId>
<instanceState>
<code>0</code>
def client = new HttpClient() <name>pending</name>
… </instanceState>
def responseStream = <dnsName/>
…
getMethod.getResponseBodyAsStream()
</RunInstancesResponse>
def parser = new XmlParser(false, false)
def response = parser.parseText(responseStream)

def newServers = response.instancesSet.item.collect {
new EC2Server(this, awsProperties, ssh,
it.instanceId.text(),
it.instanceState.name.text())
}

Slide 31

Metaclasses enhance JDK classes
groovy:000> new File("/Users/cer/.bashrc").text
===> export PATH=/Users/cer/tools/apache-maven-3.0/bin:
$PATH

groovy:000> l = [1,2,3]
===> [1, 2, 3]
groovy:000> l.getClass()
===> class java.util.ArrayList
groovy:000> l.getClass().metaClass.foo = { delegate.size }
===> groovysh_evaluate$_run_closure1@13a66c87
groovy:000> l.foo()
===> 3

Slide 32

Builders
def report(String path, hosts, cpuCount, threadCount) {
def builder = new groovy.xml.MarkupBuilder(new OutputStreamWriter(new FileOutputStream(path)))
builder.performanceReport {
cpus cpuCount
threads threadCount
hosts.entrySet().each {hostName, metric ->
host {
name hostName <performanceReport>
cpuUtil metric.getAverageBusy() <cpus>1</cpus>
} <threads>10</threads>
}
<host>
requests {
timing.sort{ a, b-> a.key <=> b.key}.each{ requestName, metric-> <name>database</name>
request { <cpuUtil>3.27</cpuUtil>
name requestName </host>
art metric.average() <host>
errors metric.errorPercentage()
<name>tomcat0</name>
}
} <cpuUtil>94.32</cpuUtil>
} </host>
def durationValue = (endTime – startTime)/1000.0 …
duration durationValue <duration>557.943</duration>
def tpsValue = transactionCount/ durationValue
<tps>10.753786677133686</tps>
tps tpsValue
art averageResponseTime() <art>916.6578333333</art>
} </performanceReport>
}

4/3/11 Copyright (c) 2011 Chris Richardson. All rights reserved. Slide 33

Grails plugins use metaclasses
o  Key part of the Grails plugins
o  Plugins can dynamically add methods,
e.g. GORM persistence methods
o  Injected methods are closures that
have access to the Spring application
context
n  Less need for traditional dependency
injection
n  Alternative to AOP static crosscutting or
(non-POJO) inheritance

Slide 34

Grails/GORM persistence methods
class Customer { Implemented using
String name metaclasses and method
} missing

Customer c = new Customer("John Doe")

if (!c.save())
fail "validation failed: ${c.errors}"

Customer c2 = Customer.get(c.id)

c2.delete()

assertNull Customer.get(c.id)

def customers = Customer.findAllByName(“Fred”)

Slide 35

Commentable plugin example
class CommentableGrailsPlugin {

def doWithDynamicMethods = { ctx ->

for(domainClass in application.domainClasses) {
if(Commentable.class.isAssignableFrom(domainClass.clazz)) {

domainClass.clazz.metaClass {
'static' {
getRecentComments {-> … }
}

addComment { poster, String text -> }

getComments = {-> }
…
}

Slide 36

Grails DSL for ORM
class Customer {

static transients = ["networth"]

static mapping = {
id column: ‘customer_id’ Closures
table ‘crc_customer’
columns {

}
name column: ‘customer_name’ Method
} missing
def getNetworth() { … }
}
...

Slide 37

Grails Controllers -
o  Accessing request/response
o  Databinding
n  b.properties = params
n  new B(params)
o  …

Slide 38

Agenda
time checking

Slide 39

Getting complex Groovy code to
work can be frustrating

o  Dynamic language = less information
for IDE:
n  Limited compile-time checking
n  Limited refactorings
n  Limited completion
n  No todo list of errors

Slide 40

Groovy fans say "write unit tests“
BUT…

Groovy

When you have typos
versus

Java

Slide 41

Unit tests don't always catch errors
void testPollStatus_discoveringNewServer() {
mockEC2RequestExecutor.demand.executeRequest {params ->
….
}
def mockEC2Server = new MockFor(EC2Server.class)
….
mockEC2Server.use {
mockEC2RequestExecutor.use {
ec2 = new EC2(awsProperties)
ec2.pollStatus() public class EC2 {
assertEquals 1, ec2.servers.size()
} public pollStatus() {
} def params = ['Action':
} 'DescribeInstances']
def p =
requestor.executeRequest(params)
class EC2RequestExecutor { …
}
public Node executeEc2Request(Map …
parameters) { } Method signature changes are
… often missed
}

Slide 42

The trouble with duck typing
o  Interface between components not
defined in a single place
o  It is scattered throughout callers
o  Difficult to understand
o  Difficult to change

Slide 43

The outcome: fear of change

Did my tests
catch all
the obvious errors?

Slide 44

Agenda
o  Scala: expressiveness and
compile-time checking

Slide 45

Distilling a language

Builders

Dynamic methods
and properties

Gpath-based
XML handling
Dynamic features

Static features

Closures

Groovy
Literals for
collections
GStrings

Slide 46

Scala – a modern (2003) static
language
o  Object-oriented
n  Pure object-oriented language
n  All values are objects “I can honestly say if someone had
n  Class-based shown me the Programming in
Scala book by by Martin Odersky,
o  Functional Lex Spoon & Bill Venners back in
n  Functions are values 2003 I'd probably have never
created Groovy.”
n  Higher-order functions
n  Currying James Strachan, Creator of Groovy
http://macstrac.blogspot.com/
o  Statically typed 2009/04/scala-as-long-term-
n  Expressive replacement-for.html

n  Type inference
o  Extensible to support domain specific languages
n  Methods as infix and postfix operators
n  Automatic closure construction

Slide 47

The good news
o  Fully interoperable with Java
o  Interactive shell
o  Rich, extensible language
o  Closures
o  Good XML processing
o  Named arguments and default
parameter values
o  Option class avoids null references
o  Traits provide “multiple inheritance”
Slide 48

Scala command line

Slide 49

Simple example
object StringFunctions {

def reverseSentence(sentence : String) = sentence.split(" ").reverse.mkString(" ")

}

import org.junit._
import Assert._

@Test
class StringFunctionsTest {

@Test
def testReverseSentence() = {
val input = "Hello New York"
val expected = "York New Hello"
assertEquals(expected, StringFunctions.reverseSentence(input))
}
}

Slide 50

Collection literals example
@Test
class ScalaCollectionExamplesTest {

@Test
def listExample() = {
val myList = List(1, 2, 3)
assertEquals(3, myList.length)
assertEquals(List(0, 1, 2, 3), 0::myList)
assertEquals(List(2, 3), myList.tail)
assertEquals(1, myList.head)
}

@Test
def testMapExampleList() : Unit = {
val myMap = Map( "x" -> 1, "y" -> 2, "z" -> 3)
assertEquals(1, myMap("x"))
}
}

Slide 51

Closure example

@Test
class ScalaCollectionExamplesTest {

@Test
def testMapList() = {
assertEquals(List(2,4,6), myList.map ( x => x * 2))
assertEquals(List(2,4,6), myList.map ( _ * 2))
}

@Test
def testFilterList() = {
assertEquals(List(1, 3), myList.filter( _ % 2 == 1));
}

}

Slide 52

Defining new control constructs 1
class ExampleControlConstructExampleTest {

def withOutputStream[T](f : File)(body : InputStream => T) = {
val fis = new FileInputStream(f)
try {
body(fis)
} finally {
fis.close
}
}

@Test
def withOpenFileExample {
withOutputStream(new File("/Users/cer/.bashrc")) {fis =>
println(fis.available)
}
}

Slide 53

Defining new control constructs 2

By-name parameter

var assertsEnabled = false

def myAssert(message : String, assertion : => Boolean) {
if (assertsEnabled) {
if (!assertion)
throw new RuntimeException(message)
}
}

@Test
def assertionsDisabled = {
myAssert("never", { println("you will not see me") ; false })
}

Slide 54

Pattern matching
case class Organization(name: String,
industry: Industry.Value, revenue: Double, assets: Double, liabilities: Double) {

}

object Industry extends Enumeration {
val Banking, GreenEnergy, IT, Other = Value

}

class LoanApprovalPolicy {

def isLoanApproved(organization : Organization) = organization match {
case Organization(_, Industry.GreenEnergy, _, _, _) => true
case Organization(_, Industry.Banking, _, assets, liabilities)
if liabilities < assets * 1.5 => true
case Organization(_, Industry.Banking, _, _, _) => false
case Organization(_, _, _, assets, liabilities) if assets > liabilities => true
case _ => false
}

}

Slide 55

XML generation
@Test
class ScalaXmlExampleTest {

@Test
def xmlLiterals() {
val now = new Date()
val loanRequestor =
new Organization("Community Bank, Inc", Industry.Banking, 10, 10, 5)

val doc = <loanRequest>
<time>{now.getTime()}</time>
<requester>{loanRequestor.name}</requester>
<industry>{loanRequestor.industry}</industry>
<revenue>{loanRequestor.revenue}</revenue>
<assets>{loanRequestor.assets}</assets>
<liabilities>{loanRequestor.liabilities}</liabilities>
</loanRequest>

val docAsString : String = doc.toString()
println(docAsString)
}

Slide 56

XML parsing
class ScalaXmlExampleTest {

@Test
def xmlProcessing() {
val doc = xml.XML.loadString(docAsString)
assertEquals ("Community Bank, Inc", (doc "requester").text)

doc match {
case <loanRequest>{children @ _* }</loanRequest> =>
for (x <- children if !x.isInstanceOf[scala.xml.Text]) {
processChildren(x)
}
}
}

def processChildren(node : scala.xml.Node ) = {
node match {
case <time>{value}</time> => println(value.text)
case <requester>{value}</requester> => println(value.text)
…
}
}

Slide 57

DSL Example
class OrganizationDslTest {
val o : Organization = (Organization
called "Bernie Madoff & Associates"
in Industry.Banking
withRevenuesOf (10 million)
withAssetsOf (10 billion)
withLiabilitiesOf (30 billion))

}
object Organization {
def called(name : String) = new OrganizationBuilder(name)

class OrganizationBuilder(name : String) {
var industry : Industry.Value = null
def in(industry : Industry.Value) = { this.industry = industry; this }
def withRevenuesOf(…) = …
…
def make()= new Organization(name, industry, revenue, assets, liabilities)
}

implicit def organizationBuilderToOrganization(builder : OrganizationBuilder)
= builder.make
}

Slide 58

Named arguments and default values

o  Arguments can be named
o  Parameters can have default values
o  Benefits:
n  Clearer code – Booleans aren’t so evil
n  Avoids confusing overloaded methods/
constructors
n  Less need for the Builder pattern

def foo(x: Int = 10, y: Int = 20) = x * y

foo() should be(200)
foo(y = 30, x = 50) should be(1500)
foo(x = 50) should be(1000)

Slide 59

Constructing object hierarchies
object ExampleSwingApp extends SimpleGUIApplication {

def top = new MainFrame {
title = "Example Swing App" Defines method declared
abstract in superclass
val button = new Button {
text = "press"
} Sets property
contents = button

var count = 0 Statements/
listenTo(button)
reactions += { expressions that
case ButtonClicked(b) => are executed
count = count + 1
Console.println(count);
b.text = "press: " + count Nesting of new Class {
}
} }
mirrors object hierarchy
}

Slide 60

Pimping existing classes
scala> 5.times { println("Hello") }
<console>:6: error: value times is not a member of Int
5.times { println("Hello") }
^
scala> class IntWithTimes(n: Int) {
| def times(body: => Unit) = for (x <- 1 to n) body
| }
defined class IntWithTimes

scala> implicit def toIntWithTimes(n: Int) = new IntWithTimes(n)
toIntWithTimes: (n: Int)IntWithTimes

scala> 5.times { println("Hello") }
Hello
Hello
Hello Implicit conversion
Hello
Hello

Slide 61

Implicit parameters 1
@Test
def implicitParamsImplicitVar {
implicit val defaultValue = 8

def m(i : Int) (implicit j : Int) = i * j

assertEquals(48, m(6))

assertEquals(42, m(6)(7))

}
Slide 62

Implicit parameters 2
scala> implicit def toInt : Int = 5
toInt: Int

scala> def m(i : Int) (implicit j : Int) = i * j
m: (i: Int)(implicit j: Int)Int

scala> m(6)
res6: Int = 30

scala> m(6)(7)
res7: Int = 42
Slide 63

Null References

My Billion Dollar
mistake

Slide 64

Use Options instead of null…
var x : Option[Int] = Some(5)
var y : Option[Int] = None
var z : Option[Int] = Some(7)

assertEquals(5, x.get)
try {
y.get
fail
} catch {
case _ : NoSuchElementException =>
}

assertEquals(5, x.getOrElse(10))
assertEquals(10, y.getOrElse(10))

Slide 65

… more options
assertEquals(Some(7), x.map(_ + 2))
assertEquals(None, y.map(_ + 2))
assertEquals(None, x.filter(_ == 2))
assertEquals(Some(5), x.filter(_ == 5))

assertEquals(Some(35),
for (a <- x ; b <- z) yield a * b)

Slide 66

Traits
o  Consist of members
n  methods
n  fields
n  types
o  The members can be abstract
o  Multiple traits can be mixed into a class
n  Alternative to multiple inheritance
n  Class must define abstract members
o  Uses:
n  Modularizing applications
n  Enriching interfaces
n  Decorating classes

Slide 67

RESTful WS using Spring MVC and
Scala
o  Handling HTTP request Photo management
n  Extract values from URL
n  Invoke appropriate handler
application
n  Generate response (XML)
o  Sounds like a job for Scala's GET /album
n  Pattern matching GET /album/<albumId>
n  XML support POST /album
…

<album>
<id>6699</id>
GET /album/6699 <photos>…</photos>
…
</album>

Slide 68

Pattern matching HTTP Requests
o  Match against
n  URL – represented as a List
n  Method
n  …
o  Returns XML
request match {
case Request(List("album"), "GET", _, _) =>
listAlbums
case Request(List("album", albumId), "GET", _, _) =>
findAlbum(albumId)
}
def findAlbum(albumId : String) = {
def album = albumService.findAlbum(albumId)
<album>
<id>{album.getId()}</id>
<photos>…</photos>
…
</album>
}

Slide 69

The Extractor

object Request {
def unapply(request : HttpServletRequest)
: Option[ (List[String], String, RequestBody, Binder)]
={
val path =
List.fromArray(request.getPathInfo().split("/")).tail
Some(path,
request.getMethod,
new RequestBody(request),
new Binder(request))
}
}

Slide 70

Package as WSEndpoints
trait WsEndpoint extends PartialFunction[HttpServletRequest, Node] {

def isDefinedAt(request : HttpServletRequest) = wsDispatchRules.isDefinedAt(request)

def apply(request : HttpServletRequest) = wsDispatchRules(request)

protected def wsDispatchRules : PartialFunction[HttpServletRequest, Node]

} @Component
class AlbumWsEndpoint extends WsEndpoint {

@Autowired
def setAlbumService(albumService : AlbumService) = …

def wsDispatchRules = {
case Request(List("album", albumId), "GET", _, _) => findAlbum(albumId)
…
}

def findAlbum(albumId : String) = {
def album = albumService.findAlbum(albumId)
<album>…</album>
}

Slide 71

Integrate with Spring MVC 1
@Component
class WsEndpointHandlerMapping extends HandlerMapping {

@Autowired
def setApplicationContext(applicationContext : ApplicationContext) {
this.applicationContext = applicationContext
}

def endpoints = …applicationContext.getBeansOfType(classOf[WsEndpoint])…

override def getHandler(request : HttpServletRequest) : HandlerExecutionChain = {
val endpointForRequest = endpoints.find( _.isDefinedAt(request))
endpointForRequest match {
case Some(endpoint) => new HandlerExecutionChain(endpoint)
case None => null
}
}

Maps HTTP request to WSEndpoint

Slide 72

Integrate with Spring MVC 2
@Component
class WsEndpointHandlerAdapter extends HandlerAdapter {

override def supports(handler : Object) = {
handler.isInstanceOf[WsEndpoint]
}

override def handle(request : HttpServletRequest ,
response : HttpServletResponse , handler : Object ) : ModelAndView = {

val endpoint = handler.asInstanceOf[WsEndpoint]

val node = endpoint(request)
response.setContentType("text/xml")
response.getWriter.print(node.toString)

null
}

Invokes WSEndpoint and generates HTTP response

Slide 73

Scala and Spring DI
o  Scala and Spring DI play together
o  @Component on Scala classes works
o  @Autowired fields:
n  Annotation ends up on field and
generated methods è fail
n  Need to define and annotate setters

Slide 74

Agenda
time checking

Slide 75

Really bad news
o  Scala IDEs are ok and slowly
improving BUT have a really really
long way to go compared to Java
o  Limited refactoring => painful to
maintain code quality?

Slide 76

More bad news…
o  Expressiveness comes from complex
language features
o  Generics are better than Java but still
complex
BUT
o  The investment in learning these
features is repaid with every
application you write

Slide 77

…more bad news
o  Lift is not Grails (or Rails)
o  A subset of the Scala community has
a functional programming background
=> Obsessed with their Monads
o  Implicit conversions make it possible
to write “incomprehensible” code
o  Scala Collections != Java Collections
n  BUT Implicit conversions simplify
interoperability

Slide 78

Summary
o  The Java Language has stagnated and
need to be replaced
o  Dynamic languages are one option
o  But many benefits of those languages
are due to good language design
o  Scala = expressiveness + static typing
o  But Grails derives much of its power
from the dynamic features of Groovy
o  Therefore, choose the right tool:
n  Web application è Use Groovy and Grails
n  Complex business logic/middleware è
Consider Scala

Slide 79

Summary – Pessimistic view
o  Languages used by mainstream
developers have improved
substantially
o  But are those languages that much
better than the 25+ year old “ivory
tower” languages: LISP, Smalltalk,
ML, … ?

Slide 80

Final thoughts
o  Learn some languages including
Haskell
o  Watch the 50 in 50 presentation by
Guy Steele and Dick Gabriel:
http://blog.jaoo.dk/2008/11/21/art-
and-code-obscure-or-beautiful-code/

"Those who cannot remember the past
are condemned to repeat it”
http://en.wikipedia.org/wiki/George_Santayana

Slide 81

The end
Twitter:

@crichardson

Send email:

chris@chrisrichardson.net

Slide 82

In pursuit of expressivity: Groovy and Scala compared

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