A First Date With Scala

A first date with Scala Franco Lombardo XP User Group - Bergamo http://www.francolombardo.net Edward Hopper - Summertime

First of all, you must know… http://www.francolombardo.net … I’m not a Guru!!!

Scala: passport
IT was born in 2001 at the Ecoles Polytechniques fédérales de Lausanne
Firs public release in 2004
Ideated by Martin Odersky, the father of Java “Generics”
The compiler generates .class file for the JVM (someone says there’s also a version for .NET  )
http://www.francolombardo.net Claudio Destito – Auto-ritratto

Scala = SCAlable LAnguage http://www.francolombardo.net In order to “scale” you need solid basis!
Widespread virtual machine
Libraries/frameworks/application servers created in Java are immediately available in Scala
Base syntax very close to Java, so it’s very appealing for a great number of programmers
The base of Scala is Java

Here is our star: Hello World!!! http://www.francolombardo.net package hello import java . util .Date object Hello extends Application { println ( "Yet another Hello World program" ) val jvmVer = System . getProperty ( "java.version" ) println ( "Running on JMV " + jvmVer ) println ( "On " + new Date ()) } Yet another Hello World program Running on JMV 1.6.0_13 On Sat May 23 17:36:34 CEST 2009 Output

Here is our star: Hello World!!! http://www.francolombardo.net package hello import java . util .Date object Hello extends Application { println ( "Yet another Hello World program" ) val jvmVer = System . getProperty ( "java.version" ) println ( "Running on JMV " + jvmVer ) println ( "On " + new Date ()) } Basic syntax very close to Java

Here is our star: Hello World!!! http://www.francolombardo.net package hello import java . util .Date object Hello extends Application { println ( "Yet another Hello World program" ) val jvmVer = System . getProperty ( "java.version" ) println ( "Running on JMV " + jvmVer ) println ( "On " + new Date ()) } Object from java.lang are imported by default

Here is our star: Hello World!!! http://www.francolombardo.net package hello import java . util .Date object Hello extends Application { println ( "Yet another Hello World program" ) val jvmVer = System . getProperty ( "java.version" ) println ( "Running on JMV " + jvmVer ) println ( "On " + new Date ()) } Object from Java libraries are easly avaible

Here is our star: Hello World!!! http://www.francolombardo.net package hello import java . util .Date object Hello extends Application { println ( "Yet another Hello World program" ) val jvmVer = System . getProperty ( "java.version" ) println ( "Running on JMV " + jvmVer ) println ( "On " + new Date ()) } Simplified syntax: semicolon is not mandatory

Here is our star: Hello World!!! http://www.francolombardo.net package hello import java . util .Date object Hello extends Application { println ( "Yet another Hello World program" ) val jvmVer = System . getProperty ( "java.version" ) println ( "Running on JMV " + jvmVer ) println ( "On " + new Date ()) } Simplified syntax: less code

Here is our star: Hello World!!! http://www.francolombardo.net package hello import java . util .Date object Hello extends Application { println ( "Yet another Hello World program" ) val jvmVer = System . getProperty ( "java.version" ) println ( "Running on JMV " + jvmVer ) println ( "On " + new Date ()) } Simplified syntax: type inference

Type inference: have you ever seen it? http://www.francolombardo.net //Java return customer.getOrder(40) .getRow(20) .getItem() .getWeight() * 2.5; (OK, in this example we do not obey Demeter law, but it’s only an example…  )

Tools for scalability: static typing http://www.francolombardo.net
The compiler gives lots of automatic tests on the code for free
Enhanced refactorings
Better average performances (OK, not always, but Scala substituted Ruby at Twitter for performances issues)
Static typing is a clear and automatic tool for code documentation

Tools for scalbility: “pure” Object Orientation http://www.francolombardo.net
Every value is an object
Every operation is a method call (so we can redefine operators easily)
2 + 5 //equals to... 2. + (5)
Syntactic tricks to improve readability:
Starting method names with letters is not mandatory
In some cases points and parenthesis are not mandatory

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net
A problem of big Object Oriented systems: objects that can become very fat
Fernando Botero – Bailerina na barra

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net //Java public class Customer { public String name() { //some implementation… } public String address() { //some implementation }
Fat objects: the Customer

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net //Java public class Customer { //… public Bank preferredBank() { //some implementation… } public Solvability solvability() { //some implementation… }
Fat objects: the Customer for accounting purposes

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net //Java public class Customer { //… public Agent zoneAgent() { //some implementation… } public Boolean isToSendEMails() { //some implementation… }
Fat Objects: the Customer for the CRM system

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net //Java public class Customer { //… public Carrier[] carriers() { //some implementation… } public Discount[] discounts() { //some implementation… }
Fat objects: the Customer for orders

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net //Java public class Customer { //… public Priority schedulingPriority() { //some implementation… } public TechnicalInfo[] standards() { //some implementation… }
Fat objects: the Customer for production

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net
Fat objects: does inheritance help?
Customer name() address() AccountingCustomer preferredBank() solvability() CRMCustomer zoneAgent() isToSendEMails() ProductionCustomer schedulingPriority() standards()
What if I needed the preferred bank in the CRM module?

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net //Scala case class Customer ( name : String , address : String ) trait accountingCstomer { def preferredBank = //some implementation def solvability = //some implementation } trait CRMCustomer { def zoneAgent = //some implementation def isToSendEMail = //implementation }
Fat objects: decomposing with Traits

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net //Scala val customer = new Customer ( “Lombardo ltd" , “ Wall Street, 1" ) with CRMCustomer with AccountingCustomer
Fat objects: decomposing with Traits
“ Dynamic” type composition

Tools for scalability: functional programming http://www.francolombardo.net
Functions are base objects of the language, as strings and numbers
Therefore functions can be arguments and return values of method calls
You can decompose the system in generic functions which you can reuse reducing duplications Higher order functions

Tools for scalability: functional programming http://www.francolombardo.net
Functions don’t have a state: their results are based only on input values.
A function evaluation has no side effects beside the computation of the result
Reusing functions is simpler if we don’t need to reproduce a particular state
We can test code more easily
Referential transparency

Tools for scalability: functional programming http://www.francolombardo.net An example: numeric integration…

Tools for scalability: functional programming http://www.francolombardo.net … I was joking, let’s use a more “concrete” example //Here’s an order row case class OrderRow ( description : String , price : Double , qty : Double ) { def amount = price * qty } //And an order val order = List ( OrderRow ( "Beer" , 5.0, 2), OrderRow ( "Chips" , 2.5, 1))

Tools for scalability: functional programming http://www.francolombardo.net //A traditional approach to VAT computation var total = 0.0 for ( row <- order ) { total += row . amount * 0.2 } println ( "VAT (Total): " + total );

We merge 3 operations
Loop

Loop
Accumulation

Loop
Accumulation
Computation

Tools for scalability: functional programming http://www.francolombardo.net //The computation: it’s a function we can assign val vat = ( row : OrderRow ) => row . amount * 0.2 //Composition of accumulation and computation def composition ( aggr : ( Double , Double ) => Double , calculus : ( OrderRow => Double )) ( partial : Double , row : OrderRow ) = aggr ( partial , calculus ( row )) val totalVat = order . foldLeft (0.0)( composition (_ + _, vat )) Let’s decompose these 3 operations

Tools for scalability: functional programming http://www.francolombardo.net val totalVat = order . foldLeft (0.0)( composition (_ + _, vat )) Let’s decompose these 3 operations
Loop
//Java B b = start; for ( final A a : listOfA) { b = method( b , a ); } return b; //Scala listOfA . foldLeft(start)(method)

Loop
Accumulation

Loop
Accumulation
Computation

Tools for scalability: functional programming http://www.francolombardo.net val totalAmount = order . foldLeft (0.0)( composition (_ + _, _. amount )) val maxAmount = order . foldLeft (0.0)( composition ( Math . max , _. amount )) val maxVat = order . foldLeft (0.0)( composition ( Math . max , vat )) We can recompose them in a different way

Structural types http://www.francolombardo.net def deleteAllRows ( statement : java . sql . Statement ) = statement . execute ( "DELETE FROM MYTABLE" ) How can we test this method? An hand written mock? The java.sql.Statement interface has 41 methods!!! (Well, I don’t like frameworks…)

Structural types http://www.francolombardo.net def deleteAllRows ( statement : { def execute ( sql : String): Boolean }) = statement . execute ( "DELETE FROM MYTABLE" ) Let’s modify our method declaring only what we need

Structural types http://www.francolombardo.net def testDeleteAllRows () { val mockStatement = new { def execute ( sql : String ) = { println ( sql ) //Oppure qualsiasi cosa x test true //Valore di ritorno di execute } } deleteAllRows ( mockStatement ) } Now we can test our code easily

Implicit conversions http://www.francolombardo.net class RemoteStatement { def remoteExecute ( sql : String ) = { println ( sql + " executed remotely :-)" ) true } } What if we’d need to use a library for doing remote queries?

Implicit conversions http://www.francolombardo.net implicit def normalize ( remote : RemoteStatement ) = new { def execute ( sql : String ) = remote . remoteExecute ( sql ) } //I can use it even if it is not the exact type! deleteAllRows ( new RemoteStatement ) No problem: we can convert on the fly!

Domain Specific Languages http://www.francolombardo.net val tenDollars = (4.0 USD ) + (6.0 USD ) I’d like to write something like this Andy Warhol– Dollar sign

Domain Specific Languages http://www.francolombardo.net abstract class Currency { def name : String ; override def toString = name } object Euro extends Currency { def name = "EUR" } object Dollar extends Currency { def name = "USD" } No problem!

Domain Specific Languages http://www.francolombardo.net case class Money [ C <: Currency ]( amount : Double , currency : C ) { def + ( otherMoney : Money [ C ]) = new Money ( amount + otherMoney . amount , currency ) override def toString = amount + " " + currency } No problem!

Domain Specific Languages http://www.francolombardo.net object Money { implicit def doubleConverter ( d : Double ) = new { def EUR = { new Money ( d , Euro ) } def USD = { new Money ( d , Dollar ) } } No problem!

Domain Specific Languages http://www.francolombardo.net object Playground extends BasicClass { def main ( args : Array [ String ]) { 10 PRINT "SCALA ROCKS!" 20 GOTO 10 RUN } } With these tools we can write very exciting DSLs! Example created by Szymon Jachim See http://www.scala-lang.org/node/1403

A First Date With Scala

by Franco Lombardo on Jan 24, 2010

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