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 ,[object Object],[object Object],[object Object],[object Object],http://www.francolombardo.net Claudio Destito – Auto-ritratto

Scala = SCAlable LAnguage http://www.francolombardo.net In order to “scale” you need solid basis! ,[object Object],[object Object],[object Object],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 ,[object Object],[object Object],[object Object],[object Object]

Tools for scalbility: “pure” Object Orientation http://www.francolombardo.net ,[object Object],[object Object],2 + 5 //equals to... 2. + (5) ,[object Object],[object Object],[object Object]

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net ,[object Object],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 } ,[object Object]

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… } ,[object Object]

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… } ,[object Object]

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… } ,[object Object]

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… } ,[object Object]

Tools for scalability: Object Orientation with Traits http://www.francolombardo.net ,[object Object],Customer name() address() AccountingCustomer preferredBank() solvability() CRMCustomer zoneAgent() isToSendEMails() ProductionCustomer schedulingPriority() standards() ,[object Object]

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 } ,[object Object]

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 ,[object Object],“ Dynamic” type composition

Tools for scalability: functional programming http://www.francolombardo.net ,[object Object],[object Object],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 ,[object Object],[object Object],[object Object],[object Object],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 );

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 ); ,[object Object],[object Object]

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 ); ,[object Object],[object Object],[object Object]

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 ); ,[object Object],[object Object],[object Object],[object Object]

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 ,[object Object],//Java B b = start; for ( final A a : listOfA) { b = method( b , a ); } return b; //Scala listOfA . foldLeft(start)(method)

Tools for scalability: functional programming http://www.francolombardo.net val totalVat = order . foldLeft (0.0)( composition (_ + _, vat )) Let’s decompose these 3 operations ,[object Object],[object Object]

Tools for scalability: functional programming http://www.francolombardo.net val totalVat = order . foldLeft (0.0)( composition (_ + _, vat )) Let’s decompose these 3 operations ,[object Object],[object Object],[object Object]

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

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