Why Functional Programming ?

Concurrency and Parallelism

The root of the problem

                        
var x = 0
async { x = x+1 }
async { x = x*2 }
// Can give 0,1,2
                        
                    

• It helps to encapsulate state in actors or transactions, but the fundamental problem stays the same.
• non-determinism = parallel processing + mutable state
• To get a deterministic processing, avoid the mutable state!
• Avoiding mutable state means programming functionally.

Definition of a Pure Function

A pure function is a function where the return value is only determined by its input values, without observable side effects.

                    
def example(x:Double):Double = x* sqrt(2)

def example(x:Double):Double = x* 1.4142135623730951


                

Why Scala ?

Scala is a Unifier

Scala's ToolBox

Few Examples

A Class ...

Java :

                            
public class Person {
public final String name;
public final int age;
Person(String name, int age) {
this.name = name;
this.age = age;
}
}
                
                        

Scala :

                            
class Person(val name:String,val age:Int)
                
                        

... and its usage

Java :

                          
import java.util.ArrayList;
...
Person[] people;
Person[] minors;
Person[] adults;
{
ArrayList<Person> minorsList = new ArrayList<Person>();
ArrayList<Person> adultsList = new ArrayList<Person>();
for(int i = 0 ; i< people.length; i++)
(people[i].age < 18 ? minorsList:adultsList).add(people[i]);
minors = minorsList;
adults = adultsList;
}

                      

Scala :

              
val people : Array[Person]
val (minor, adults) = people partion (_.age < 18)

          

Going Parallel

Java :

?

Scala :

                            
val people : Array[Person]
val (minor, adults) = people.par partion (_.age < 18)
                
                        

Things I miss so much in Java

I don't remember the last time I saw a

                    
java.lang.NullPointerException
	at $line23.$read$$iw$$iw$.liftedTree1$1(:13)
	at $line23.$read$$iw$$iw$.(:13)
	at $line23.$read$$iw$$iw$.()
	at $line23.$eval$.$print$lzycompute(:7)
	at $line23.$eval$.$print(:6)
	at $line23.$eval.$print()
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
                    
                

Thanks to how the values from JavaMethods are returned within the Option Class.
Note : Ignoring it and directly calling a the get Function can still lead to faulty code.

The Painful way of writing parallel and concurrent applications

No more Runnable Blocks of code.

Getting Started

Using Scala

• Scala Interpreter
• Scala Scripts
• Compiled Scala Code
• Build Tools Maven, sbt

Installing Scala

Scala Interpreter

                    
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_79).
Type in expressions for evaluation. Or try :help.

scala> println("Hello World")
Hello World

scala>
                    
                

Scala Interpreter

$ scala
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_79).
Type in expressions for evaluation. Or try :help.

scala> object HelloWorld {
     |   def main(args: Array[String]): Unit = {
     |     println("Hello, world! "+args.mkString(",") )
     |   }
     | }
defined object HelloWorld

scala> HelloWorld.main(Array())
Hello, world!

scala> HelloWorld.main(Array("hi","there"))
Hello, world! hi,there

Scala Scripts

                    
$ cat hello.scala
println("hello world")

$ scala hello.scala
hello world
                    
                

Scala Scripts

$ cat hello.scala
object HelloWorld {
  def main(args: Array[String]): Unit = {
    println("Hello, world! "+args.mkString(",") )
  }
}
HelloWorld.main(Array())
HelloWorld.main(Array("hi","there"))

vishnu@HMECL001076 ~/learning
$ scala hello.scala
Hello, world!
Hello, world! hi,there

Compiled Scala Code

                    
$ cat HelloWorld.scala
object HelloWorld {
  def main(args: Array[String]): Unit = {
    println("Hello, world! "+args.mkString(",") )
  }
}

$ scalac HelloWorld.scala

$ scala HelloWorld
Hello, world!

$ scala HelloWorld hey how are you
Hello, world! hey,how,are,you
                    
                

Build Tools - Maven

$ mvn archetype:generate \
-DarchetypeGroupId=net.alchim31.maven \
-DarchetypeArtifactId=scala-archetype-simple \
-DarchetypeVersion=1.5 \

$ tree simple
simple
|-- pom.xml
`-- src
    |-- main
    |   `-- scala
    |       `-- test
    |           `-- App.scala
    `-- test
        `-- scala
            `-- samples
                |-- junit.scala
                |-- scalatest.scala
                `-- specs.scala

7 directories, 5 files

mvn exec:java -Dexec.mainClass="test.App"

To create a java scala Mixin Project check link

Build Tools - sbt

$ sbt
[info] Set current project to test (in build file:/home/vishnu/learning/test/)
> run
[info] Updating {file:/home/vishnu/learning/test/}test...
[info] Resolving org.fusesource.jansi#jansi;1.4 ...
[info] Done updating.
[info] Compiling 1 Scala source to /home/vishnu/learning/test/target/scala-2.10/classes...
[info] 'compiler-interface' not yet compiled for Scala 2.10.6. Compiling...
[info]   Compilation completed in 16.501 s
[info] Running HelloWorld
Hello, world!
[success] Total time: 20 s, completed 22 Mar, 2016 2:27:10 PM
> run hey how are you
[info] Running HelloWorld hey how are you
Hello, world! hey,how,are,you
[success] Total time: 0 s, completed 22 Mar, 2016 2:27:59 PM
>

Sbt Installation Guide

Build Tools - sbt Dependencies

$ cat build.sbt
name := "sbtExample"

version := "1.0"

scalaVersion := "2.10.0"

resolvers += "Typesafe Repository" at "http://repo.typesafe.com/typesafe/releases/"

libraryDependencies += "com.typesafe.akka" % "akka-actor_2.10" % "2.2-M1"
libraryDependencies += "com.typesafe" % "config" % "1.3.0"