The 7th Annual IndicThreads Pune Conference was held on 14-15 December 2012. http://pune12.indicthreads.com/

1. Polyglot and Functional Programming
on the Java Virtual Machine
Mohan Kumar Muddana
InfoStretch Pvt. Ltd.

2. Some of the languages on JVM
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3. Why so many languages on JVM
• JVM has grown over the years and has become a
mature platform.
• Very wide industry adoption.
• Availability of large developer community.
• Plethora of Tools and Technologies
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4. Polyglot Programming on the JVM
• Different languages bring in different features.
• Wider choice of availability.
• Better programming features.
• Imperative and Functional
• Interoperability between the languages.
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5. JVM – The Polyglot Platform
JVM was initially built for (Java) Classes.
How does it handles so different kind of language
constructs.
the Da Vinci Machine Project
(http://openjdk.java.net/projects/mlvm/)
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6. How JVM incorporates and consumes
Java 6 : - JSR 223: Scripting for the Java Platform
(Groovy, Jython, JRuby)
Java 7 :- JSR 292: Supporting Dynamically Typed Languages on
the JavaTM Platform
MLVM – Multi Language Virtual Machine.
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7. History of Functional Programming
Alonzo Church
Creator of λ-Calculus – 1940
λ-Calculus is the basis of all functional programming languages
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8. History of Functional Programming
• What does it fundamentally say-
A function is an action that is applied to one thing (the argument) to
obtain another thing (the value).
• Two Fundamental functions in λ-Calculus
(I, the identity function). I is defined as follows: (Ix) is x, whatever
x may be.
(H). H is defined as the function that always returns the identity
function. That is, (Hx) is I for whatever x may be.
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9. Functional Programming
• Derives from mathematics, Lambda Calculus
• Built on the basis of functions
• Immutability and Concurrency
• Data Structures with strong filtering
• Lesser code with fewer language constructs.
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10. Functional Programming Languages
• Lisp – One of t he oldest programming language
(Lisp is functional language)
• Haskel – Pure functional language
• Erlang - Massively scalable soft real-time systems with
requirements on high availability and fault tolerant systems.
• Scheme, ML, OCAML etc.
• And many others 10

11. Scala Language aka Scalable
Why Scala
Ordersky says:
"We wanted to make Java more expressive, so people
could be more productive" and write at a higher level
of abstraction with fewer lines of code, Odersky says.
How it is scalable
Getting Started
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12. Scala – Object World
• Scala is Pure Object Oriented Language
• Traits
• Case Classes and Pattern Matching
• Type safety and Type Erasure
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13. Scala – Classes and Objects
Everything is objects.
Store objects into a variables.
class HelloWorld() {
val welcome: String = “Hello World ”
def msg(name: String) = println(welcome + name)
}
val h = new HelloWorld()
h.msg(“Your Name”)
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14. Scala – Traits
It is an kind of an Interface with fields and concrete methods.
Unlike Class inheritance, a class can mix (Mixins) in any number
of traits.
trait PolyglotProgrammer {
def polyProg() {
println("Polyglot Programmer")
}
}
class Person extends PolyglotProgrammer with
PolyglotSpeaker {}
val person = new Person
println(person.polyProg)
println(person.polySpeaker) 14

15. Scala – Case Classes and Pattern Match
• Case classes provides pattern matching on objects
without large amount of boilerplate code.
• Case classes are normal classes with a case modifier as
prefix.
case Class Rectangle(length: Int, breadth: Int)
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16. Scala – Case Classes and Pattern Match
Case Classes comes with additional conventions
- The compiler adds a factory method with the name
of the class, so no need to create an instance with
new operator.
- All arguments in the parameter list gets implicit val
prefix, so they are maintained as fields.
- Compiler adds natural implementations of toString,
hashCode and equals.
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17. Groovy
Groovy is the closest to Java language.
Supports both static and dynamic type binding.
Powerful XML processing constructs.
Very good support for regular expressions
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18. Groovy – XML Processing
def writer = new StringWriter();
def xml = new groovy.xml.MarkupBuilder(writer);
xml.person(id:2) {
name 'Gweneth‘ age 1
}
println writer.toString();
<person id='2'>
<name>Gweneth</name>
<age>1</age>
</person>
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19. Groovy Beans - Conciseness
class Person {
private String name
private int age
}
def p = new Person(name: “Ramu”, age: 15)
(No syntactic ceremony)
@Immutable annotation to make it immutable
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20. Groovy – Elvis Operator ?:
In Java if/else construct would look like this
String tradeStatus = "Active";
String status = tradeStatus != null ? tradeStatus :
“Inactive";
In Groovy
String tradeStatus = "Active"
String status = tradeStatus ?: "Inactive“
Groovy coerces the String into a boolean; assuming the
String was null, it will convert to the Boolean value of
false. No ceremonial Null Pointer check 20

21. Groovy – DSL
DSL with the help of identity: The Context Method
Trade trade = new Trade()
trade.identity {
setOrderId(1)
setOrderStatus(false)
setEquityName("Fictious Inc")
setQuantity(100)
setPrice(17.25)
}
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22. Clojure a Lisp dialect on JVM
Why Clojure
– Predominately functional language
– Stateless
– Homoiconicity
– (Un)digestive Syntax – Which you might fall in love
later
– Persistent data structures
– STM
– Atom and Agents
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23. Clojure – Functional
Predominately Functional language
(defn name doc-string? attr-map? [params*] body)
(defn helloworld [username] “returns a String hello message”
(str “Hello,” username))
Dynamic Language
Resolves types at runtime
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24. Clojure – Homoiconicity
Representation of its own data structures and atomic
values or more casually code-as-data and data-as-code
(defn average [numbers]
(/ (apply + numbers) (count numbers)))
Stateless
This definition is a list of data structure containing symbols,
values, vectors and another list consists of function body
(+ 7 3) on REPL yields
=> (+ 7 3)
10 24

25. Clojure – Homoiconicity
Data as Code – Consider map
(def names {"Rich" "Hickey" "Martin" "Odersky"})
Now use names as function
(names “Rich”) will result in Hickey
else more verbose get
(get names “Rich” “None”)
Any Clojure data structure can be a key in a map
(def names {:Lisp “Rich” :Scala “Martin”})
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26. Clojure – Parenthesis and Prefix Notation
A method in general of most of the languages
methodName(arg1, arg2, arg3);
A clojure function has prefix notation
(function-name arg1 arg2 arg3)
Imagine if you want to operate on a large list of values
sum (60+80+90+120)
Whereas in Lisp or Clojure syntax
(apply + [60 80 90 120])
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27. Clojure – Functional
First Class Functions
Functions that always return the same result when passed the
same arguments
Functions exist as data (function value).
Higher Order Functions
Take other functions as arguments or return a function as a
result.
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28. Clojure – Collections
All of them are immutable, heterogeneous and persistent.
Heterogeneous means that they can hold any kind of
object.
Being persistent means that old versions of them are
preserved when new versions are created.
Very rich data structures as well functions to operate on.
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29. Clojure – Sequences
• In Clojure, all these data structures can be accessed
through a single abstraction: the sequence (seq)
• Every aggregate data structure in Clojure can be viewed
as a sequence
• (first {"Rich" "Hickey" "Martin" "Odersky"})
• (rest {"Rich" "Hickey" "Martin" "Odersky"})
• (cons [“James" “Gosling“] {"Rich" "Hickey" "Martin"
"Odersky"})
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30. Clojure – STM
Software Transactional Manager is the way to handle
concurrency of mutable data in Clojure.
STM is very optimistic.
Alternative to lock based synchronization.
Mutual Exclusion
Every read and write that it is performing is in a log.
Onus is on reader which will commit to the shared memory in
case no modifications are done during the time, else would
re-execute the transaction.
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31. Clojure – STM
Mutable References
ref, deref or @ , ref-set, dosync
Need to be explicit when mutable data is required.
(def value (ref 100))
ref wraps and protects access to its internal state.
Even to read, it needs to deref
(deref value) or @value
As the value is in STM
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32. Clojure – STM
STM provides similar transaction properties of a Database
But STM handles in memory, so can’t guarantee durability.
• Updates are atomic. If you update more than one ref in a
transaction, the cumulative effect of all the updates will
appear as a single instantaneous event to anyone not inside
your transaction.
Stateless
• Updates are consistent. Refs can specify validation functions.
If any of these functions fail, the entire transaction will fail.
• Updates are isolated. Running transactions cannot see
partially completed results from other transactions.
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33. Clojure – STM
Agents
Agents provide independent, asynchronous change of individual
locations.
Agents are bound to a single storage location for their
lifetime, and only allow mutation of that location (to a new
state) to occur as a result of an action.
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34. Interop – Clojure to Java
Clojure is complied and generates Bytecode
Clojure embraces Java and its libraries. Idiomatic
Clojure code can call Java libraries directly
Creating Java instances and accessing its methods.
In REPL:
Stateless
(def cal (java.util.Calendar/getInstance)
(. cal getTime)
Code:
(import [java.util.Calendar])
(defn cal (.getInstance java.util.Calendar))
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35. Interop – Java to Clojure
Clojure is implemented as Java class library.
Embed Clojure using load code and call functions
import clojure.lang.RT;
import clojure.lang.Var;
public class Foo {
public static void main(String[] args) throws Exception {
Stateless
RT.loadResourceScript("foo.clj"); Var foo = RT.var("user", "foo");
Object result = foo.invoke("Hi", "there");
System.out.println(result);
}
}
(“user” being the namespace and “foo” being the function from Clj)
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36. Polylingualism
Sapir-Whorf Hypothesis
According to the first, linguistic determinism,
our thinking is determined by language.
According to the second, linguistic relativity,
people who speak different languages
perceive and think about the world quite
differently.
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37. References
Programming in Scala (Martin Odersky, Lex Spoon, Bill Venners)
Clojure Programming (Chas Emerick, Brian Carper, Christophe
Grand)
Programming Clojure (Stuart Halloway)
Well Gounded Java Developer (Benjamin Evans, Martin Verburg)
Programming in Groovy (Venkat Subramaniam)
Wikipedia.org
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38. Thank you !
gmail mohanjune@gmail.com
twitter @mohanjune Stateless
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