FUNCTIONAL PROGRAMMING (FP)

Prateek Jain

(prateekjainaa@gmail.com)

Is OOPs, oops?• NullPointer problems• Inheritance issues• “new” operator issues

Is OOPs, oops?• NullPointer problems• Inheritance issues• “new” operator issues

Is OOPs, oops?• NullPointer problems• Inheritance issues• “new” operator issues

Why FP?• I Have to Be Good at Writing Concurrent Programs• Most Programs Are Just Data Management Problems

(why ORM?)• More Modular• I Have to Work Faster and Faster• Code is written once, but read many times

Why FP?• I Have to Be Good at Writing Concurrent Programs• Most Programs Are Just Data Management Problems

(why ORM?)• More Modular• I Have to Work Faster and Faster• Code is written once, but read many times

Why FP?• I Have to Be Good at Writing Concurrent Programs• Most Programs Are Just Data Management Problems

(why ORM?)• More Modular• I Have to Work Faster and Faster• Code is written once, but read many times

Why FP?• I Have to Be Good at Writing Concurrent Programs• Most Programs Are Just Data Management Problems

(why ORM?)• More Modular• I Have to Work Faster and Faster• Code is written once, but read many times

Why FP?• I Have to Be Good at Writing Concurrent Programs• Most Programs Are Just Data Management Problems

(why ORM?)• More Modular• I Have to Work Faster and Faster• Code is written once, but read many times

What is FP?• RECURSION• ABSTRACTION• HIGHER ORDER FUNCTIONS

• IMPACT MOST PROGRAMMING LANGUAGES

PROGRAMS AS FUNCTIONS• PROGRAM = DESCRIPTION OF A SPECIFIC

COMPUTATION• Y = F(X)• F: X ->Y

• MATHEMATICS• VARIABLES = ACTUAL VALUES• NO MEMEORY ALLOCATION CONCEPT

• IMPERATIVE LANGUAGE• VARIABLES = MEMORY LOCATIONS + VALUES

PROGRAMS AS FUNCTIONS• NO LOOPS BUT RECURSION

• NO VARIABLE EXCEPT AS A NAME FOR A VALUE

• NO ASSIGNMENT OPERATION (x = x+1 , MEANINGLESS)

• ONLY CONSTANTS, PARAMETERS AND VALUES

EXAMPLE

Void GCD (int u, int v, int* x) {

Int y, t, z;

z = u;

y = v;

While (y!=0) {

…

…

}

…

}

Void GCD(int u, int v) {

if(v==0)

return u;

else

return GCD(v, u%v);

}

imperative functional

NO VARIABLES, NO ASSIGNMENT

• No notion of the internal state of a function.• Referential Transparency.• Value Semantics.

FP vs Others• Recursions instead of loops• Pattern matching instead of “if”• Pattern matching instead of state machines• Information transformation instead of sequence of tasks

FP vs Others• Persistent data structures• Powerful concurrency constructs : Actors• Software transactional memory• Avoid “Null”

What it really means?• Immutability is good

What it really means?• Immutability is good• No bugs (due to nasty side effects)

What it really means?• Immutability is good• No bugs (due to nasty side effects)• Benefits from concurrency

What it really means?• Immutability is good• No bugs (due to nasty side effects)• Benefits from concurrency• No more messy loops

What it really means?• Immutability is good• No bugs (due to nasty side effects)• Benefits from concurrency• No more messy loops• Lazy evaluation

FP Examples• Erlang, Haskell, Clojure• F#• JAVA 8 (prject lambda), Scala, Groovy• R, Mathematica etc. (specialized languages)

CAUTION CAUTION

Maintaining, Maintainability• Use functional style where\till it makes the intent more

readable.

Sort(extract(filter(having(on(Pet.class).getSpecies(), is(Dog)), pets, on(Pet.class).getName()), on(String.class));

List<Pet> dogs = filter(having(on(Pet.class).getSpecies(), is(Dog)), pets);List<String> dogNames = extract(dogs, on(Pet.class).getName());List<String> sortedDogNames = sort(dogNames, on(String.class));

Maintaining, Maintainability• One liners are always not better.

Private Converter<Pet, VetStay> toVetStay () {@override public VetStay converter(Pet pet) {

return new VetStay(pet, new Date(), “….”);}});

Convert(pets, new Convert<Pet, VetStay>() {@override public VetStay converter(Pet pet) {

return new VetStay(pet, new Date(), “….”);}});

Convert(pets, toVetStay());

FP - Adoption• Facebook (tchat), Linkedin uses Erlang• Twitter, UBS, Credit Suisse uses Scala

QUESTIONS?Questions?

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