Functional programming

FUNCTIONAL
PROGRAMMING (FP)
Prateek Jain
(prateekjainaa@gmail.com)

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

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

imperative

functional

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);
}

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

• No bugs (due to nasty side effects)

• Benefits from concurrency

• No more messy loops

• No more messy loops
• Lazy evaluation

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

Maintaining, Maintainability
• Use functional style wheretill 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.

Convert(pets, new Convert<Pet, VetStay>() {
@override public VetStay converter(Pet pet) {
return new VetStay(pet, new Date(), “….”);
}});

Private Converter<Pet, VetStay> toVetStay () {
@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

Functional programming

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