Functional programming is a programming paradigm that uses functions and avoids side effects, mutable data, and state changes. It is based on lambda calculus. Popular functional programming concepts include map, reduce, filter, take, drop, head, and tail. Functional programming languages like Haskell, ML, Scala, Clojure, and F# are used by companies like Twitter, Facebook, and Apache for applications involving big data processing.

1. Functional Programming
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2. What is Functional programming(FP)?
• Programming paradigm(inside Declarative (contrast: Imperative) )
• Use function
• Original is Lambda calculus
• Avoid side effect
• Avoid changing state
• Avoid mutable data

3. Lambda calculus
• def sum (a, b) = a + b
• Lambda calculus version = (a -> (b -> a + b))
• Currying(function return function) example
Sum = (a -> (b -> a + b))
Sum3 = Sum(3) = (3 -> (b -> 3 + b)) = (b -> 3 + b)
Sum34 = Sum(3)(4) = Sum3(4) = (4 -> 3 + 4) = 7

4. OOP vs FP (factorial)
OOP
result: BigInt = 1;
for (int i = 1; i<=n;i++) {
result = result * i;
}
FP
def fac(n): BigInt = {
if(n == 1) return 1;
else return n * fac(n-1);
}
tail call optimize version
def fac(n, result = 1): BigInt = {
if(n == 1) return result;
else return fac(n-1, result * n);
}

5. Popular function in FP
• Map => List(1,2,3).map(a => a+1) = List(2,3,4)
• Reduce => List(1,2,3).reduce(_ + _) = 6
• Filter => List(1,2,3).filter(a => a <= 2) = List(1,2)
• Take => List(1,2,3).take(1) = List(1)
• Drop => List(1,2,3).drop(1) = List(2,3)
• Head => List(1,2,3).head => List(1)
• Tail => List(1,2,3).tail => List(2,3)

6. side effect
What is side effect
• I/O**
• Database connection**
• Http request**
• Random number
(**) next slide
Example
Don’t have side effect
def sum(a, b): Int = return a + b;
Have side effect
sideEffectNum = random();
def sumSideEffect(a, b): Int =
return a + b + sideEffectNum;
Solved
sideEffectNum = random();
def sumSideEffect(a, b, sideEffectNum): Int =
return a + b + sideEffectNum;

7. How to solve “side effect”?
• Functor
• Applicative
• Monad

8. Functor
http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html

9. Applicative
http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html

10. Monad
http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html

11. FP Language
Pure
• ML
• Haskell
Impure
• Scala(my work)
• Clojure
• Lisp
• F#

12. Who use FP?
• Twitter Finagle (scala)
• Apache Spark, Flink (scala)
• Facebook anti spam (haskell)
• Datomic Immutable database (clojure)

13. Interested in
• Category theory (functor, applicative, monad)
• Lambda calculus <- the most interested

14. Q & A