# functional programming

    Endre Galaczi

    🐦  @galacziendre
    ✉️  galacziendre@gmail.com

# functional programming - intro

functional programming style
evaluation of mathematical functions
avoid changing-state and mutable data

# functional programming - first-class functions

    var triple = function(n) {
        return n * 3;
    }

functions can be passed as arguments

# functional programming - higher-order functions

map, filter, reduce

    var numbers = [1,2,3].map(triple)

instead of:

    input = [1,2,3]
    numbers = []
    for(var i = 0; i<=numbers.length; i++){
        numbers.push(input[i])
     }

# functional programming - pure functions - no mutation, no assignment

    var a = 5;
    a = a + 1

no mutating global state

    function 💩 triple(n){

        times_called = times_called + 1;

        return n * 3;
    }

# functional programming - recursion

no mutation -> no loops
no while
no for

✨  recursion ✨

    function count_to_a_chicken(n){
        if(n==0) return "🐔 ";
        else return count_to_chicken(n-1);
    }

# tail call optimisation

stack overflow

    factorial 0 = 1
    factorial n = n * factorial n-1

    factorial(10) =
    10 * factorial(9) =
    10 * (9 * factorial(8)) =
    10 * (9 * (8 * factorial(7))) =
    10 * (9 * (8 * (7 * factorial(6))) =
    ...
    10 * (9 * (8 * (7 * (6 * (5 * (4 * (3 * (2 * 1))))))))


# tail call optimisation

    factorial 0 i = i
    factorial n i = factorial (n-1) (i*n)

    factorial(4,1)=
    (factorial(3,4))=
    ((factorial(2,12)))=
    (((factorial(1,24))))=
    ((((factorial(0,24)))))=
    ((((24))))=

es6 tco

# functional programming - expressions over statements

encourages writing code using
expressions rather than statements

    int total = 0;
    for(int i = 1; i<=10; i++){
        total = total + 1;
    }

    sum [1..10]

# functional programming - composition

    hatch = 🍳  -> 🐔
    clone = 🐔  -> 🐔 🐔

    superhatcher x = clone(clone(hatch(x)))
    superhatcher = clone.clone.hatch

    superhatcher 🍳   ->  🐔 🐔 🐔 🐔

# functional programming - composition

    var triple = function(n) {
        return n * 3;
    }
    var numbers = [1,2,3].map(triple)


    input = [1,2,3]
    numbers = []
    for(var i = 0; i<=numbers.length; i++){
        numbers.push(input[i])
    }


# programming is about making things simpler

    "There are only two hard things in Computer Science:
        cache invalidation and naming things."

    -- Phil Karlton


#  why is it suddenly so popular?

    memory is cheap

    cpu is growing in # cores not in GHz

    functionally pure code is easier to understand

    pure functions are easier to unit test

# unit testing pure functions

    var mistery_func = function(a, b) {

        //lol I don't know why we need this
        //but it breaks otherwise

        write_to_file(random_filename(), "💩 ");
        return a * 2 + b * 5;
    }

side effects need to be cleaned up
or need to be mocked

# do you unit test?

learn by example or from your own mistakes

tests as communication

# Joel Test

Do you use source control?
Can you make a build in one step?
Do you make daily builds?
Do you have a bug database?
Do you fix bugs before writing new code?
Do you have an up-to-date schedule?
Do you have a spec?
Do programmers have quiet working conditions?
Do you use the best tools money can buy?
Do you have testers?
Do new candidates write code during their interview?
Do you do hallway usability testing?


- do you always write tests?
- do you have mandatory code review?


# Getting practical

    Islands of functional purity

    You will probably never end up programming in a purely functional language

    Like in javascript you are not going to start using recursion everywhere

    Even if you never work in a functional language professionally,
    understanding functional programming will make you a better developer.

    It will give you a new perspective on your code and programming in general.

# A little bit of haskell

    sum [1..12]

    factorial 0 = 1
    factorial n = n * factorial (n - 1)

        let right_triangles = [(x, y, z) |
             x**2 + y**2 = z**2,
             x <- [1..20],
             y <- [1..20],
             z <- [1..20] ]


# Thanks for listening

    Endre Galaczi

    🐦  @galacziendre
    ✉️  galacziendre@gmail.com