functional-programming

Learn about functional programming

Material

• https://www.quora.com/What-is-a-side-effect-in-programming
• https://egghead.io/lessons/javascript-redux-pure-and-impure-functions
• https://wiki.haskell.org/Pointfree
• https://medium.com/@cscalfani/so-you-want-to-be-a-functional-programmer-part-1-1f15e387e536#.g1itz0qga

Pure Functions

• Have inputs and outputs well defined

function square(x){
  return x * x;
}

• Don't have side-effects
• Whenever your entry is 5 return will be the same.

func(3); // Result WHENEVER is 4
function func(x) {
  return x + 1;
}

Hight Order Functions

• The functions that:
  • They operate on other functions
  • Received as a parameters
  • Return functions

  let calculate = function(fn, x, y){
    return fn(x, y);
  }
  

Map

• The function invoke a callback and return one new array with result, apply item by item

const numbers = [1, 2, 3];
const square = x => x * x;
const squaredNumbers = numbers.map(square); // [1, 4, 9]

Filter

• The function invoke a callback and return one new array with result, return the filter the elements in initial array with base in function calback.

const numbers = [1, 4, 7, 10];
const isBiggerThanFour = value => value > 4 ;
const numbersBiggerThanFour = numbers.filter(isBiggerThanFour); // [7, 10]

Reduce

• The function received with parameters callback functions and a initial value, transform the array in unique value

const numbers = [1, 2, 3];
const sum = (x, y) => x + y ;
const numbersSum = numbers.reduce(sum, 0); //6

Currying

• Transform the function with multiples parameters in the sequency the functions that have one parameter

const greeting = greet => name => `${greet} ${name}`;
const hello = greeting('Hello');

hello('World'); // Hello World

Compose

• Compose small functions and create complex functions, more reuse

const compose = (f, g) => x => f(g(x));

const toUpperCase = x => x.toUpperCase();
const exclaim = x => x + '!';

const angry = compose(toUpperCase, exclaim);

angry('ahhh'); // AHHH!