Functional Programming Concepts and Examples

functional-programming

// Pure Functions
// Programming without side-effects

Remove:
1. Assignments
2. Loops (use recursion instead)
3. Freeze all array literals and object literals (remove Date and Math.random)

// MONAD

1. is an object
2. constructor is a monad because it's a function that returns a monad

// Functional Programming Patterns

// src: https://medium.com/@chetcorcos/functional-programming-for-javascript-people-1915d8775504#.y9ecpc487
// src: http://fr.umio.us/why-ramda/

// Compose Functions - second function is evaluated first
const add1 = a => a+1
const times2 = a => a *2
const compose = (a, b) => c => a(b(c))
const add1OfTimes2 = compose(add1, times2)
// console.log(add1OfTimes2(5))

// Pipe (piping an array of functions)
const pipe = fns => x => fns.reduce((v, f) => f(v), x)
const times2Add1 = pipe([times2, add1])
// console.log(times2Add1(5))

// Pipe example 2
const formalGreeting = name => `Hello ${name}`
const casualGreeting = name => `sub ${name}`
const male = name => `Mr. ${name}`
const female = name => `Mrs. ${name}`
const doctor = name => `Dr. ${name}`
const  phd = name => `${name} PhD`
const md = name => `${name} M.D.`

// ext 1 
// console.log(formalGreeting(male(phd("Chet"))))
// ext 2
const identity = x => x
const greet = (name, options) => {
  return pipe([
    //greeting
    options.formal ? formalGreeting : casualGreeting,
    //prefix
    options.doctor ? doctor : 
      options.male ? male :
        options.female ? female :
          identity,
    //suffix
    options.phd ? phd :
      options.md ? md :
        identity
    ])(name)
}
// console.log(greet('Chet', {formal: true, male: true, phd: true}))

// Function currying
const users = [{name: 'chet', age:25}, {name:'joe', age:24}]
R.pipe(
  R.sortBy(R.prop('age')), // sort user by the age property
  R.map(R.prop('name')),   // get each name property
  R.join(', '),            // join the names with a comma
)(users)
// => "joe, chet"

// monad
list = [-1,0,1]
list.map(inc) // => [0,1,2]
list.map(isZero) // => [false, true, false]