Как функциональное программирование может сделать нашу жизнь проще

var calculateTotalPrice = data => {
  // ...
};

fs.readFile('./offers.csv', 'utf8', calculateTotalPrice);

var someGlobalFlag = true;

var swapState = data => {
    if (data.needSwapState) {
        someGlobalFlag = !someGlobalFlag;
    }
};

var someGlobalFlag = true;

var swapState = (data, flag) => {
    if (data.needSwapState) {
        return flag ? true : false;
    }
};

var newGlobalFlag = swapState(data, someGlobalFlag);

var toUpperCaseAll = (list) => {
    for (let i = 0; i < list.length; i++) {
        list[i] = list[i].toUpperCase();
    }

    return list;
};

toUpperCaseAll(['mike', 'leo', 'raf', 'don']);
// ['MIKE', 'LEO', 'RAF', 'DON']

var toUpperCaseAll = (list) => {
    var result = [];

    for (let i = 0; i < list.length; i++) {
        result[i] = list[i].toUpperCase();
    }

    return result;
};

toUpperCaseAll(['mike', 'leo', 'raf', 'don']);
// ['MIKE', 'LEO', 'RAF', 'DON']

var add = (a, b) => a + b;

add(2, 5) // 5
add(2, 5) // 5
add(2, 5) // 5

a + b



var add = (a, b) => a + b;

add(2, 5);

var five = 5;



var five = () => 5;

var transformList = (callback, list) => {
    var result = [];

    for (let value of list) {
        let newValue = callback(value);

        result.push(newValue);
    }

    return result;
}

function transformList(callback, [x, list]) {
    if (!x) return [];

    return [callback(x),  . . . transformList(callback, list)];
}

var add = function (x) {
    return function (y) {
        return x + y;
    };
};

var increment = add(1);
var addTen = add(10);

var add3 = (a, b, c) =>  a + b + c;
var add3Curryed = curry(add3);

add3Curryed(1, 2, 3); // 6
add3Curryed(1)(2, 3); // 6
add3Curryed(1, 2)(3); // 6
add3Curryed(1)(2)(3); // 6

curry = func => {
    var arity = func.length;

    return function f1(. . . args) {
        if (args.length >= arity) {
            return func.apply(null, args);
        } else{
            return function f2(. . . nargs) {
              return f1.apply(null, args.concat(nargs));
            };
        }
    };
};

var capitalize = str => str.charAt(0).toUpperCase() + str.slice(1);
var trim = str => str.trim();


var formatStr = compose(capitalize, trim);

var split = curry((separator, str) => str.split(separator));
var join = curry((separator, list) => list.join(separator));
var map = curry((func, list) => list.map(func));
var toLowerCase = str => str.toLowerCase();


var toSlug = compose(
  encodeURIComponent,
  joinString('-'),
  map(stringToLowerCase),
  splitString(' ')
);

var compose = (. . . funcs) =>
    (value) => funcs.reverse().reduce((v,fn) => fn(v), value);

var getRows = pluck('rows');
var getNames = pluck('name');

var renderUsersTable = Engine.render('users-table');

var getUsersFromDb = compose(getRows, User.findAll);

var cleanUpData = compose(capitalise, getNames);

var makePage = compose(renderUsersTable, map(cleanUpData), getUsersFromDb);


makePage( {limit: 20} );

var Container = function(x) {
    this.__value = x;
};

Container.of = x => new Container(x);

Container.prototype.map = func => {
    return Container.of(func(this.__value));
};

Container.of(2).map(function(two){ return two + 2 });
// Container(4)

var getUsersFromDb = compose(getRows, User.findAll);

var getUsersFromDb = compose(getRows, Maybe.of, User.findAll);

var getUsersFromDb = compose(map(getRows), Maybe.of, User.findAll);

getUsersFromDb() // Maybe([{name: 'leo', name: 'don']);
getUsersFromDb() // Maybe(null);

var Maybe = function(x) {
    this.__value = x;
}

Maybe.of = function(x) {
    return new Maybe(x);
}

Maybe.prototype.isNothing = function() {
    return (this.__value === null || this.__value === undefined);
}

Maybe.prototype.map = function(f) {
    return this.isNothing() ? Maybe.of(null) : Maybe.of(f(this.__value));
}

var renderUsersTable = Engine.render('users-table');

var makePage = compose(
  map(renderUsersTable),
  map(cleanUpData),
  getUsersFromDb
);

var errorMessage = 'sorry there is no users';
var errorContainer = Either.Left(errorMessage);

var getUsers = cond([
    [isArrayLike, Either.Right],
    [T, always(errorContainer)]
]);

var getUsersFromDb = compose(map(getRows), getUsers, User.findAll);

var renderUsersTable = Engine.render('users-table');
var renderErrorMessage = Engine.render('error-message');

var render = Either.either(renderErrorMessage, renderUsersTable);

var makePage = compose(render, getUsersFromDb);

var getUsersFromDb = compose(getRows, User.findAll);

var findAll = params =>
    Future((reject, resolve) =>
        User.findAll(params)
            .then(resolve)
            .catch(reject));

var getUsersFromDb = compose(map(getRows), findAll);

var makePage = compose(map(cleanUpData), getUsersFromDb);

var page = makePage({limit: 20});

page.fork(renderErrorMessage, renderUsersTable);

var getRows = pluck('rows');
var getNames = pluck('name');
var cleanUpData = compose(capitalise, getNames);

var renderUsersTable = Engine.render('users-table');
var renderErrorMessage = Engine.render('error-message');

var render = Either.either(renderErrorMessage, renderUsersTable);

var render = . . .

var errorMessage = 'sorry there is no users';
var errorContainer = Either.Left(errorMessage);

var getUsers = cond([
  [isArrayLike, Either.Right],
  [T, always(errorContainer)]
]);

var findAll = params =>
    Future((reject, resolve) =>
        User.findAll(params).then(resolve).catch(reject));

var getUsersFromDb = compose(flatMap(getRows), map(getUsers), findAll);

var render = . . .
var getUsersFromDb = . . .

var makePage = compose(map(render), flatMap(cleanUpData), getUsersFromDb);

var page = makePage({limit: 20});

page.fork(logError, insertToDOM);