1. Hardcore functional
programming
in Javascript
Leonardo Garcia Crespo
@leogcrespo
gh/leoasis

2. Agenda
● Currying & Partial Application
● Function Composition
● Fantasy Land
● Functors
● Monads

3. Currying

4. Currying:
f(x, y, z) = r
f(x) -> g(y) -> h(z) => r

5. function add(a) {
return function(b) {
return a + b;
};
}
var add1 = add(1);
add1(3); // 4

6. function makeFullName(first) {
return function(middle) {
return function(last) {
return first + ' ' + middle + ' ' + last;
};
};
}
var makeJerry = makeFullName('Jerry');
makeJerry('Lee')('Lewis');

7. var add1 = add(1);
add1(2); // 3
add(1)(2); // 3
add(1, 2); // ?

8. _.curry(fn)

9. var add = _.curry(function(a, b) {
return a + b;
});
var add1 = add(1);
add1(2); // 3
add(1)(2); // 3

10. Partial Application

11. var add1 = add(1);
var add1 = add.bind(null, 1);
var add1 = _.partial(add, 1);

12. Function Composition
(f . g)(x) = f(g(x))
_.compose(f, g)(x) == f(g(x))

13. _.compose(reverse, toUpper)

14. _.compose(reverse, toUpper)
"Hello"

15. _.compose(reverse, toUpper)
"Hello"

16. _.compose(reverse, toUpper)
"HELLO"

17. _.compose(reverse, toUpper)
"HELLO"

18. _.compose(reverse, toUpper)
"HELLO"

19. _.compose(reverse, toUpper)
"OLLEH"

20. _.compose(reverse, toUpper)
"OLLEH"

21. _.compose(reverse, toUpper)
"OLLEH"

22. Problem with _
Data comes first:
_.map(list, fn)
_.reduce(list, fn)
_.pluck(list, prop)
...

23. If data came last:
var add1ToAll = _.map(add1);
var names = _.map(get('name'));

24. Examples

25. Fantasy Land

26. Functor
Semigroup
Monoid
Apply
Chain
Applicative
Monad

27. Map for lists:
map :: (a -> b) -> [a] -> [b]
map(add1, [1, 2, 3]) // [2, 3, 4]

28. Functors
map :: (a -> b) -> F a -> F b
where F is a functor

29. Laws
Identity:
map(id, F a) == F a
map(id) == id
Composition:
map(f . g) == map(f) . map(g)

30. Maybe Functor
map(add1, Maybe(2)); // Maybe(3)
map(add1, Maybe(null)); // Maybe(null)

31. Promise Functor
map(add1, Promise(2)); // Promise(3)
map(add1, delay(3)); // Promise(...4)

32. Examples

33. Monads
A functor that also has:
of :: a -> M a
flatMap :: (a -> M b) -> M a -> M b
or chain :: M a -> (a -> M b) -> M b

34. Laws
Left Identity:
of(a).chain(f) == f(a)
Right Identity:
M(a).chain(of) == M(a)
Associativity:
M(a).chain(f).chain(g) == M(a).chain(x -> f(x).chain(g))

35. List Monad
Array.of(1); // [1]
var withNeg = function(a) { return [-a, a]; };
flatMap(withNeg, [1, 2, 3]); // [-1, 1, -2, 2, -3, 3]
[1, 2, 3].chain(withNeg);

36. Maybe Monad
Maybe.of(1); // Maybe(1)
var getName = function(obj) { return Maybe(obj.name); };
flatMap(getName, Maybe({name: 'John'})) // Maybe('John')
Maybe({name: 'John'}).chain(getName);
flatMap(getName, Maybe({age: 20})); // Maybe(null)

37. Promise Monad
Promise.of(1); // Promise(1)
// findTodo :: Number -> Promise(todo)
var findTodo = function(id) {
return fetchFromDb('todo', id);
};
flatMap(findTodo, Promise.of(123)); // Promise(...todo)
Promise.of(123).chain(getName);

38. Examples

39. Next
● Monoids
● Applicative Functors
● Learn a FP language!
● A LOT more than this

40. Why?
● Declarative (what vs how)
● Common Patterns
● Small functions
● Oriented to fns, not data
● Correctness (mathematics)

41. Resources
● http://learnyouahaskell.com/
● Hey Underscore! You're Doing it wrong (https:
//www.youtube.com/watch?v=m3svKOdZijA)
● https://github.com/fantasyland/fantasy-land
● http://functionaltalks.org
● Brian McKenna (@puffnfresh) & Brian Lonsdorf
(@drboolean) on Twitter

42. Thanks!
Qs?
Leonardo Garcia Crespo
@leogcrespo
gh/leoasis