mapValues -> Functor for functions

01_mapValues.js

// (a -> a) -> (a -> b) -> (a -> b)
var mapValue = transform => fn => v => fn(transform(v));

// (string -> string)  - so string is a above
var louder = x => x.toUpperCase();
// (string -> Element)  - so element is b above
var hi = (m) => ({ type: 'h1', value: m })

// (string -> Element)  - so we end up with a (a -> b) again
var titleLoud = mapValue(louder)(h1);

// Element
var hi = titleLoud('hi');

// Composition - pretty similar, (f ་ g)(x)  = f(g(x))
var compose = f => g => x => f(g(x)));

02_functor.hs

-- Functor has only one typeclass method - fmap :: (a -> b) -> f a -> f b

-- the 'f' for function will be the type 'function that takes one argument', which is written like: `((->) r)`, confusingly! Ideally
-- it'd be written like (r ->) but syntax doesn't allow
 
-- instance Functor ((->) r) where
--   fmap f g = (\x -> f (g x))
  

addOne x = x + 1
timesTwo x = x * 2

-- 11
main = print $ fmap addOne timesTwo $ 5