store-comonad.js

/*
data Store s a = Store (s -> a) s

instance Functor (Store s) where
  fmap f (Store g s) = Store (f . g) s

instance Extend (Store s) where
  duplicate (Store f s) = Store (Store f) s

instance Comonad (Store s) where
  extract (Store f s) = f s
*/
// also see: http://package.elm-lang.org/packages/joneshf/elm-comonad/1.0.0/Store

// compose all the things
const o = f => g => x => f(g(x));

// here :: s
// view :: s -> a
const Store = here => view => ({  
  map:       f  => Store(here)(o(f)(view)),
  duplicate: () => Store(here)(next => Store(next)(view)),
  extract:   () => view(here),
  move:      s  => Store(s)(view) 
});

store-comonad2.js

// closer to the Elm impl
const Store = (here, view) => ({ here, view });

const o = f => g => x => f(g(x));

const map       = f => s  => Store(s.here, o(f)(s.view));
const duplicate = s       => Store(s.here, next => Store(next, s.view));
const extract   = s       => s.view(s.here);
const move      = s => sa => duplicate(sa).view(s);

// we can derive the `extend` (the inverse of monadic `bind`) from the above:
// extend :: Comonad w => (w a -> b) -> w a -> w b
const extend f  = o(map(f))(duplicate);