lens.mjs

import S from 'sanctuary';
import Identity from 'sanctuary-identity';


//    lens :: (s -> a) -> (a -> s -> s) -> Lens s a
const lens = getter => setter => f => s => (
  S.map (v => setter (v) (s))
        (f (getter (s)))
);

//    view :: Lens s a -> s -> a
const view = l => x => (l (S.Left) (x)).value;

//    over :: Lens s a -> (a -> a) -> s -> s
const over = l => f => x => S.extract (l (y => Identity (f (y))) (x));

//    email :: Lens User String
const email = lens (user => user.email) (email => user => ({...user, email}));

//    user :: User
const user = {id: 1, email: 'dc@davidchambers.me'};

console.log (view (email) (user));
console.log (over (email) (S.toUpper) (user));