JavaScript: Lenses (Functional Programming)

fp-lenses.js

// FP Lenses
const lens = get => set => ({ get, set });
const view = lens => obj => lens.get(obj);
const set = lens => val => obj => lens.set(val)(obj);
const over = lens => fn => obj => set(lens)(fn(view(lens)(obj)))(obj);
const lensProp = key => lens(prop(key))(assoc(key));

fp-lib.js

// Generic FP utils
const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));
const map = fn => list => list.map(fn);
const filter = fn => list => list.filter(fn);
const lt = left => right => left < right;
const add = left => right => left + right;
const upper = str => str.toUpperCase();
const prop = key => obj => obj[key];
const assoc = key => val => obj => Object.assign({}, obj, {[key]: val});

usage.js

/**
 * Example usage: Object
 */
const amountLens = lens(prop('amount'))(assoc('amount'));
over(amountLens)(add(5))({ x: 1, amount: 10 });
//=> { x: 1, amount: 15 }

/**
 * Example usage: Array
 */
const assocArray = idx => val => arr => {
  const clone = arr.slice();
  clone[idx] = val;
  return clone;
}
const headLens = lens(prop(0))(assocArray(0));
over(headLens)(upper)(['first', 'second']);
//=> [ 'FIRST', 'second' ]

/**
 * Example usage: Chaining
 */
const moneyLens = lensProp('money');
const data = [{ money: 42 }, { money: 1024 }, { money: 1337 }];
compose(
  map(over(moneyLens)(add('€ '))),
  filter(compose(lt(100), view(moneyLens)))
)(data);
//=> [ { money: '€ 1024' }, { money: '€ 1337' } ]

/**
 * Example usage: Composition
 */
const article = { title: 'FP ftw!', comments: [{ t: 'boo!' }, { t: 'yay!' }] };
over(lensProp('comments'))(map(over(lensProp('t'))(upper)))(article);
//=> { title: 'FP ftw!', comments: [{ t: 'BOO!' }, { t: 'YAY!' }]}

one minor piece of feedback i have - the compose function here has the behavior of the pipe function. the difference is that compose executes functions from left to right (using reduceRight), while pipe executes from right to left.

your implementation isn't breaking any sort of law, i just thought i would mention it 🙂