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!' }]}

This implementation mutates the original data.

just put Object.assign({}, object, {[key]: value}) to copy the objects so the purists feel comfortable. That's the one page introduction for a senior developer .Thanks

@leihuang69 & @dimitris-papadimitriou-chr Thanks for your feedback, you're totally right. Mutating is not FP-like. I've updated the implementation.

awesome gist! my program runs within a specialized execution context built to mitigate supply chain attacks. this often leads me to the task of rolling my own function implementation, which currently relates to lens logic, but thanks to you I've been spared 🙂 @branneman

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 🙂