How to lift and compose functions together?

Question

I have a list of people in this structure:

const people = [
  {name: 'jenny', friends: ['jeff']},
  {name: 'frank', friends: ['jeff', 'ross']},
  {name: 'sarah', friends: []},
  {name: 'jeff', friends: ['jenny', 'frank']},
  {name: 'russ', friends: []},
  {name: 'calvin', friends: []},
  {name: 'ross', friends: ['frank']},
];

I would like to filter out people two ways: with and without friends; moreover I would like the Predicate of the Array.filter to be lifted, like so:

const peopleWithoutFriends = people.filter(withoutFriends);
console.log(peopleWithoutFriends);

const peopleWithFriends = people.filter(withFriends);
console.log(peopleWithFriends);

I can achieve this behavior by explicitly writing a by function like this:

const by = x => i => {
  return Boolean(get(i, x));
};
const withFriends = by('friends.length');
const peopleWithFriends = people.filter(withFriends);
console.log(peopleWithFriends);

Problem: If I wanted the inverse I would need to explicitly write a whole new function for peopleWithoutFriends

const notBy = x => i => {
  return !Boolean(get(i, x));
};

const withOutFriends = notBy('friends.length');
const peopleWithoutFriends = people.filter(withOutFriends);

I do not want to write my by function twice. I would rather compose smaller functions together.

Question:

How can I write and use small functions like: flow Booleanget curry not and compose withFriends and withOutFriends Predicates for my Array.filter over the list of people.

Repl: https://repl.it/@matthewharwood/ChiefWelloffPaintprogram

const {flow, get, curry} = require('lodash');

const people = [
  {name: 'jenny', friends: ['jeff']},
  {name: 'frank', friends: ['jeff', 'ross']},
  {name: 'sarah', friends: []},
  {name: 'jeff', friends: ['jenny', 'frank']},
  {name: 'russ', friends: []},
  {name: 'calvin', friends: []},
  {name: 'ross', friends: ['frank']},
];
const not = i => !i;

const withFriends = i => flow(
  Boolean,
  get(i, 'friends.length'), // arity of this is 2 so might be harder to lift, is it possible tho with curry?
); // No idea what i'm doing here.


const peopleWithFriends = people.filter(withFriends);
console.log(peopleWithFriends);

const withoutFriends = flow(not, withFriends);
const peopleWithoutFriends = people.filter(withoutFriends);
console.log(peopleWithoutFriends);

you are probably interested in `lodash/fp` as well – Mulan May 18 '19 at 21:52 — Mulan, May 18 '19 at 21:52

Ori Drori · Accepted Answer · 2019-05-19T05:53:23.867

Since the result of with/without friends functions is a Boolean, you can negate (or complement) the result of the one to get the other. In addition the arity of the functions is 1 (the object they operate on).

Lodash/fp:

const { flow, get, isEmpty, negate } = _;

const people = [
  {name: 'jenny', friends: ['jeff']},
  {name: 'frank', friends: ['jeff', 'ross']},
  {name: 'sarah', friends: []},
  {name: 'jeff', friends: ['jenny', 'frank']},
  {name: 'russ', friends: []},
  {name: 'calvin', friends: []},
  {name: 'ross', friends: ['frank']},
];

const withoutFriends = flow(get('friends'), isEmpty); // create a function that gets the friends array, and check if it is empty
const withFriends = negate(withoutFriends); // negate the result of withoutFriends

const peopleWithFriends = people.filter(withFriends);
console.log(peopleWithFriends);

const peopleWithoutFriends = people.filter(withoutFriends);
console.log(peopleWithoutFriends);

<script src='https://cdn.jsdelivr.net/g/lodash@4(lodash.min.js+lodash.fp.min.js)'></script>

Ramda:

const { pipe, prop, isEmpty, complement } = R;

const people = [
  {name: 'jenny', friends: ['jeff']},
  {name: 'frank', friends: ['jeff', 'ross']},
  {name: 'sarah', friends: []},
  {name: 'jeff', friends: ['jenny', 'frank']},
  {name: 'russ', friends: []},
  {name: 'calvin', friends: []},
  {name: 'ross', friends: ['frank']},
];

const withoutFriends = pipe(prop('friends'), isEmpty); // create a function that gets the friends array, and check if it is empty
const withFriends = complement(withoutFriends); // negate the result of withoutFriends

const peopleWithFriends = people.filter(withFriends);
console.log(peopleWithFriends);

const peopleWithoutFriends = people.filter(withoutFriends);
console.log(peopleWithoutFriends);

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.26.1/ramda.js"></script>

Notes:

Both _.flow() and R.pipe execute the sequence from left to right (top to bottom). The functions _.compose() and R.compose order is reversed.
The 1st function in flow/pipe/compose gets everything that is passed to the composed function. The other functions in the sequence always get a single param (the result of the previous function)/.
Both Ramda and Lodash have a reject method, which is the opposite of filter, if the predicate returns true, the item is removed. For example, R.reject(foo, xs) is equivalent to R.filter(R.complement(foo), xs). (noted by @ScottSauyet's in this comment)

In functional programming sometimes the terms are more confusing than the actual application :) — Ori Drori, May 18 '19 at 19:29
*"Both `flow` and `pipe` execute the sequence from right to left..."* - I think you mean *"left-to-right"*. — Mulan, May 18 '19 at 21:51
Also note that Ramda's `reject` could be useful here. `R.reject(foo, xs)` is equivalent to `R.filter(R.complement(foo), xs)`. — Scott Sauyet, May 18 '19 at 22:56

How to lift and compose functions together?

Question:

1 Answers1