lifting.js

import R from "ramda"

const subLens = (by, val, def) => {
  const orDefault = val => val === undefined ? def : val
  const matches = R.compose(R.equals(val), by)
  return R.lens(
    s => orDefault(R.find(matches, s)),
    (a, s) => {
      const idx = R.findIndex(matches, s)
      return idx >= 0 ? R.set(R.lensIndex(idx), a, s) : s
    })
}

// liftListBy :: (s -> a) -> Atom [{a,b}] -> (a -> Atom b -> Atom c) -> Atom [Atom(c)]
export const liftListBy = R.curry((by, listAtom, subModel, defaultValue = {}) =>
  listAtom
    .scan((cache, m) => {
      const itemsByKey = R.indexBy(by, m)
      const added = m.map(by).filter(key => !(key in cache)).map(key =>
        subModel(key, listAtom.lens(subLens(by, key, defaultValue))))
      const removed =
        R.keys(cache).filter(key => !(key in itemsByKey))
      return R.isEmpty(added) && R.isEmpty(removed) ? cache : R.pipe(
        R.merge(R.indexBy(by, added)),
        R.omit(removed)
      )(cache)
    }, {})
    .skipDuplicates()
    .map(R.values)
    .toProperty())

export const liftListById = liftListBy(R.prop("id"))