garbage.js

const memoizeArraySelector = (baseSelector) => {
  let _last = [ ]
  return (state) => {
    const result = baseSelector(state)
    if (arrayEquals(result, _last)) return _last
    _last = result
    return result
  }
}

export const mapSelector = f => baseSelector => createSelector(baseSelector, f)
export const mapSelectorForArray = f => baseSelector => memoizeArraySelector(mapSelector(f)(baseSelector))

export const filteredBy = predicate => mapSelectorForArray(data => _.filter(data, predicate))
export const rejectedBy = predicate => mapSelectorForArray(data => _.reject(data, predicate))
export const sortedBy = sortBy => mapSelectorForArray(data => _.sortBy(data, sortBy))
export const sortedByOrder = (sortByOrder, direction) => mapSelectorForArray(data => _.orderBy(data, sortByOrder, direction))

@iboss-ptk

• About flow type, yep I agreed that Flow/TypeScript type notation makes it look very scary.

• About making memoizeSelector work with both array and non-array, I like it a lot.

• Yes, it can be a Reader monad. For example, using createSelector to combine selectors can be considered a monadic operation:

  // Assume a selector is a fantasy-land monad instead of a (s => a) function.
  const combine2 = s1 => s2 => f => s1.chain(v1 => s2.map(v2 => f(v1, v2)))