RFC: `createMap`

[Kelin2025]

Case

Sometimes you need to calculate data for every record in KV

Worth mentioning:

• Calculated data might be anything. E.g. it can be a sum of two fields, some string based on certain conditions. Literally anything. So API should be low-level rather than something rigid like createSwitch etc.

Solution

import { createListApi, createMapApi } from '@keyval/core'

type StatsWidgetState = {
  widgetId: string;
  ethValue: number;
  txPerDay: number
}

const $statsWidgets = createListApi<StatsWidgetState>({
  keygen: item => item.widgetId
})

const $roles = createMap($statsWidgets, {
  fn: stats => {
    if (stats.ethValue > 10) { return 'whale' }
    if (stats.txPerDay > 10) { return 'day_trader' }
    return 'stalling'
  }
})

Also, there's an optional source field

const $roles = createMap($statsWidget, {
  source: $preferredRole,
  fn: (stats, preferredRole) => {
    const roles = []
    /* calculate roles */
    if (roles.includes(preferredRole)) { return preferredRole }
    return roles[0] ?? 'stalling'
  }
})

[Minhir]

There is could be a case where you don't need all your key-values. For example, you want to have only roles.includes(preferredRole) values in map. In such case, we can create filter api as well. Also, if your kv changes very often, filter could save a lot of updates.

Additionally, I can imagine the case with desired updates only triggered by clock.

createMap($statsWidgets, {
  clock,
  filter,
  fn
})

That thing looks very similar to sample. That could be a good reason to create sourcebale API for sample and use kv as source.

Anyway, I think it would be nice to have createMap with clock, filter and fn as first implementation.

[Minhir]

Can we keep only one extended function for this?

createSelection(k$v, {source, filter, map})

[Kelin2025]

I think we can merge createSelection + createMap into one method if both fn and filter will be optional. Will you help with TS @Minhir?