[RFC] atom effects

Continuing from discussions in the discord channel. Recoil proposes Atom Effects. We don’t need to provide the same API, and probably it’s impossible as there are some conceptual differences between jotai and recoil. (It is interesting to me that things get to be seen differently in jotai from recoil, because originally jotai is modeled after recoil.)

However, there are good use cases. It seems like there are three possibilities for each use case.

1. can be implemented as derived atoms
2. can’t be implemented as derived atoms, but something can be added in jotai core
3. can never be implemented in jotai

Collecting use cases might be important. Please share your ideas.

For 1, we could create new functions in jotai/utils: withEffect and atomWithEffect.

Any thoughts? volunteers?

[dai-shi]

Hm, okay debugLabel is already a camel case.

Here’s version 3.

import { atom } from 'jotai'

const dataAtom = atom(null)
dataAtom.onMount = (get, set) => {
  const unsubscribe = someStore.subscribe((nextData) => {
    set(dataAtom, nextData)
  })
  return unsubscribe
}

[dai-shi]

While I would try to avoid adding a new feature in core, this seems unavoidable.

• We discussed some patterns in #234, but useEffect based solution only works for non-base atoms
• We started working on #248, and support for subscription for base atoms seems crucial

---

So, here’s the proposal.

import { atom } from 'jotai'

const dataAtom = atom(null)
dataAtom.effects = [
  (get, set) => {
    const unsubscribe = someStore.subscribe((nextData) => {
      set(dataAtom, nextData)
    })
    return unsubscribe
  }
]

// effects are invoked in commit phase, when it first have a dependent
// and will be cleaned up when there are no dependents (even if it's a very short period.)
// `get` can read only from the committed state, wip state is just ignored.