Document the use of a function as the second parameter of `set`

[Mon]

Summary

While exploring the atom page, I discovered that it's possible to pass a function as the second parameter of set. This function will be automatically invoked, receiving the atom's current value as its first parameter.

const countAtom = atom(1)
const derivedAtom = atom(
  (get) => get(countAtom),
  (get, set, action) => {
    set(countAtom, (c) => c + 1)
  },
)

It would be better if we document the behavior explicitly in the "derived atoms" section of the page.

https://github.com/pmndrs/jotai/blob/234dff9/docs/core/atom.mdx#L26-L50

const writeOnlyAtom = atom(
  null, // it's a convention to pass `null` for the first argument
  (get, set, update) => {
    // `update` is any single value we receive for updating this atom
    set(priceAtom, get(priceAtom) - update.discount)
+   // We can also pass a function as the second parameter
+   // The function will be invoked, receiving the atom's current value as its first parameter
+   set(priceAtom, (price) => price - update.discount)
  },
)

Link to reproduction

https://jotai.org/docs/core/atom

[Mon]

I can open a PR if you like the idea.

[Daishi Kato]

I discovered that it's possible to pass a function as the second parameter of set.

Internally, it will simply invoke the target atom's write function. So, it is possible because countAtom accepts such a function.

I can open a PR if you like the idea.

Yes, feel free to open a PR please.