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fix: Wrong variance on the WritableAtom Result type parameter #3135

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fix: Wrong variance on the WritableAtom Result type parameter #3135

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2ae3fac
fix: Wrong variance on the WritableAtom Result type parameter
atomanyih Aug 26, 2025
02133d1
use unknown instead of void per feedback
atomanyih Aug 27, 2025
cffb1ae
Merge branch 'main' into main
dai-shi Oct 16, 2025
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8 changes: 4 additions & 4 deletions src/vanilla/atom.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -58,7 +58,7 @@ export interface Atom<Value> {

export interface WritableAtom<Value, Args extends unknown[], Result>
extends Atom<Value> {
read: Read<Value, SetAtom<Args, Result>>
read: Read<Value, SetAtom<Args, unknown>>
write: Write<Args, Result>
onMount?: OnMount<Args, Result>
}
Expand All @@ -75,7 +75,7 @@ let keyCount = 0 // global key count for all atoms

// writable derived atom
export function atom<Value, Args extends unknown[], Result>(
read: Read<Value, SetAtom<Args, Result>>,
read: Read<Value, SetAtom<Args, unknown>>,
write: Write<Args, Result>,
): WritableAtom<Value, Args, Result>

Expand All @@ -98,7 +98,7 @@ export function atom<Value>(
): PrimitiveAtom<Value> & WithInitialValue<Value>

export function atom<Value, Args extends unknown[], Result>(
read?: Value | Read<Value, SetAtom<Args, Result>>,
read?: Value | Read<Value, SetAtom<Args, unknown>>,
write?: Write<Args, Result>,
) {
const key = `atom${++keyCount}`
Expand All @@ -110,7 +110,7 @@ export function atom<Value, Args extends unknown[], Result>(
},
} as WritableAtom<Value, Args, Result> & { init?: Value | undefined }
if (typeof read === 'function') {
config.read = read as Read<Value, SetAtom<Args, Result>>
config.read = read as Read<Value, SetAtom<Args, unknown>>
} else {
config.init = read
config.read = defaultRead
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9 changes: 9 additions & 0 deletions tests/vanilla/types.test.tsx
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Expand Up @@ -67,3 +67,12 @@ it('type utils should work', () => {
}
expect(Component).toBeDefined()
})

it('WritableAtom Result type should be covariant', () => {
function Component() {
const writableAtom = atom(null, () => null)
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@dai-shi dai-shi Aug 27, 2025

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This test doesn't represent your original reproduction. Can you do something similar to your repro in the PR description?

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@atomanyih atomanyih Aug 27, 2025

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Which part of the original reproduction do you mean? () => null or [someArg: number]?

The someArg was actually a mistake. That wouldn't be assignable anyway (the arguments should be contravariant). I fixed the example in my local reproduction and updated the PR description

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@dai-shi dai-shi Aug 28, 2025

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I mean in this case, we could do:

Suggested change
const writableAtom = atom(null, () => null)
const writableAtom = atom(null, () => null as null | number)

I think we need the actual case in the test:

export const createWritableAtom = (): WritableAtom<
  null,
  [],
  number | null
> =>
  atom(
    () => null,
    (get, set, someArg: string) => {
      return null;
    },
  );

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@dai-shi dai-shi Oct 18, 2025

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☝️ Does it make sense?


expectType<WritableAtom<null, [], number | null>>(writableAtom)
}
expect(Component).toBeDefined()
})