Describe the problem
When declaring a derived store that has null as the initial value and has two or more stores as dependencies, I can't specify
the type that the derived store contains without first having so specify the type of the dependency array, which is somewhat frustrating.
// ideal situation (does not currently work)
// the type of the the store's value is `DerivedData | null`
// and the type of the dependecies is inferred
const myData = derived<DerivedData | null>([a, b, c], ([$a, $b, $c], set) => {
if(!$a || !$b || !$c) {
set(null);
return;
}
const { data, dispose } = generateData($a, $b, $c);
set(data);
return dispose;
});
// reality
// one way to work around the problem
// manually typing the `set` parameter will yield the same result, but it's more verbose
const myData = derived([a, b, c], ([$a, $b, $c], set: (value: DerivedData | null) => void) => {
if(!$a || !$b || !$c) {
set(null);
return;
}
const { data, dispose } = generateData($a, $b, $c);
set(data);
return dispose;
});
// another workaround
// like before, it yields the same result, but it's also more verbose than it should be
const deps = [a, b, c] as const
const myData = derived<typeof deps, DerivedData | null>(deps, ([$a, $b, $c], set) => {
if(!$a || !$b || !$c) {
set(null);
return;
}
const { data, dispose } = generateData($a, $b, $c);
set(data);
return dispose;
});Describe the proposed solution
Swapping the order of the generic parameters of the derived function would solve the problem.
|
/** |
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* @template {import('./private.js').Stores} S |
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* @template T |
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* @param {S} stores |
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* @param {Function} fn |
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* @param {T} [initial_value] |
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* @returns {import('./public.js').Readable<T>} |
|
*/ |
|
export function derived(stores, fn, initial_value) { |
Alternatives considered
The alternatives were listed above.
Importance
would make my life easier
Describe the problem
When declaring a derived store that has
nullas the initial value and has two or more stores as dependencies, I can't specifythe type that the derived store contains without first having so specify the type of the dependency array, which is somewhat frustrating.
Describe the proposed solution
Swapping the order of the generic parameters of the
derivedfunction would solve the problem.svelte/packages/svelte/src/runtime/store/index.js
Lines 124 to 132 in f4c4d99
Alternatives considered
The alternatives were listed above.
Importance
would make my life easier