Self referencing derived store

[antony]

Describe the bug
This is half bug, half proposal.

The current implementation of derived stores doesn't provide a way to use the store's existing value in the new value (a-la .update).

You can see in the REPL my workaround for this is to use get(storeName) which... sort of works.

The issue is that you get a console error which pertains to whatever the error you previously encountered was, which is written every time the store updates.

Logs

 Error: Uncaught (in promise): thir is not defined

To Reproduce
https://svelte.dev/repl/df25b5983b8d464fb1e47af7a1d12d61?version=3.22.3

Expected behavior
The current behaviour is correct, aside from the error.

Information about your Environment:

• Svelte version 3.22.3

Severity
I do need to use the current store value in a derived store, as my derived store builds a set of results which need to be concatenated. I can work around it though, so:

low

[pushkine]

https://svelte.dev/repl/15e9932df3d34a50a2fe9d745a38b727?version=3.22.3

[antony]

wow @pushkine - that's a nice workaround! time to refactor again :D

[Conduitry]

@antony I'm not seeing any warnings or exceptions with your REPL.

[antony]

@Conduitry break it - change console to consdole or something, and then hit the button a few times. It'll print the error for ever more.

[tanhauhau]

can't replicate the bug, removing the bug label

[rmunn]

Looks like #6737 is related to this; either a duplicate, or an overlapping request. If the optional second parameter of derived could be either set or (set, update) then this use case would be solved, too:

export const first = writable({ value: 1 })
const second = derived([ first ], ([ $first ], set) => {
	set({ value: $first.value + 1 })
})
export const third = derived([ second ], async ([ $second ], (set, update)) => {
	console.log('deriving third value')
	update($third => ({ $third.value + $second.value }))
}, { value: 1 })