What is the use-case for synchronous fetcher?

[koba04]

According to the type definition and implementation, the fetcher function seems to support a non-promise function.
What is the use-case for the synchronous fetcher?
I've seen synchronous fetchers in tests; Is it only for testing?
If there is no use-case, could I drop the synchronous fetcher from the type definition?

• Type Definition

swr/src/types.ts

Line 1 in a9c10f7

export type fetcherFn<Data> = ((...args: any) => Data | Promise<Data>) | null

• A test

swr/test/use-swr-suspense.test.tsx

Lines 81 to 95 in a9c10f7

	it('should work for non-promises', async () => {
	function Section() {
	const { data } = useSWR('suspense-4', () => 'hello', {
	suspense: true
	})
	return <div>{data}</div>
	}
	render(
	<Suspense fallback={<div>fallback</div>}>
	<Section />
	</Suspense>
	)
	await screen.findByText('hello')
	})

• Implementation checking if the value is a promise

swr/src/use-swr.ts

Lines 779 to 785 in a9c10f7

	if (
	CONCURRENT_PROMISES[key] &&
	typeof CONCURRENT_PROMISES[key].then === 'function'
	) {
	// if it is a promise
	throw CONCURRENT_PROMISES[key]
	}

[huozhi]

In my understanding, swr can be used as a state managment library for various cases, not only for handling network requests. The revalidate action could mean "retrieve state from data source", which can be either sync or async.

Imagine we're building a todo list app, swr hooks can still be a great helper to access and cache states of todo items.

// getTodoItems can be a function reading todos from local states
const {data:todoItems} = useSWR('todos', getTodoItems)

here's another example in swr codebase https://github.com/vercel/swr/tree/master/examples/storage-tab-sync

[koba04]

Thank you! I didn't know the example.
I was confused about the CONCURRENT_PROMISES variable might store a non-promise value, so it would be nice if I can refactor the code, but I understood it's intentional. Thank you!

[morgs32]

It would be great if useSWR would immediately return data when the fetcher is synchronous. It seems to always return undefined. Any way to do that besides passing default data to SWRConfig??