Observing promises

observePromise connects a Promise to a state updater — typically React setState.

observePromise

import { Empty, observePromise } from '@ekz/async-data';
import React from 'react';

function UserPanel() {
    const [data, setData] = React.useState(() => Empty<User>());

    React.useEffect(() => {
        const cancel = observePromise(
            fetchUser(id),
            (update) => setData((current) => update(current)),
        );

        return cancel;
    }, [id]);

    // render with data.match(...)
}

The returned function sets an internal cancelled flag so late resolutions do not update state after unmount or dependency change.

Flow:

1. pending() is applied immediately
2. On resolve → ready(value)
3. On reject → fail(error)

observePromiseGS

Convenience wrapper when you have separate getter/setter:

observePromiseGS(fetchUser(id), () => data, setData);

Stale reloads

To keep showing the previous user while refetching:

setData((current) => current.pending());
observePromise(fetchUser(id), (update) => setData((current) => update(current)));

If current already holds a user, the state becomes PendingStale.

React patterns

• Return the unsubscribe function from useEffect
• Reset to Empty() when inputs change if you do not want stale UI
• Prefer match in render over branching on booleans