// Created on savesnippets.com · https://savesnippets.com/nK2xRddEB33WU6
type AsyncState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error';   error: Error };

function assertNever(x: never): never {
  throw new Error(`Unhandled variant: ${JSON.stringify(x)}`);
}

function render<T>(s: AsyncState<T>): string {
  switch (s.status) {
    case 'idle':    return '—';
    case 'loading': return 'Loading…';
    case 'success': return `OK: ${JSON.stringify(s.data)}`;
    case 'error':   return `Error: ${s.error.message}`;
    default:        return assertNever(s);   // ← compile error if you add a variant
  }
}