// A fun fact about $derive: if the deriving expression throws an error, then Svelte (at least as of
// 5.20.2) _permanently_ stops evaluating it, even if the inputs change. To prevent this, you need
// to ensure that derive expressions never raise an exception.
function tryDerive(args, func, fallback) {
  try {
    return func(...args);
  } catch (e) {
    console.debug('deriver threw exception', e, func, args);
    return fallback;
  }
}

// A "deriver" is a function that never raises; if the underlying function would raise, the
// corresponding deriver instead returns a fallback value (or `undefined`).
export function makeDeriver(func, fallback) {
  return (...args) => tryDerive(args, func, fallback);
}

// Some widely-used derivers, for convenience.
export const json = makeDeriver(JSON.parse);