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attempt / isOk / isFail ​

Problem ​

You want to call an async function and handle success and failure in one place without try/catch — returning a discriminated union instead of throwing.

Solution ​

Use attempt(fn) to receive { ok: true, value } on success or { ok: false, error } on failure. It never throws.

Use isOk(result) and isFail(result) as type-guard helpers to narrow the discriminated union.

ts
import { attempt, isOk, isFail } from '@vielzeug/arsenal';

const result = await attempt(() => fetch('/api/user').then((r) => r.json()));

if (isOk(result)) {
  console.log(result.value); // typed as the resolved value
} else {
  console.error(result.error); // the caught error
}

// Or narrow with isFail for early-return pattern
if (isFail(result)) {
  reportError(result.error);
  return;
}
console.log(result.value);

Combining with retry ​

ts
import { attempt, retry } from '@vielzeug/arsenal';

const result = await attempt(() =>
  retry((signal) => fetch('/api/data', { signal }).then((r) => r.json()), { times: 3 }),
);

if (!result.ok) {
  reportError(result.error);
}

Pitfalls ​

  • attempt wraps a single call — it does not retry on failure. Use retry for resilient calls and wrap the whole thing in attempt if you want a non-throwing result.
  • The error field is typed as unknown; narrow it before accessing properties.