03 — Wrap a throwing API

JSON.parse throws on bad input, and that failure is invisible in its signature. Wrapping it once at the boundary converts the hidden exception into an explicit Bad the caller is forced to handle.

Run it: node examples/03-wrap-throwing-api.ts

/**
 * Example 03 — turn a throwing API into a Result.
 *
 * `JSON.parse` throws on bad input, and that failure is invisible in its
 * signature. Wrapping it once at the boundary converts the hidden exception into
 * an explicit `Bad` the caller is forced to handle.
 */
import { bad, type Bad, ok, type Ok } from "simple-monad";

function parseJson<T>(raw: string): Ok<T> | Bad<"invalid_json", string> {
  try {
    return ok(JSON.parse(raw) as T);
  } catch (err) {
    return bad(
      "invalid_json",
      err instanceof Error ? err.message : String(err),
    );
  }
}

const okInput = '{"id":1}';
console.log(`Input: ${okInput}`);
const good = parseJson<{ id: number }>(okInput);
console.log(good.isOk() ? good.value : good.reason); // { id: 1 }

const badInput = "{ not json";
console.log(`Input: ${JSON.stringify(badInput)}`);
const broken = parseJson(badInput);
console.error(broken.isBad() ? `failed: ${broken.reason}` : "ok"); // failed: invalid_json
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