monet.js

```js
const { Maybe, Either } = require('monet');

function parseJson(str) {
  return Maybe.fromNull(str)
    .map(JSON.parse)                       // may throw – wrap in Try later if needed
    .cata(
      () => Either.left('invalid JSON'),
      obj => Either.right(obj)
    );
}

const payload = '{"age":30}';
const result = parseJson(payload)
  .flatMap(data => data.age >= 18 ? Either.right('adult') : Either.left('minor'));

result.cata(
  err => console.error('Error:', err),
  msg => console.log('Success:', msg)
);
```