ts_proper_option_type.ts

interface Some<V> {
  tag: 'Some';
  value: V;
}

// A nullary type, which is just a tag at runtime for our purposes.
interface None {
  tag: 'None';
}

type Option<V> = Some<V> | None;

const safeHead = (xs: number[]): Option<number> => {
  if (xs instanceof Array) {
    if (xs.length === 0) {
      return {
        tag: 'None'
      };
    } else {
      return {
        tag: 'Some',
        value: xs[0],
      }
    }
  } else {
    const _ex_patt_match: never = xs;
  }
};

const xs = [1,2,3];
const mb = safeHead(xs);