TypeScript Option<T>

option.enum.ts

enum TypeNone { }
export type None = { type: typeof TypeNone };
function None<T>(d: T): None { return { type: TypeNone } };
function isNone(x: any): x is None { return !!(x && x.type === TypeNone) };

enum TypeSome { }
export type Some<T> = { type: typeof TypeSome, data: [_: T] };
function Some<T>(d: T): Some<T> { return { type: TypeSome, data: [d] } };
function isSome(x: any): x is Some<unknown> { return !!(x && x.type === TypeSome && x.data) };

export const Option = {
  None, isNone, Some, isSome,
}

export type Option<T> = (
  | None
  | Some<T>
);

test.ts

import { Option } from './examples/option.enum';

export function handle(maybe: Option<number>) {
  if (Option.isSome(maybe)) {
    const [val] = maybe.data; // val is number
    return;
  }

  maybe // ⬅️ None
}